TOM SWIFT AND HIS AERIAL WARSHIP

or

The educational robot building kits for 14 year oldsNaval Terror of the Seas

BY

VICTOR APPLETON

CONTENTS

TOM SWIFT AND HIS AERIAL WARSHIP

CHAPTER I

TOM IS PUZZLED

"What's the matter, Tom? You look rather blue!"

"Blue! Say, Ned, I'd turn red, green, yellow, or any other color of therainbow, if I thought it would help matters any."

"Whew!"

Ned Newton, the chum and companion of Tom Swift, gave vent to a whistleof surprise, as he gazed at the young fellow sitting opposite him, neara bench covered with strange-looking tools and machinery, whileblueprints and drawings were scattered about.

Ranged on the sides of the room were models of many queer craft, mostof them flying machines of one sort or another, while through the opendoor that led into a large shed could be seen the outlines of a speedymonoplane.

"As bad as that, eh, Tom?" went on Ned. "I thought something was upwhen I first came in, but, if you'll excuse a second mention of thecolor scheme, I should say it was blue—decidedly blue. You look asthough you had lost your last friend, and I want to assure you that ifyou do feel that way, it's dead wrong. There's myself, for one, andI'm sure Mr. Damon—"

"Bless my gasoline tank!" exclaimed Tom, with a laugh, in imitation ofthe gentleman Ned Newton had mentioned, "I know that! I'm not worryingover the loss of any friends."

"And there are Eradicate, and Koku, the giant, just to mention a coupleof others," went on Ned, with a smile.

"That's enough!" exclaimed Tom. "It isn't that, I tell you."

"Well, what is it then? Here I go and get a half-holiday off from thebank, and just at the busiest time, too, to come and see you, and Ifind you in a brown study, looking as blue as indigo, and maybe you'reall yellow inside from a bilious attack, for all I know."

"Quite a combination of colors," admitted Tom. "But it isn't what youthink. It's just that I'm puzzled, Ned."

"Puzzled?" and Ned raised his eyebrows to indicate how surprised he wasthat anything should puzzle his friend.

"Yes, genuinely puzzled."

"Has anything gone wrong?" Ned asked. "No one is trying to take any ofyour pet inventions away from you, is there?"

"No, not exactly that, though it is about one of my inventions I ampuzzled. I guess I haven't shown you my very latest; have I, Ned?"

"Well, I don't know, Tom. Time was when I could keep track of you andyour inventions, but that was in your early days, when you started witha motorcycle and were glad enough to have a motorboat. But, sinceyou've taken to aerial navigation and submarine work, not to mentionone or two other lines of activity, I give up. I don't know where tolook next, Tom, for something new."

"Well, this isn't so very new," went on the young inventor, for TomSwift had designed and patented many new machines of the air, earth andwater. "I'm just trying to work out some new problems in aerialnavigation, Ned," he went on.

"I thought there weren't any more," spoke Ned, soberly enough.

"Come, now, none of that!" exclaimed Tom, with a laugh. "Why, thesurface of aerial navigation has only been scratched. The science isfar from being understood, or even made safe, not to say perfected, aswater and land travel have been. There's lots of chance yet."

"And you're working on something new?" asked Ned, as he looked aroundthe shop where he and Tom were sitting. As the young bank employee hadsaid, he had come away from the institution that afternoon to have alittle holiday with his chum, but Tom, seated in the midst of hisinventions, seemed little inclined to jollity.

Through the open windows came the hum of distant machinery, for TomSwift and his father were the heads of a company founded to manufactureand market their many inventions, and about their home were groupedseveral buildings. From a small plant the business had grown to be agreat tree, under the direction of Tom and his father.

"Yes, I'm working on something new," admitted Tom, after a moment ofsilence.

"And, Ned," he went on, "there's no reason why you shouldn't see it.I've been keeping it a bit secret, until I had it a little furtheradvanced, but I've got to a point now where I'm stuck, and perhaps itwill do me good to talk to someone about it."

"Not to talk to me, though, I'm afraid. What I don't know aboutmachinery, Tom, would fill a great many books. I don't see how I canhelp you," and Ned laughed.

"Well, perhaps you can, just the same, though you may not know a lot oftechnical things about machines. It sometimes helps me just to tell mytroubles to a disinterested person, and hear him ask questions. I'vegot dad half distracted trying to solve the problem, so I've had to letup on him for a while. Come on out and see what you make of it."

"Sure, Tom, anything to oblige. If you want me to sit in front of yourphoto-telephone, and have my picture taken, I'm agreeable, even if youshoot off a flashlight at my ear. Or, if you want me to see how long Ican stay under water without breathing I'll try that, too, provided youdon't leave me under too long, lead the way—I'm agreeable as far asI'm able, old man."

"Oh, it isn't anything like that," Tom answered with a laugh. "I mightas well give you a few hints, so you'll know what I'm driving at. ThenI'll take you out and show it to you."

"What is it—air, earth or water?" asked Ned Newton, for he knew hischum's activities led along all three lines.

"This happens to be air."

"A new balloon?"

"Something like that. I call it my aerial warship, though."

"Aerial warship, Tom! That sounds rather dangerous!"

"It will be dangerous, too, if I can get it to work. That's what it'sintended for."

"But a warship of the air!" cried Ned. "You can't mean it. A warshipcarries guns, mortars, bombs, and—"

"Yes, I know," interrupted Tom, "and I appreciate all that when Icalled my newest craft an aerial warship."

"But," objected Ned, "an aircraft that will carry big guns will be solarge that—"

"Oh, mine is large enough," Tom broke in.

"Then it's finished!" cried Ned eagerly, for he was much interested inhis chum's inventions.

"Well, not exactly," Tom said. "But what I was going to tell you wasthat all guns are not necessarily large. You can get big results withsmall guns and projectiles now, for high-powered explosives come insmall packages. So it isn't altogether a question of carrying a certainamount of weight. Of course, an aerial warship will have to be big, forit will have to carry extra machinery to give it extra speed, and itwill have to carry a certain armament, and a large crew will be needed.So, as I said, it will need to be large. But that problem isn'tworrying me."

"Well, what is it, then?" asked Ned.

"It's the recoil," said Tom, with a gesture of despair.

"The recoil?" questioned Ned, wonderingly.

"Yes, from the guns, you know. I haven't been able to overcome that,and, until I do, I'm afraid my latest invention will be a failure."

Ned shook his head.

"I'm afraid I can't help you any," he said. "The only thing I knowabout recoils is connected with an old shotgun my father used to own.

"I took that once, when he didn't know it," Ned proceeded. "It waspretty heavily loaded, for the crows had been having fun in ourcornfield, and dad had been shooting at them. This time I thought I'dtake a chance.

"Well, I fired the gun. But it must have had a double charge in it andbeen rusted at that. All I know is that after I pulled the trigger Ithought the end of the world had come. I heard a clap of thunder, andthen I went flying over backward into a blackberry patch."

"That was the recoil," said Tom.

"The what?" asked Ned.

"The recoil. The recoil of the gun knocked you over."

"Oh, yes," observed Ned, rubbing his shoulder in a reflective sort ofway. "I always thought it was something like that. But, at the time Iput it down to an explosion, and let it go at that."

"No, it wasn't an explosion, properly speaking," said Tom. "You see,when powder explodes, in a gun, or otherwise, its force is exerted inall directions, up, down and every way."

"This went mostly backward—in my direction," said Ned ruefully.

"You only thought so," returned Tom. "Most of the power went out infront, to force out the shot. Part of it, of course, was exerted on thebarrel of the gun—that was sideways—but the strength of the steelheld it in. And part of the force went backward against your shoulder.That part was the recoil, and it is the recoil of the guns I figure onputting aboard my aerial warship that is giving me such trouble."

"Is that what makes you look so blue?" asked Ned.

"That's it. I can't seem to find a way by which to take up the recoil,and the force of it, from all the guns I want to carry, will just abouttear my ship to pieces, I figure."

"Then you haven't actually tried it out yet?" asked Ned.

"Not the guns, no. I have the warship of the air nearly done, but I'veworked out on paper the problem of the guns far enough so that I knowI'm up against it. It can't be done, and an aerial warship without gunswouldn't be worth much, I'm afraid."

"I suppose not," agreed Ned. "And is it only the recoil that isbothering you?"

"Mostly. But come, take a look at my latest pet," and Tom arose to leadthe way to another shed, a large one in the distance, toward which hewaved his hand to indicate to his chum that there was housed thewonderful invention.

The two chums crossed the yard, threading their way through the variousbuildings, until they stood in front of the structure to which Tom hadcalled attention.

"It's in here," he said. "I don't mind admitting that I'm quite proudof it, Ned; that is, proud as far as I've gone. But the gun businesssure has me worried. I'm going to talk it off on you. Hello!" criedTom suddenly, as he put a key in the complicated lock on the door,"someone has been in here. I wonder who it is?"

Ned was a little startled at the look on Tom's face and the sound ofalarm in his chum's voice.

CHAPTER II

A FIRE ALARM

Tom Swift quickly opened the door of the big shed. It was built tohouse a dirigible balloon, or airship of some sort. Ned could easilytell that from his knowledge of Tom's previous inventions.

"Something wrong?" asked the young bank clerk.

"I don't know," returned Tom, and then as he looked inside the place,he breathed a sigh of relief.

"Oh, it's you, is it, Koku?" he asked, as a veritable giant of a mancame forward.

"Yes, master, it is only Koku and your father," spoke the big chap,with rather a strange accent.

"Oh, is my father here?" asked Tom. "I was wondering who had opened thedoor of this shed."

"Yes, Tom," responded the elder Swift, coming up to them, "I had a newidea in regard to some of those side guy wires, and I wanted to try itout. I brought Koku with me to use his strength on some of them."

"That's all right, Dad. Ned and I came out to wrestle with that recoilproblem again. I want to try some guns on the craft soon, but—"

"You'd better not, Tom," warned his father. "It will never work, I tellyou. You can't expect to take up quick-firing guns and bombs in anairship, and have them work properly. Better give it up."

"I never will. I'll make it work, Dad!"

"I don't believe you will, Tom. This time you have bitten off more thanyou can chew, to use a homely but expressive statement."

"Well, Dad, we'll see," began Tom easily. "There she is, Ned," he wenton. "Now, if you'll come around here..."

But Tom never finished that sentence, for at that moment there camerunning into the airship shed an elderly, short, stout, fussygentleman, followed by an aged colored man. Both of them seemed verymuch excited.

"Bless my socks, Tom!" cried the short, stout man. "There sure istrouble!"

"I should say So, Massa Tom!" added the colored man. "I done didprognosticate dat some day de combustible material of which dat shed amcomposed would conflaggrate—"

"What's the matter?" interrupted Tom, jumping forward. "Speak out!Eradicate! Mr. Damon, what is it?"

"The red shed!" cried the short little man. "The red shed, Tom!"

"It's on fire!" yelled the colored man.

"Great thunderclaps!" cried Tom. "Come on—everybody on the job!" heyelled. "Koku, pull the alarm! If that red shed goes—"

Instantly the place was in confusion. Tom and Ned, looking from awindow of the hangar, saw a billow of black smoke roll across the yard.But already the private fire bell was clanging out its warning. And,while the work of fighting the flames is under way, I will halt theprogress of this story long enough to give my new readers a little ideaof who Tom Swift is, so they may read this book more intelligently.Those of you who have perused the previous volumes may skip this part.

Tom Swift, though rather young in years, was an inventor of note. Histastes and talents were developed along the line of machinery andlocomotion. Motorcycles, automobiles, motorboats, submarine craft, and,latest of all, craft of the air, had occupied the attention of TomSwift and his father for some years.

Mr. Swift was a widower, and lived with Tom, his only son, in thevillage of Shopton, New York State. Mrs. Baggert kept house for them,and an aged colored man, Eradicate Sampson, with his mule, Boomerang,did "odd jobs" about the Shopton home and factories.

Among Tom's friends was a Mr. Wakefield Damon, from a nearby village.Mr. Damon was always blessing something, from his hat to his shoes, aharmless sort of habit that seemed to afford him much comfort. Thenthere was Ned Newton, a boyhood chum of Tom's, who worked in theShopton bank. I will just mention Mary Nestor, a young lady of Shopton,in whom Tom was more than ordinarily interested. I have spoken of Koku,the giant. He really was a giant of a man, of enormous strength, andwas one of two whom Tom had brought with him from a strange land whereTom was held captive for a time. You may read about it in a bookdevoted to those adventures.

Tom took Koku into his service, somewhat to the dismay of Eradicate,who was desperately jealous. But poor Eradicate was getting old, andcould not do as much as he thought he could. So, in a great measure,Koku replaced him, and Tom found much use for the giant's strength.

Tom had begun his inventive work when, some years before this storyopens, he had bargained for Mr. Damon's motorcycle, after that machinehad shot its owner into a tree. Mr. Damon was, naturally, perhaps, muchdisgusted, and sold the affair cheap. Tom repaired it, made someimprovements, and, in the first volume of this series, entitled "TomSwift and His Motor-cycle," you may read of his rather thrillingadventures on his speedy road-steed.

From then on Tom had passed a busy life, making many machines andhaving some thrilling times with them. Just previous to the opening ofthis story Tom had made a peculiar instrument, described in the volumeentitled "Tom Swift and His Photo-Telephone." With that a persontalking could not only see the features of the person with whom he wasconversing, but, by means of a selenium plate and a sort of camera, apermanent picture could be taken of the person at either end of thewire.

By means of this invention Tom had been able to make a picture that hadsaved a fortune. But Tom did not stop there. With him to invent was asnatural and necessary as breathing. He simply could not stop it. And sowe find him now about to show to his chum, Ned Newton, his latestpatent, an aerial warship, which, however, was not the success Tom hadhoped for.

But just at present other matters than the warship were in Tom's mind.The red shed was on fire.

That mere statement might not mean anything special to the ordinaryperson, but to Tom, his father, and those who knew about his shops, itmeant much.

"The red shed!" Tom cried. "We mustn't let that get the best of us!Everybody at work! Father, not you, though. You mustn't exciteyourself!"

Even in the midst of the alarm Tom thought of his father, for the agedman had a weak heart, and had on one occasion nearly expired, beingsaved just in time by the arrival of a doctor, whom Tom brought to thescene after a wonderful race through the air.

"But, Tom, I can help," objected the aged inventor.

"Now, you just take care of yourself, Father!" Tom cried. "There areenough of us to look after this fire, I think."

"But, Tom, it—it's the red shed!" gasped Mr. Swift.

"I realize that, Dad. But it can't have much of a start yet. Is thealarm ringing, Koku?"

"Yes, Master," replied the giant, in correct but stilted English. "Ihave set the indicator to signal the alarm in every shop on thepremises."

"That's right." Tom sprang toward the door. "Eradicate!" he called.

"Yais, sah! Heah I is!" answered the colored man. "I'll go git mahmule, Boomerang, right away, an' he—"

"Don't you bring Boomerang on the scene!" Tom yelled. "When I want thatshed kicked apart I can do it better than by using a mule's heels. Andyou know you can't do a thing with Boomerang when he sees fire."

"Now dat's so, Massa Tom. But I could put blinkers on him, an'—"

"No, you let Boomerang stay where he is. Come on, Ned. We'll see whatwe can do. Mr. Damon—"

"Yes, Tom, I'm right here," answered the peculiar man, for he had comeover from his home in Waterford to pay a visit to his friends, Tom andMr. Swift. "I'll do anything I can to help you, Tom, bless my necktie!"he went on. "Only say the word!"

"We've got to get some of the stuff out of the place!" Tom cried. "Wemay be able to save it, but I can't take a chance on putting out thefire and letting some of the things in there go up in smoke. Come on!"

Those in the shed where was housed what Tom hoped would prove to be asuccessful aerial warship rushed to the open. From the other shops andbuildings nearby were pouring men and boys, for the Swift plantemployed a number of hands now.

Above the shouts and yells, above the crackle of flames, could be heardthe clanging of the alarm bell, set ringing by Koku, who had pulled thesignal in the airship shed. From there it had gone to every building inthe plant, being relayed by the telephone operator, whose duty it wasto look after that.

"My, you've got a big enough fire-fighting force, Tom!" cried Ned inhis chum's ear.

"Yes, I guess we can master it, if it hasn't gotten the best of us.Say, it's going some, though!"

Tom pointed to where a shed, painted red—a sign of danger—could beseen partly enveloped in smoke, amid the black clouds of which shot outred tongues of flame.

"What have you got it painted red for?" Ned asked pantingly, as theyran on.

"Because—" Tom began, but the rest of the sentence was lost in a yell.

Tom had caught sight of Eradicate and the giant, Koku, unreeling from acentral standpipe a long line of hose.

"Don't take that!" Tom cried. "Don't use that hose! Drop it!"

"What's the matter? Is it rotten?" Ned wanted to know.

"No, but if they pull it out the water will be turned on automatically."

"Well, isn't that what you want at a fire—water?" Ned demanded.

"Not at this fire," was Tom's answer. "There's a lot of calcium carbidein that red shed—that's why it's red—to warn the men of danger. Youknow what happens when water gets on carbide—there's an explosion, andthere's enough carbide in that shed to send the whole works sky high.

"Drop that hose!" yelled Tom in louder tones. "Drop it, Rad—Koku! Doyou want to kill us all!"

CHAPTER III

A DESPERATE BATTLE

Tom's tones and voice were so insistent that the giant and the coloredman had no choice but to obey. They dropped the hose which, halfunreeled, lay like some twisted snake in the grass. Had it been pulledout all the way the water would have spurted from the nozzle, for itwas of the automatic variety, with which Tom had equipped all his plant.

"But what are you going to do, Tom, if you don't use water?" asked Ned,wonderingly.

"I don't know—yet, but I know water is the worst thing you can put oncarbide," returned Tom. For all he spoke Slowly his brain was workingfast. Already, even now, he was planning how best to give battle to theflames.

It needed but an instant's thought on the part of Ned to make himunderstand that Tom was right. It would be well-nigh fatal to use wateron carbide. Those of you who have bicycle lanterns, in which that notvery pleasant-smelling chemical is used, know that if a few drops ofwater are allowed to drip slowly on the gray crystals acetylene gas isgenerated, which makes a brilliant light. But, if the water drips toofast, the gas is generated too quickly, and an explosion results. Inlamps, of course, and in lighting plants where carbide is used, thereare automatic arrangements to prevent the water flowing too freely tothe chemical. But Tom knew if the hose were turned on the fire in thered shed a great explosion would result, for some of the tins ofcarbide would be melted by the heat.

Yet the fire needed to be coped with. Already the flames were comingthrough the roof, and the windows and door were spouting red fire andvolumes of smoke.

Several other employees of Tom's plant had made ready to unreel morehose, but the warning of the young inventor, shouted to Eradicate andKoku, had had its effect. Every man dropped the line he had begun tounreel.

"Ha! Massa Tom say drop de hose, but how yo' gwine t' squirt watah ona fire wifout a hose; answer me dat?" and Eradicate looked at Koku.

"Me no know," was the slow answer. "I guess Koku go pull shed down andstamp out fire."

"Huh! Maybe yo' could do dat in cannibal land, where yo' all comefrom," spoke Eradicate, "but yo' can't do dat heah! 'Sides, de red shedwill blow up soon. Dere's suffin' else in dere except carbide, an'dat's gwine t' go up soon, dat's suah!"

"Maybe you get your strong man-mule, Boomerang," suggested Koku."Nothing ever hurt him—explosion or nothing. He can kick shed all topieces, and put out fire."

"Dat's what I wanted t' do, but Massa Tom say I cain't," explained thecolored man. "Golly! Look at dat fire!"

Indeed the blaze was now assuming alarming proportions. The red shed,which was not a small structure, was blazing on all sides. About itstood the men from the various shops.

"Tom, you must do something," said Mr. Swift. "If the flames once reachthat helmanite—"

"I know, Father. But that explosive is in double vacuum containers, andit will be safe for some time yet. Besides, it's in the cellar. It'sthe carbide I'm most worried about. We daren't use water."

"But something will have to be done!" exclaimed Mr. Damon. "Bless myred necktie, if we don't—"

"Better get back a way," suggested Tom. "Something may go off!"

His words of warning had their effect, and the whole circle moved backseveral paces.

"Is there anything of value in the shed?" asked Ned.

"I should say there was!" Tom answered. "I hoped we could get some ofthem out, but we can't now—until the fire dies down a bit, at anyrate."

"Look, Tom! The pattern shop roof is catching!" shouted Mr. Swift,pointing to where a little spurt of flame showed on the roof of adistant building.

"It's from sparks!" Tom said.

"Any danger of using water there?" Ned wanted to know.

"No, use all you like! That's the only thing to do. Come on, you withthe hose!" Tom yelled. "Save the other buildings!"

"But are you going to let the red shed burn?" asked Mr. Swift. "Youknow what it means, Tom."

"Yes, Father, I know. And I'm going to fight that fire in a new way.But we must save the other buildings, too. Play water on all the othersheds and structures!" ordered the young inventor. "I'll tackle thisone myself. Oh, Ned!" he called.

"Yes," answered his chum. "What is it?"

"You take charge of protecting the place where the new aerial warshipis stored. Will you? I can't afford to lose that."

"I'll look after it, Tom. No harm in using water there, though; isthere?"

"Not if you don't use too much. Some of the woodwork isn't varnishedyet, and I wouldn't want it to be wet. But do the best you can. TakeKoku and Eradicate with you. They can't do any good here."

"Do you mean to say you're going to give up and let this burn?"

"Not a bit of it, Ned. But I have another plan I want to try. Livelynow! The wind's changing, and it's blowing over toward my aerialwarship shed. If that catches—"

Tom shook his head protestingly, and Ned set off on the run, calling tothe colored man and the giant to get out another line of hose.

"I wonder what Tom is going to do?" mused Ned, as he neared the bigshed he and the others had left on the alarm of fire.

Tom, himself, seemed in no doubt as to his procedure. With one look atthe blazing red shed, as if to form an opinion as to how much longer itcould burn without getting entirely beyond control, Tom set off on arun toward another large structure. Ned, glancing toward his chum,observed:

"The dirigible shed! I wonder what his game is? Surely that can't bein danger—it's too far off!"

Ned was right as to the last statement. The shed, where was housed agreat dirigible balloon Tom had made, but which he seldom used of late,was sufficiently removed from the zone of fire to be out of danger.

Meanwhile several members of the fire-fighting force that had beensummoned from the various shops by the alarm, had made an effort tosave from the red shed some of the more valuable of the contents. Therewere some machines in there, as well as explosives and chemicals, inaddition to the store of carbide.

But the fire was now too hot to enable much to be done in the way ofsalvage. One or two small things were carried out from a littleaddition to the main structure, and then the rescuers were driven backby the heat of the flames, as well as by the rolling clouds of blacksmoke.

"Keep away!" warned Mr. Swift. "It will explode soon. Keep back!"

"That's right!" added Mr. Damon. "Bless my powder-horn! We may all begoing sky-high soon, and without aid from any of Tom Swift'saeroplanes, either."

Warned by the aged inventor, the throng of men began slowly moving awayfrom the immediate neighborhood of the blazing shed. Though it mayseem to the reader that some time has elapsed since the first soundingof the alarm, all that I have set down took place in a very shortperiod—hardly three minutes elapsing since Tom and the others camerushing out of the aerial warship building.

Suddenly a cry arose from the crowd of men near the red shed. Ned, whostood ready with several lines of hose, in charge of Koku, Eradicateand others, to turn them on the airship shed, in case of need, lookedin the direction of the excited throng.

The young bank clerk saw a strange sight. From the top of the dirigibleballoon shed a long, black, cigar-shaped body arose, floating graduallyupward. The very roof of the shed slid back out of the way, as Tompressed the operating lever, and the dirigible was free to rise—asfree as though it had been in an open field.

"He's going up!" cried Ned in surprise. "Making an ascent at a timelike this, when he ought to stay here to fight the fire! What's gotteninto Tom, I'd like to know? I wonder if he can be—"

Ned did not finish his half-formed sentence. A dreadful thought cameinto his mind. What if the sudden fire, and the threatened danger, aswell as the prospective loss that confronted Tom, had affected his mind?

"It certainly looks so," mused Ned, as he saw the big balloon floatfree from the shed. There was no doubt but that Tom was in it. He couldbe seen standing within the pilot-house, operating the various wheelsand levers that controlled the ship of the air.

"What can he be up to?" marveled Ned. "Is he going to run away from thefire?"

Koku, Eradicate and several others were attracted by the sight of thegreat dirigible, now a considerable distance up in the air. Certainlyit looked as though Tom Swift were running away. Yet Ned knew his chumbetter than that.

Then, as they watched, Ned and the others saw the direction of theballoon change. She turned around in response to the influence of therudders and propellers, and was headed straight for the blazing shed,but some distance above it.

"What can he be planning?" wondered Ned.

He did not have long to wait to find out.

An instant later Tom's plan was made clear to his chum. He saw Tomcircling over the burning red shed, and then the bank clerk saw whatlooked like fine rain dropping from the lower part of the balloonstraight into the flames.

"He can't be dousing water on from up above there," reasoned Ned."Pouring water on carbide from a height is just as bad as spurting iton from a hose, though perhaps not so dangerous to the persons doingit. But it can't be—"

"By Jove!" suddenly exclaimed Ned, as he had a better view of what wasgoing on. "It's sand, that's what it is! Tom is giving battle to theflames with sand from the ballast bags of the dirigible! Hurray? That'sthe ticket! Sand! The only thing safe to use in case of an explosivechemical fire.

"Fine for you. Tom Swift! Fine!"

CHAPTER IV

SUSPICIONS

High up aloft, over the blazing red shed, with its dangerous contentsthat any moment might explode, Tom Swift continued to hold his bigdirigible balloon as near the flames as possible. And as he stoodoutside on the small deck in front of the pilot-house, where werelocated the various controls, the young inventor pulled the levers thatemptied bag after bag of fine sand on the spouting flames that,already, were beginning to die down as a result of this effectualquenching.

"Tom's done the trick!" yelled Ned, paying little attention now to thebig airship shed, since he saw that the danger was about over.

"Dhat's what he suah hab done!" agreed Eradicate. "Mah ole muleBoomerang couldn't 'a' done any better."

"Huh! Your mule afraid of fire," remarked Koku.

"What's dat? Mah mule afraid ob fire?" cried the colored man. "Lookheah, yo' great, big, overgrowed specimen ob an equilateral quadruped,I'll hab yo' all understand dat when yo' all speaks dat way about afriend ob mine dat yo'—"

"That'll do, Rad!" broke in Ned, with a laugh. He knew that when Tom'shelper grew excited on the subject of his mule there was no stoppinghim, and Boomerang was a point on which Eradicate and Koku were alwaysarguing. "The fire is under control now."

"Yes, it seems to have gone visiting," observed Koku.

"Visiting?" queried Ned, in some surprise.

"Yes, that is, it is going out," went on Koku.

"Oh, I understand!" laughed Ned. "Yes, and I hope it doesn't pay usanother visit soon. Oh, look at Tom, would you!" he cried, for theyoung aviator had swung his ship about over the flames, to bringanother row of sand bags directly above a place where the fire washottest.

Down showered more sand from the bags which Tom opened. No fire couldlong continue to blaze under that treatment. The supply of air was cutoff, and without that no fire can exist. Water would have been worsethan useless, because of the carbide, but the sand covered it up sothat it was made perfectly harmless.

Moving slowly, the airship hovered over every part of the now slowlyexpiring flames, the burned opening in the roof of the shed making itpossible for the sand to reach the spots where it was most needed. Theflames died out in section after section, until no more could beseen—only clouds of black smoke.

"How is it now?" came Tom's voice, as he spoke from the deck of theballoon through a megaphone.

"Almost out," answered Mr. Damon. "A little more sand, Tom."

The eccentric man had caught up a piece of paper and, rolling it into acone, made an improvised megaphone of that.

"Haven't much more sand left," was Tom's comment, as he sent down alast shower. "That will have to do. Hustle that carbide and otherexplosive stuff out of there now, while you have a chance."

"That's it!" cried Ned, who caught his chums meaning. "Come on, Koku.There's work for you."

"Me like work," answered the giant, stretching out his great arms.

The last of the sand had completely smothered the fire, and Tom,observing from aloft that his work was well done, moved away in thedirigible, sending it to a landing space some little distance away fromthe shed whence it had arisen. It was impossible to drop it back againthrough the roof of the hangar, as the balloon was of such bulk thateven a little breeze would deflect it so that it could not beaccurately anchored. But Tom had it under very good control, and soonit was being held down on the ground by some of his helpers.

As all the sand ballast had been allowed to run out Tom was obliged toopen the gas-valves and let some of the lifting vapor escape, or hecould not have descended.

"Come on, now!" cried the inventor, as he leaped from the deck of hissky craft. "Let's clean out the red shed. That fire is only smothered,and there may be sparks smoldering under that sand, which will burstinto flame, if we're not careful. Let's get the explosives out of theway."

"Bless my insurance policy, yes," exclaimed Mr. Damon. "That was a finemove of yours."

"It was the only way I could think of to put out the fire," Tomreplied. "I knew water was out of the question, and sand was the nextthing."

"But I didn't know where to get any until I happened to think of theballast bags of my dirigible. Then I knew, if I could get above thefire, I could do the trick. I had to fly pretty high, though, as thefire was hot, and I was afraid it might explode the gas bag and wreckme."

"You were taking a chance," remarked Ned.

"Oh, well, you have to take chances in this business," observed Tom,with a smile. "Now, then, let's finish this work."

The sand, falling from the ballast bags of the dirigible, had soeffectually quenched the fire that it was soon cool enough to permitclose approach. Koku, Tom and some of the men who best knew how tohandle the explosives, were soon engaged in the work of salvage.

"I wish I could help you, Tom," said his aged father. "I don't seemable to do anything but stand here and look on," and he gazed about himrather sadly.

"Never you mind, Dad!" Tom exclaimed. "We'll get along all right now.You'd better go up to the house. Mr. Damon will go with you."

"Yes, of course!" exclaimed the odd man, catching a wink from Tom, whowanted his father not to get too excited on account of his weak heart."Come along, Professor Swift. The danger is all over."

"All right," assented the aged inventor, with a look at the stillsmoking shed.

"And, Dad, when you haven't anything else to do," went on Tom, ratherwhimsically, "you might be thinking up some plan to take up the recoilof those guns on my aerial warship. I confess I'm clean stumped on thatpoint."

"Your aerial warship will never be a success," declared Mr. Swift. "Youmight as well give that up, Tom."

"Don't you believe it, Dad!" cried Tom, with more of a jolly air of onechum toward another than as though the talk was between father and son."You solve the recoil problem for me, and I'll take care of the rest,and make the air warship sail. But we've got something else to do justnow. Lively, boys."

While Mr. Swift, taking Mr. Damon's arm, walked toward the house, Tom,Ned, Koku, and some of the workmen began carrying out the explosiveswhich had so narrowly escaped the fire. With long hooks the men pulledthe shed apart, where the side walls had partly been burned through.Tom maintained an efficient firefighting force at his works, and themen had the proper tools with which to work.

Soon large openings were made on three sides of the red shed, orrather, what was left of it, and through these the dangerous chemicalsand carbide, in sheet-iron cans, were carried out to a place of safety.In a little while nothing remained but a heap of hot sand, some charredembers and certain material that had been burned.

"Much loss, Tom?" asked Ned, as they surveyed the ruins. They were bothblack and grimy, tired and dirty, but there was a great sense ofsatisfaction.

"Well, yes, there's more lost than I like to think of," answered Tomslowly, "but it would have been a heap sight worse if the stuff hadgone up. Still, I can replace what I've lost, except a few models Ikept in this place. I really oughtn't to have stored them here, butsince I've been working on my new aerial warship I have sort of letother matters slide. I intended to make the red shed nothing but astorehouse for explosive chemicals, but I still had some of my plansand models in it when it caught."

"Only for the sand the whole place might have gone," said Ned in a lowvoice.

"Yes. It's lucky I had plenty of ballast aboard the dirigible. Yousee, I've been running it alone lately, and I had to take on plenty ofsand to make up for the weight of the several passengers I usuallycarry. So I had plenty of stuff to shower down on the fire. I wonderhow it started, anyhow? I must investigate this."

"Mr. Damon and Eradicate seem to have seen it first," remarked Ned.

"Yes. At least they gave the alarm. Guess I'll ask Eradicate how hehappened to notice. Oh, I say, Rad!" Tom called to the colored man.

"Yais, sah, Massa Tom! I'se comin'!" the darky cried, as he finishedpiling up, at a safe distance from the fire, a number of cans ofcarbide.

"How'd you happen to see the red shed ablaze?" Tom asked.

"Why, it was jest dish yeah way, Massa Tom," began the colored man. "Ihad jest been feedin' mah mule, Boomerang. He were pow'ful hungry,Boomerang were, an', when I give him some oats, wif a carrot sliced upin 'em—no, hole on—did I gib him a carrot t'day, or was ityist'day?—I done fo'got. No, it were yist'day I done gib him decarrot, I 'member now, 'case—"

"Oh, never mind the carrot, or Boomerang, either, Rad!" broke in Tom,"I'm asking you about the fire."

"An' I'se tellin' yo', Massa Tom," declared Eradicate, with a ratherreproachful look at his master. "But I wanted t' do it right an'proper. I were comin' from Boomerang's stable, an' I see suffin' redspoutin' up at one corner ob de red shed. I knowed it were fire rightaway, an' I yelled."

"Yes, I heard you yell," Tom said. "But what I wanted to know is, didyou see anyone near the red shed at the time?"

"No, Massa Tom, I done didn't."

"I wonder if Mr. Damon did? I must ask him," went on the younginventor. "Come, on, Ned, we'll go up to the house. Everything is allright here, I think. Whew! But that was some excitement. And I didn'tshow you my aerial warship after all! Nor have you settled that recoilproblem for me."

"Time enough, I guess," responded Ned. "You sure did have a luckyescape, Tom."

"That's right. Well, Koku, what is it?" for the giant had approached,holding out something in his hand.

"Koku found this in red shed," went on the giant, holding out a round,blackened object. "Maybe him powder; go bang-bang!"

"Oh, you think it's something explosive, eh?" asked Tom, as he took theobject from the giant.

"Koku no think much," was the answer. "Him look funny."

Tom did not speak for a moment. Then he cried:

"Look funny! I should say it did! See here, Ned, if this isn'tsuspicious I'll eat my hat!" and Tom beckoned excitedly to his chum,who had walked on a little in advance.

CHAPTER V

A QUEER STRANGER

What Tom Swift held in his hand looked like a small cannon ball, but itcould not have been solid or the young aviator would not so easily haveheld it out at arm's length for his friend Ned Newton to look at.

"This puts a different face on it, Ned," Tom went on, as he turned theobject over.

"Is that likely to go off?" the bank clerk asked, as he came to a halta little distance from his friend.

"Go off? No, it's done all the damage it could, I guess."

"Damage? It looks to me as though it had suffered the most damageitself. What is it, one of your models? Looks like a bomb to me."

"And that's what it is, Ned."

"Not one of those you're going to use on your aerial warship, is it,Tom?"

"Not exactly. I never saw this before, but it's what started the firein the red shed all right; I'm sure of that."

"Do you really mean it?" cried Ned.

"I sure do."

"Well, if that's the case, I wouldn't leave such dangerous thingsaround where there are explosives, Tom."

"I didn't, Ned. I wouldn't have had this within a hundred miles of myshed, if I could have had my way. It's a fire bomb, and it was set togo off at a certain time. Only I think something went wrong, and thebomb started a fire ahead of time.

"If it had worked at night, when we were all asleep, we might not haveput the fire out so easily. This sure is suspicious! I'm glad you foundthis, Koku."

Tom was carefully examining the bomb, as Ned had correctly named it.The bank clerk, now that he was assured by his chum that the object haddone all the harm it could, approached closer.

What he saw was merely a hollow shell of iron, with a small opening init, as though intended for a place through which to put a charge ofexplosives and a fuse.

"But there was no explosion, Tom," explained Ned.

"I know it," said Tom quietly. "It wasn't an explosive bomb. Smellthat!"

He held the object under Ned's nose so suddenly that the young bankclerk jumped back.

"Oh, don't get nervous," laughed Tom. "It can't hurt you now. But whatdoes that smell like?"

Ned sniffed, sniffed again, thought for a moment, and then sniffed athird time.

"Why," he said slowly, "I don't just know the name of it, but it's thatfunny stuff you mix up sometimes to put in the oxygen tanks when we goup in the rarefied atmosphere in the balloon or airship."

"Manganese and potash," spoke Tom. "That and two or three other thingsthat form a chemical combination which goes off by itself ofspontaneous combustion after a certain time. Only the person who putthis bomb together didn't get the chemical mixture just right, and itwent off ahead of time; for which we have to be duly thankful."

"Do you really think that, Tom?" cried Ned.

"I'm positive of it," was the quiet answer.

"Why—why—that would mean some one tried to set fire to the red shed,Tom!"

"They not only tried it, but did it," responded Tom, more coolly thanseemed natural under the circumstances. "Only for the fact that themixture went off before it was intended to, and found us all alert andready—well, I don't like to think what might have happened," and Tomcast a look about at his group of buildings with their valuablecontents.

"You mean some one purposely put that bomb in the red shed, Tom?"

"That's exactly what I mean. Some enemy, who wanted to do me an injury,planned this thing deliberately. He filled this steel shell withchemicals which, of themselves, after a certain time, would send out ahot tongue of flame through this hole," and Tom pointed to the openingin the round steel shell.

"He knew the fire would be practically unquenchable by ordinary means,and he counted on its soon eating its way into the carbide and otherexplosives. Only it didn't."

"Why, Tom!" cried Ned. "It was just like one of those alarm-clockdynamite bombs—set to go off at a certain time."

"Exactly," Tom said, "only this was more delicate, and, if it hadworked properly, there wouldn't have been a vestige left to give us aclue. But the fire, thanks to the ballast sand in the dirigible, wasput out in time. The fuse burned itself out, but I can tell by thesmell that chemicals were in it. That's all, Koku," he went on to thegiant who had stood waiting, not understanding all the talk between Tomand Ned. "I'll take care of this now."

"Bad man put it there?" asked the giant, who at least comprehended thatsomething was wrong.

"Well, yes, I guess you could say it was a bad man," replied Tom.

"Ha! If Koku find bad man—bad for that man!" muttered the giant, as heclasped his two enormous hands together, as though they were already onthe fellow who had tried to do Tom Swift such an injury.

"I wouldn't like to be that man, if Koku catches him," observed Ned."Have you any idea who it could be, Tom?"

"Not the least. Of course I know I have enemies, Ned. Every successfulinventor has persons who imagine he has stolen their ideas, whether hehas ever seen them or not. It may have been one of those persons, orsome half-mad crank, who was jealous. It would be impossible to say,Ned."

"It wouldn't be Andy Foger, would it?"

"No; I don't believe Andy has been in this neighborhood for some time.The last lesson we gave him sickened him, I guess."

"How about those diamond-makers, whose secret you discovered? Theywouldn't be trying to get back at you, would they?"

"Well, it's possible, Ned. But I don't imagine so. They seem to havebeen pretty well broken up. No, I don't believe it was thediamond-makers who put this fire bomb in the red shed. Their line ofactivities didn't include this branch. It takes a chemist to know justhow to blend the things contained in the bomb, and even a good chemistis likely to fail—as this one did, as far as time went."

"What are you going to do about it?" Ned asked.

"I don't know," and Tom spoke slowly, "I hoped I was done with all thatsort of thing," he went on; "fighting enemies whom I have neverknowingly injured. But it seems they are still after me. Well, Ned,this gives us something to do, at all events."

"You mean trying to find out who these fellows are?"

"Yes; that is, if you are willing to help."

"Well, I guess I am!" cried the bank clerk with sparkling eyes. "Iwouldn't ask anything better. We've been in things like this before,Tom, and we'll go in again—and win! I'll help you all I can. Now,let's see if we can pick up any other clues. This is like old times!"and Ned laughed, for he, like Tom, enjoyed a good "fight," and one inwhich the odds were against them.

"We sure will have our hands full," declared the young inventor."Trying to solve the problem of carrying guns on an aerial warship, andfinding out who set this fire."

"Then you're not going to give up your aerial warship idea?"

"No, indeed!" Tom cried. "What made you think that?"

"Well, the way your father spoke—"

"Oh, dear old dad!" exclaimed Tom affectionately. "I don't want toargue with him, but he's dead wrong!"

"Then you are going to make a go of it?"

"I sure am, Ned! All I have to solve is the recoil proposition, and, assoon as we get straightened out from this fire, we'll tackle thatproblem again—you and I. But I sure would like to know who put this inmy red shed," and Tom looked in a puzzled manner at the empty fire bombhe still held.

Tom paused, on his way to the house, to put the bomb in one of hisoffices.

"No use letting dad know about this," he went on. "It would only besomething else for him to worry about."

"That's right," agreed Ned.

By this time nearly all evidences of the fire, except for the blackenedruins of the shed, had been cleared away. High in the air hung a cloudof black smoke, caused by some chemicals that had burned harmlesslysave for that pall. Tom Swift had indeed had a lucky escape.

The young inventor, finding his father quieted down and conversingeasily with Mr. Damon, who was blessing everything he could think of,motioned to Ned to follow him out of the house again.

"We'll leave dad here," said Tom, "and do a little investigating on ourown account. We'll look for clues while they're fresh."

But, it must be confessed, after Tom and Ned had spent the rest of thatday in and about the burned shed, they were little wiser than when theystarted. They found the place where the fire bomb had evidently beenplaced, right inside the main entrance to the shed. Tom knew it hadbeen there because there were peculiar marks on the charred wood, and acertain queer smell of chemicals that confirmed his belief.

"They put the bomb there to prevent anyone going in at the first alarmand saving anything," Tom said. "They didn't count on the roof burningthrough first, giving me a chance to use the sand. I made the roof ofthe red shed flimsy just on that account, so the force of the explosionif one ever came, would be mostly upward. You know the expanding gases,caused by an explosion or by rapid combustion, always do just aselectricity does, seek the shortest and easiest route. In this case Imade the roof the easiest route."

"A lucky provision," observed Ned.

That night Tom had to confess himself beaten, as far as finding clueswas concerned. The empty fire bomb was the only one, and that seemedvalueless.

Close questioning of the workmen failed to disclose anything. Tom wasparticularly anxious to discover if any mysterious strangers had beenseen about the works. There was a strict rule about admitting them tothe plant, however, and it could not be learned that this had beenviolated.

"Well, we'll just have to lay that aside for a while," Tom said thenext day, when Ned again came to pay a visit. "Now, what do you say totackling, with me, that recoil problem on the aerial warship?"

"I'm ready, if you are," Ned agreed, "though I know about as much ofthose things as a snake does about dancing. But I'm game."

The two friends walked out toward the shed where Tom's new craft washoused. As yet Ned had not seen it. On the way they saw Eradicatewalking along, talking to himself, as he often did.

"I wonder what he has on his mind," remarked Ned musingly.

"Something does seem to be worrying him," agreed Tom.

As they neared the colored man, they could hear him saying:

"He suah did hab nerve, dat's what he did! De idea ob askin' me all demquestions, an' den wantin' t' know if I'd sell him!"

"What's that, Eradicate?" asked Tom.

"Oh, it's a man I met when I were comin' back from de ash dump,"Eradicate explained. One of the colored man's duties was to cart ashesaway from Tom's various shops, and dump them in a certain swampy lot.With an old ramshackle cart, and his mule, Boomerang, Eradicate didthis task to perfection.

"A man—what sort of a man?" asked Tom, always ready to be suspiciousof anything unusual.

"He were a queer man," went on the aged colored helper. "First hestopped me an' asted me fo' a ride. He was a dressed-up gen'man, too,an' I were suah s'prised at him wantin' t' set in mah ole ash cart,"said Eradicate. "But I done was polite t' him, an' fixed a blanket so'she wouldn't git too dirty. Den he asted me ef I didn't wuk fo' yo',Massa Tom, an' of course I says as how I did. Den he asted me about defire, an' how much damage it done, an' how we put it out. An' he end upby sayin' he'd laik t' buy mah mule, Boomerang, an' he wants t' comeheah dis arternoon an' talk t' me about it."

"He does, eh?" cried Tom. "What sort of a man was he, Rad?"

"Well, a gen'man sort ob man, Massa Tom. Stranger t' me. I nebber seedhim afo'. He suah was monstrous polite t' ole black Eradicate, an' hegib me a half-dollar, too, jest fo' a little ride. But I aint' gwine t'sell Boomerang, no indeedy, I ain't!" and Eradicate shook his gray,kinky head decidedly.

"Ned, there may be something in this!" said Tom, in an excited whisperto his chum. "I don't like the idea of a mysterious strangerquestioning Eradicate!"

CHAPTER VI

THE AERIAL WARSHIP

Ned Newton looked at Tom questioningly. Then he glanced at theunsuspicious colored man, who was industriously polishing thehalf-dollar the mysterious stranger had given him.

"Rad, just exactly what sort of a man was this one you speak of?" askedTom.

"Why, he were a gen'man—"

"Yes, I know that much. You've said it before. But was he anEnglishman, an American—or—"

Tom paused and waited for an answer.

"I think he were a Frenchman," spoke Eradicate. "I done didn't see himeat no frogs' laigs, but he smoked a cigarette dat had a funny smell,and he suah was monstrous polite. He suah was a Frenchman. I think."

Tom and Ned laughed at Eradicate's description of the man, but Tom'sface was soon grave again.

"Tell us more about him, Rad," he suggested. "Did he seem especiallyinterested in the fire?"

"No, sah, Massa Tom, he seemed laik he was more special interested inmah mule, Boomerang. He done asted how long I had him, an' how much Iwanted fo' him, an' how old he was."

"But every once in a while he put in some question about the fire, orabout our shops, didn't he, Rad?" Tom wanted to know.

The colored man scratched his kinky head, and glanced with a queer lookat Tom.

"How yo' all done guess dat?" he asked.

"Answer my question," insisted Tom.

"Yes, sah, he done did ask about yo', and de wuks, ebery now and den,"Rad confessed. "But how yo' all knowed dat, Massa Tom, when I werea-tellin' yo' all about him astin' fo' mah mule, done gets me—dat'swhat it suah does."

"Never mind, Rad. He asked questions about the plant, that's all I wantto know. But you didn't tell him much, did you?"

Eradicate looked reproachfully at his master.

"Yo' all done knows me bettah dan dat, Massa Tom," the old colored mansaid. "Yo' all know yo' done gib orders fo' nobody t' talk about yo'projections."

"Yes, I know I gave those orders," Tom said, with a smile, "but I wantto make sure that they have been followed."

"Well, I done follered 'em, Massa Tom."

"Then you didn't tell this queer stranger, Frenchman, or whatever heis, much about my place?"

"I didn't tell him nuffin', sah. I done frowed dust in his eyes."

Ned uttered an exclamation of surprise.

"Eradicate is speaking figuratively," Tom said, with a laugh.

"Dat's what I means," the colored man went on. "I done fooled him. Whenhe asted me about de fire I said it didn't do no damage at all—in fackdat we'd rather hab de fire dan not hab it, 'case it done gib us achance t' practice our hose drill."

"That's good," laughed Tom. "What else?"

"Well, he done sort ob hinted t' me ef we all knowed how de fire donestart. I says as how we did, dat we done start it ourse'ves fo'practice, an dat we done expected it all along, an' were ready fo' it.Course I knows dat were a sort of fairy story, Massa Tom, but den datcigarette-smokin' Frenchman didn't hab no right t' asted me so manyquestions, did he?"

"No, indeed, Rad. And I'm glad you didn't give him straight answers. Sohe's coming here later on, is he?"

"T' see ef I wants t' sell mah mule, Boomerang, yais, sah. I sort obthought maybe you'd want t' hab a look at dat man, so I tole him t'come on. Course I doan't want t' sell Boomerang, but ef he was t' offerme a big lot ob money fo' him I'd take it."

"Of course," Tom answered. "Very well, Rad. You may go on now, anddon't say anything to anyone about what you have told me."

"I won't, Massa Tom," promised the colored man, as he went offmuttering to himself.

"Well, what do you make of it, Tom?" asked Ned of his chum, as theywalked on toward the shed of the new, big aerial warship.

"I don't know just what to think, Ned. Of course things like this havehappened before—persons trying to worm secrets out of Eradicate, orsome of the other men."

"They never succeeded in getting much, I'm glad to say, but it alwayskeeps me worried for fear something will happen," Tom concluded.

"But about this Frenchman?"

"Well, he must be a new one. And, now I come to think of it, I did hearsome of the men speaking about a foreigner—a stranger—being aroundtown last week. It was just a casual reference, and I paid littleattention to it. Now it looks as though there might be something in it."

"Do you think he'll come to bargain with Eradicate about the mule?" Nedasked.

"Hardly. That was only talk to make Eradicate unsuspicious. Thestranger, whoever he was, sized Rad up partly right. I surmised, whenRad said he asked a lot of questions about the mule, that was only todivert suspicion, and that he'd come back to the subject of the fireevery chance he got."

"And you were right."

"Yes, so it seems. But I don't believe the fellow will come aroundhere. It would be too risky. All the same, we'll be prepared for him.I'll just rig up one of my photo-telephone machines, so that, if hedoes come to have a talk with Rad, we can both see and hear him."

"That's great, Tom! But do you think this fellow had anything to dowith the fire?"

"I don't know. He knew about it, of course. This isn't the first firewe've had in the works, and, though we always fight them ourselves,still news of it will leak out to the town. So he could easily haveknown about it. And he might be in with those who set it, for I firmlybelieve the fire was set by someone who has an object in injuring me."

"It's too bad!" declared Ned. "Seems as though they might let youalone, if they haven't gumption enough to invent things for themselves."

"Well, don't worry. Maybe it will come out all right," returned Tom."Now, let's go and have a look at my aerial warship. I haven't shown itto you yet. Then we'll get ready for that mysterious Frenchman, if hecomes—but I don't believe he will."

The young inventor unlocked the door of the shed where he kept hislatest "pet," and at the sight which met his eyes Ned Newton uttered anexclamation of surprise.

"Tom, what is it?" he cried in an awed voice.

"My aerial warship!" was the quiet answer.

Ned Newton gave vent to a long whistle, and then began a detailedexamination of the wonderful craft he saw before him. That is, he madeas detailed an examination as was possible under the circumstances, forit was a long time before the young bank clerk fully appreciated allTom Swift had accomplished in building the Mars, which was the warlikename painted in red letters on the big gas container that tugged andswayed overhead.

"Tom, however did you do it?" gasped Ned at length.

"By hard work," was the modest reply. "I've been at this for a longertime than you'd suppose, working on it at odd moments. I had a lot ofhelp, too, or I never could have done it. And now it is nearly allfinished, as far as the ship itself is concerned. The only thing thatbothers me is to provide for the recoil of the guns I want to carry.Maybe you can help me with that. Come on, now, I'll explain how theaffair works, and what I hope to accomplish with it."

In brief Tom's aerial warship was a sort of German Zeppelin type ofdirigible balloon, rising in the air by means of a gas container, or,rather, several of them, for the section for holding the lifting gaselement was divided by bulkheads.

The chief difference between dirigible balloons and ordinaryaeroplanes, as you all know, is that the former are lifted from theearth by a gas, such as hydrogen, which is lighter than air, while theaeroplane lifts itself by getting into motion, when broad, flat planes,or surfaces, hold it up, just as a flat stone is held up when you sailit through the air. The moment the stone, or aeroplane, loses itsforward motion, it begins to fall.

This is not so with a dirigible balloon. It is held in the air by meansof the lifting gas, and once so in the air can be sent in any directionby means of propellers and rudders.

Tom's aerial warship contained many new features. While it was as largeas some of the war-type Zeppelins, it differed from them materially.But the details would be of more interest to a scientific builder ofsuch things than to the ordinary reader, so I will not weary you withthem.

Sufficient to say that Tom's craft consisted first of a greatsemi-rigid bag, or envelope, made of specially prepared oiled silk andaluminum, to hold the gas, which was manufactured on board. There werea number of gas-tight compartments, so that if one, or even if a numberof them burst, or were shot by an enemy, the craft would still remainafloat.

Below the big gas bag was the ship proper, a light but strong and rigidframework about which were built enclosed cabins. These cabins, orcompartments, housed the driving machinery, the gas-generating plant,living, sleeping and dining quarters, and a pilot-house, whence theship could be controlled.

But this was not all.

Ned, making a tour of the Mars, as she swayed gently in the big shed,saw where several aluminum pedestals were mounted, fore and aft and oneither beam of the ship.

"They look just like places where you intend to mount guns," said Nedto Tom.

"And that's exactly what they are," the young inventor replied. "Ihave the guns nearly ready for mounting, but I can't seem to think of away of providing for the recoil. And if I don't take care of that, I'mlikely to find my ship coming apart under me, after we bombard theenemy with a broadside or two."

"Then you intend to fight with this ship?" asked Ned.

"Well, no; not exactly personally. I was thinking of offering it to theUnited States Government. Foreign nations are getting ready largefleets of aerial warships, so why shouldn't we? Matters in Europe aremighty uncertain. There may be a great war there in which aerial craftwill play a big part. I am conceited enough to think I can build onethat will measure up to the foreign ones, and I'll soon be in aposition to know."

"What do you mean?"

"I mean I have already communicated with our government experts, andthey are soon to come and inspect this craft. I have sent them wordthat it is about finished. There is only the matter of the guns, andsome of the ordnance officers may be able to help me out with asuggestion, for I admit I am stuck!" exclaimed Tom.

"Then you're going to do the same with this aerial warship as you didwith your big lantern and that immense gun you perfected?" asked Ned.

"That's right," confirmed Tom. My former readers will know to what NedNewton referred, and those of you who do not may learn the details ofhow Tom helped Uncle Sam, by reading the previous volumes, "Tom Swiftand His Great Searchlight," and "Tom Swift and His Giant Cannon."

"When do you expect the government experts?" Ned asked.

"Within a few days, now. But I'll have to hustle to get ready for them,as this fire has put me back. There are quite a number of details Ineed to change. Well, now, let me explain about that gun recoilbusiness. Maybe you can help me."

"Fire away," laughed Ned. "I'll do the best I can."

Tom led the way from the main shed, where the aerial warship washoused, to a small private office. As Ned entered, the door, pulled bya strong spring, swung after him. He held back his hand to prevent itfrom slamming, but there was no need, for a patent arrangement took upall the force, and the door closed gently. Ned looked around, not muchsurprised, for the same sort of door-check was in use at his bank. Buta sudden idea came to him.

"There you are, Tom!" he cried. "Why not take up the recoil of the gunson your aerial warship by some such device as that?" and Ned pointed tothe door-check.

CHAPTER VII

WARNINGS

For a moment or two Tom Swift did not seem to comprehend what Ned hadsaid. He remained staring, first at his chum, who stood pointing, andfrom him Tom's gaze wandered to the top of the door. It may have been,and probably was, that Tom was thinking of other matters at thatinstant. But Ned said again:

"Wouldn't that do, Tom? Check the recoil of the gun with whatever stuffis in that arrangement!"

A sudden change came over Tom's face. It was lighted up with a gleam ofunderstanding.

"By Jove, Ned, old man!" he cried. "I believe you've struck it! And tothink that has been under my nose, or, rather, over my head, all thiswhile, and I never thought of it. Hurray! That will solve the problem!"

"Do you think it will?" asked Ned, glad that he had contributedsomething, if only an idea, to Tom's aerial warship.

"I'm almost sure it will. I'll give it a trial right away."

"What's in that door-check?" Ned asked. "I never stopped before tothink what useful things they are, though at the bank, with the big,heavy doors, they are mighty useful."

"They are a combination of springs and hydrostatic valves," began Tom.

"Good-night!" laughed Ned. "Excuse the slang, Tom, but what in theworld is a hydrostatic valve?"

"A valve through which liquids pass. In this door-check there may be amixture of water, alcohol and glycerine, the alcohol to preventfreezing in cold weather, and the glycerine to give body to the mixtureso it will not flow through the valves too freely."

"And do you think you can put something like that on your guns, so therecoil will be taken up?" Ned wanted to know.

"I think so," spoke Tom. "I'm going to work on it right away, and we'llsoon see how it will turn out. It's mighty lucky you thought of that,for I sure was up against it, as the boys say."

"It just seemed to come to me," spoke Ned, "seeing how easily the doorclosed."

"If the thing works I'll give you due credit for it," promised Tom."Now, I've got to figure out how much force a modified hydrostaticvalve check like that will take up, and how much recoil my biggest gunwill have."

"Then you're going to put several guns on the Mars?" asked Ned.

"Yes, four quick-firers, at least, two on each side, and heavier gunsat the bow and stern, to throw explosive shells in a horizontal orupward direction. For a downward direction we won't need any guns, wecan simply drop the bombs, or shells, from a release clutch."

"Drop them on other air craft?" Ned wanted to know.

"Well, if it's necessary, yes. Though I guess there won't be muchchance of doing that to a rival aeroplane or dirigible. But in flyingover cities or forts, explosive bombs can be dropped very nicely. Foruse in attacking other air craft I am going to depend on my lateralfire, from the guns mounted on either beam, and in the bow and stern."

"You speak as though you, yourself, were going into a battle of theair," said Ned.

"No, I don't believe I'll go that far," Tom replied. "Though, if thegovernment wants my craft, I may have to go aloft and fire shots attargets for them to show them how things work.

"Please don't think that I am in favor of war, Ned," went on Tomearnestly. "I hate it, and I wish the time would come when all nationswould disarm. But if the other countries are laying themselves out tohave aerial battleships, it is time the United States did also. We mustnot be left behind, especially in view of what is taking place inEurope."

"I suppose that's right," agreed Ned. "Have you any of your guns ready?"

"Yes, all but the mounting of them on the supports aboard the Mars. Ihaven't dared do that yet, and fire them, until I provided some meansof taking up the recoil. Now I'm going to get right to work on thatproblem."

There was considerable detailed figuring and computation work ahead ofTom Swift, and I will not weary you by going into the details of highermathematics. Even Ned lost interest after the start of the problem,though he was interested when Tom took down the door-check and beganmeasuring the amount of force it would take up, computing it on scalesand spring balances.

Once this had been done, and Tom had figured just how much force couldbe expected to be taken up by a larger check, with stronger hydrostaticvalves, the young inventor explained:

"And now to see how much recoil force my guns develop!"

"Are you really going to fire the guns?" asked Ned.

"Surely," answered Tom. "That's the only way to get at real results.I'll have the guns taken out and mounted in a big field. Then we'llfire them, and measure the recoil."

"Well, that may be some fun," spoke Ned, with a grin. "More fun thanall these figures," and he looked at the mass of details on Tom's desk.

This was the second or third day after the fire in the red shed, and inthe interim Tom had been busy making computations. These were aboutfinished. Meanwhile further investigation had been made of cluesleading to the origin of the blaze in the shed, but nothing had beenlearned.

A photo-telephone had been installed near Eradicate's quarters, in thehope that the mysterious stranger might keep his promise, and come tosee about the mule. In that case something would have been learnedabout him. But, as Tom feared, the man did not appear.

Ned was much interested in the guns, and, a little later, he helped Tomand Koku mount them in a vacant lot. The giant's strength came in handyin handling the big parts.

Mr. Swift strolled past, as the guns were being mounted for thepreliminary test, and inquired what his son was doing.

"It will never work, Tom, never!" declared the aged inventor, wheninformed. "You can't take up those guns in your air craft, and firethem with any degree of safety."

"You wait, Dad," laughed Tom. "You haven't yet seen how the Newtonhydrostatic recoil operates."

Ned smiled with pleasure at this.

It took nearly a week to get all the guns mounted, for some of themrequired considerable work, and it was also necessary to attach gaugesto them to register the recoil and pressure. In the meanwhile Tom hadbeen in further communication with government experts who were soon tocall on him to inspect the aerial warship, with a view to purchase.

"When are they coming?" asked Ned, as he and Tom went out one morningto make the first test of the guns.

"They will be here any day, now. They didn't set any definite date. Isuppose they want to take us unawares, to see that I don't 'frame-up'any game on them. Well, I'll be ready any time they come. Now, Koku,bring along those shells, and don't drop any of them, for that newpowder is freakish stuff."

"Me no drop any, Master," spoke the giant, as he lifted the boxes ofexplosives in his strong arms.

The largest gun was loaded and aimed at a distant hill, for Tom knewthat if the recoil apparatus would take care of the excess force of hislargest gun, the problem of the smaller ones would be easy to solve.

"Here, Rad, where are you going?" Tom asked, as he noticed the coloredman walking away, after having completed a task assigned to him.

"Where's I gwine, Massa Tom?"

"Yes, Rad, that's what I asked you."

"I—I'se gwine t' feed mah mule, Boomerang," said the colored manslowly. "It's his eatin' time, jest now, Massa Tom."

"Nonsense! It isn't anywhere near noon yet."

"Yais, sab, Massa Tom, I knows dat," said Eradicate, as he carefullyedged away from the big gun, "but I'se done changed de eatin' hours obdat mule. He had a little touch ob indigestion de udder day, an' I'sefeedin' him diff'rent now. So I guess as how yo'll hab t' 'scuse menow, Massa Tom."

"Oh, well, trot along," laughed the young inventor. "I guess we won'tneed you. Is everything all right there, Koku?"

"All right, Master."

"Now, Ned, if you'll stand here," went on Tom, "and note the extremepoint to which the hand on the pressure gauge goes, I'll be obliged toyou. Just jot it down on this pad."

"Here comes someone," remarked the bank clerk, as he saw that hispencil was sharpened. He pointed to the field back of them.

"It's Mr. Damon," observed Tom. "We'll wait until he arrives. He'll beinterested in this."

"Bless my collar button, Tom! What's going on?" asked the eccentricman, as he came up. "Has war been declared?"

"Just practicing," replied the young inventor. "Getting ready to putthe armament on my aerial warship."

"Well, as long as I'm behind the guns I'm all right, I suppose?"

"Perfectly," Tom replied. "Now then, Ned, I think we'll fire."

There was a moment of inspection, to see that nothing had beenforgotten, and then the big gun was discharged. There was a loudreport, not as heavy, though, as Ned had expected, but there was nopuff of smoke, for Tom was using smokeless powder. Only a little flashof flame was observed.

"Catch the figure, Ned!" Tom cried.

"I have it!" was the answer. "Eighty thousand!"

"Good! And I can build a recoil check that will take up to one hundredand twenty thousand pounds pressure. That ought to be margin of safetyenough. Now we'll try another shot."

The echoes of the first had hardly died away before the second gun wasready for the test. That, too, was satisfactory, and then the smallerones were operated. These were not quite so satisfactory, as the recoildeveloped was larger, in proportion to their size, than Tom had figured.

"But I can easily put a larger hydrostatic check on them," he said."Now, we'll fire by batteries, and see what the total is."

Then began a perfect bombardment of the distant hillside, servicecharges being used, and explosive shells sent out so that dirt,stones and gravel flew in all directions. Danger signs and flags hadbeen posted, and a cordon of Tom's men kept spectators away from thehill, so no one would be in the danger zone.

The young inventor was busy making some calculations after the last ofthe firing had been completed. Koku was packing up the unfired shells,and Mr. Damon was blessing his ear-drums, and the pieces of cotton hehad stuffed in to protect them, when a tall, erect man was observedstrolling over the fields in the direction of the guns.

"Somebody's coming, Tom," warned Ned.

"Yes, and a stranger, too," observed Tom. "I wonder if that can beEradicate's Frenchman?"

But a look at the stranger's face disproved that surmise. He had afrank and pleasant countenance, obviously American.

"I beg your pardon," he began, addressing everyone in general, "but Iam looking for Tom Swift. I was told he was here."

"I am Tom Swift," replied our hero.

"Ah! Well, I am Lieutenant Marbury, with whom you had somecorrespondence recently about—"

"Oh, yes, Lieutenant Marbury, of the United States Navy," interruptedTom. "I'm glad to see you," he went on, holding out his hand. "We arejust completing some tests with the guns. You called, I presume, inreference to my aerial warship?"

"That is it—yes. Have you it ready for a trial flight?"

"Well, almost. It can be made ready in a few hours. You see, I havebeen delayed. There was a fire in the plant."

"A fire!" exclaimed the officer in surprise. "How was that? We heardnothing of it in Washington."

"No, I kept it rather quiet," Tom explained. "We had reason to suspectthat it was a fire purposely set, in a shed where I kept a quantity ofexplosives."

"Ha!" exclaimed Lieutenant Marbury. "This fits in with what I haveheard. And did you not receive warning?" he asked Tom.

"Warning? No. Of what?"

"Of foreign spies!" was the unexpected answer. "I am sorry. Some ofour Secret Service men unearthed something of a plot against you, and Ipresumed you had been told to watch out. If you had, the fire might nothave occurred. There must have been some error in Washington. But letme tell you now, Tom Swift—be on your guard!"

CHAPTER VIII

A SUSPECTED PLOT

The officer's words were so filled with meaning that Tom started. NedNewton, too, showed the effect he felt.

"Do you really mean that?" asked the young inventor, looking around tomake sure his father was not present. On account of Professor Swift'sweak heart, Tom wished to spare him all possible worry.

"I certainly do mean it," insisted Lieutenant Marbury. "And, while I amrather amazed at the news of the fire, for I did not think the plotterswould be so bold as that, it is in line with what I expected, and whatwe suspected in Washington."

"And that was—what?" asked Tom.

"The existence of a well-laid plot, not only against our government,but against you!"

"And why have they singled me out?" Tom demanded.

"I might as well tell it from the beginning," the officer went on. "Aslong as you have not received any official warning from Washington youhad better hear the whole story. But are you sure you had no word?"

"Well, now, I won't be so sure," Tom confessed. "I have been workingvery hard, the last two days, making some intricate calculations. Ihave rather neglected my mail, to tell you the truth.

"And, come to think of it, there were several letters received with theWashington postmark. But, I supposed they had to do with some of mypatents, and I only casually glanced over them. There was one letter,though, that I couldn't make head or tail of."

"Ha! That was it!" cried the lieutenant. "It was the warning in cipheror code. I didn't think they would neglect to send it to you."

"But what good would it do me if I couldn't read it?" asked Tom.

"You must also have received a method of deciphering the message," theofficer said. "Probably you overlooked that. The Secret Service mensent you the warning in code, so it would not be found out by theplotters, and, to make sure you could understand it, a method oftranslating the cipher was sent in a separate envelope. It is too badyou missed it."

"Yes, for I might have been on my guard," agreed Tom. "The red shedmight not have burned, but, as it was, only slight damage was done."

"Owing to the fact that Tom put the fire out with sand ballast from hisdirigible!" cried Ned. "You should have seen it!"

"I should have liked to be here," the lieutenant spoke. "But, if I wereyou, Tom Swift, I would take means to prevent a repetition of suchthings."

"I shall," Tom decided. "But, if we want to talk, we had better go tomy office, where we can be more private. I don't want the workmen tohear too much."

Now that the firing was over, a number of Tom's men from the shops hadassembled around the cannon. Most of them, the young inventor felt,could be trusted, but in so large a gathering one could never be sure.

"Did you come on from Washington yesterday?" asked Tom, as he, Ned andthe officer strolled toward the shed where was housed the aerialwarship.

"Yes, and I spent the night in New York. I arrived in town a short timeago, and came right on out here. At your house I was told you were overin the fields conducting experiments, so I came on here."

"Glad you did," Tom said. "I'll soon have something to show you, Ihope. But I am interested in hearing the details of this suspectedplot. Are you sure one exists?"

"Perfectly sure," was the answer. "We don't know all the details yet,nor who are concerned in it, but we are working on the case. The SecretService has several agents in the field.

"We are convinced in Washington," went on Lieutenant Marbury, when he,Tom and Ned were seated in the private office, "that foreign spies areat work against you and against our government."

"Why against me?" asked Tom, in wonder.

"Because of the inventions you have perfected and turned over to UncleSam—notably the giant cannon, which rivals anything foreign Europeanpowers have, and the great searchlight, which proved so effectiveagainst the border smugglers. The success of those two alone, to saynothing of your submarine, has not only made foreign nations jealous,but they fear you—and us," the officer went on.

"Well, if they only take it out in fear—"

"But they won't!" interrupted the officer—"They are seeking to destroythose inventions. More than once, of late, we have nipped a plot justin time."

"Have they really tried to damage the big gun?" asked Tom, referring toone he had built and set up at Panama.

"They have. And now this fire proves that they are taking othermeasures—they are working directly against you."

"Why, I wonder?"

"Either to prevent you from making further inventions, or to stop youfrom completing your latest—the aerial warship."

"But I didn't know the foreign governments knew about that," Tomexclaimed. "It was a secret."

"Few secrets are safe from foreign Spies," declared Lieutenant Marbury."They have a great ferreting-out system on the other side. We are justbeginning to appreciate it. But our own men have not been idle."

"Have they really learned anything?" Tom asked. "Nothing definiteenough to warrant us in acting," was the answer of the government man."But we know enough to let us see that the plot is far-reaching."

"Are the French in it?" asked Ned impulsively.

"The French! Why do you ask that?"

"Tell him about Eradicate, and the man who wanted to buy the mule,Tom," suggested Ned.

Thereupon the young inventor mentioned the story told by Eradicate. Healso brought out the fire-bomb, and explained his theory as to how ithad operated to set the red shed ablaze.

"I think you are right," said Lieutenant Marbury. "And, as regards theFrench, I might say they are not the only nation banded to obtain oursecrets—yours and the government's!"

"But I thought the French and the English were friendly toward us!" Nedexclaimed.

"So they are, in a certain measure," the officer went on. "And Russiais, too. But, in all foreign countries there are two parties, the warparty, as it might be called, and the peace element.

"But I might add that it is neither France, England, nor Russia that wemust fear. It is a certain other great nation, which at present I willnot name."

"And you think spies set this fire?"

"I certainly do."

"But what measures shall I adopt against this plot?" Tom asked.

"We will talk that over," said Lieutenant Marbury. "But, before I gointo details, I want to give you another warning. You must be verycareful about—"

A sudden knock on the door interrupted the speaker.

CHAPTER IX

THE RECOIL CHECK

"Who is that?" asked Ned Newton, with a quick glance at his chum.

"I don't know," Tom answered. "I left orders we weren't to be disturbedunless it was something important."

"May be something has happened," suggested the navy officer, "anotherfire, perhaps, or a—"

"It isn't a fire," Tom answered. "The automatic alarm would be ringingbefore this in that case."

The knock was repeated. Tom went softly to the door and opened itquickly, to disclose, standing in the corridor, one of the messengersemployed about the shops.

"Well, what is it?" asked Tom a bit sharply.

"Oh, if you please, Mr. Swift," said the boy, "a man has applied forwork at the main office, and you know you left orders there that if anymachinists came along, we were to—"

"Oh, so I did," Tom exclaimed. "I had forgotten about that," he went onto Lieutenant Marbury and Ned. "I am in need of helpers to rush throughthe finishing touches on my aerial warship, and I left word, if anyapplied, as they often do, coming here from other cities, that I wantedto see them. How many are there?" Tom asked of the messenger.

"Two, this time. They both say they're good mechanics."

"That's what they all say," interposed Tom, with a smile. "But, thoughthey may be good mechanics in their own line, they need to have specialqualifications to work on airships. Tell them to wait, Rodney," Tomwent on to the lad, "and I'll see them presently."

As the boy went away, and Tom closed the door, he turned to LieutenantMarbury.

"You were about to give me another warning when that interruption came.You might complete it now."

"Yes, it was another warning," spoke the officer, "and one I hope youwill heed. It concerns yourself, personally."

"Do you mean he is in danger?" asked Ned quickly.

"That's exactly what I do mean," was the prompt reply. "In danger ofpersonal injury, if not something worse."

Tom did not seem as alarmed as he might reasonably have been under thecircumstances.

"Danger, eh?" he repeated coolly. "On the part of whom?"

"That's just where I can't warn you," the officer replied. "I can onlygive you that hint, and beg of you to be careful."

"Do you mean you are not allowed to tell?" asked Ned.

"No, indeed; it isn't that!" the lieutenant hastened to assure theyoung man. "I would gladly tell, if I knew. But this plot, like theother one, directed against the inventions themselves, is so shroudedin mystery that I cannot get to the bottom of it.

"Our Secret Service men have been working on it for some time, not onlyin order to protect you, because of what you have done for thegovernment, but because Uncle Sam wishes to protect his own property,especially the searchlight and the big cannon. But, though our agentshave worked hard, they have not been able to get any clues that wouldput them on the right trail.

"So we can only warn you to be careful, and this I do in allearnestness. That was part of my errand in coming here, though, ofcourse, I am anxious to inspect the new aerial warship you haveconstructed. So watch out for two things—your inventions, and, morethan all, your life!"

"Do you really think they would do me bodily harm?" Tom asked, a trifleskeptical.

"I certainly do. These foreign spies are desperate. If they cannotsecure the use of these inventions to their own country, they aredetermined not to let this country have the benefit of them."

"Well, I'll be careful," Tom promised. "I'm no more anxious than anyoneelse to run my head into danger, and I certainly don't want any of myshops or inventions destroyed. The fire in the red shed was as close asI want anything to come."

"That's right!" agreed Ned. "And, if there's anything I can do, Tom,don't hesitate to call on me."

"All right, old man. I won't forget. And now, perhaps, you would liketo see the Mars," he said to the lieutenant.

"I certainly would," was the ready answer. "But hadn't you better seethose men who are waiting to find out about positions here?"

"There's no hurry about them," Tom said. "We have applicants every day,and it's earlier than the hour when I usually see them. They can wait.Now I want your opinion on my new craft. But, you must remember thatit is not yet completed, and only recently did I begin to solve theproblem of mounting the guns. So be a little easy with yourcriticisms."

Followed by Ned and Lieutenant Marbury, Tom led the way into the bigairship shed. There, swaying about at its moorings, was the immenseaerial warship. To Ned's eyes it looked complete enough, but, when Tompointed out the various parts, and explained to the government officerhow it was going to work, Ned understood that considerable yet remainedto be done on it.

Tom showed his official guest how a new system of elevation anddepressing rudders had been adopted, how a new type of propeller was tobe used and indicated several other improvements. The lower, or cabin,part of the aircraft could be entered by mounting a short ladder fromthe ground, and Tom took Ned and Lieutenant Marbury through theengine-room and other compartments of the Mars.

"It certainly is most complete," the officer observed. "And when youget the guns mounted I shall be glad to make an official test. Youunderstand," he went on, to Tom, "that we are vitally interested in theguns, since we now have many aircraft that can be used purely forscouting purposes. What we want is something for offense, a veritablenaval terror of the seas."

"I understand," Tom answered. "And I am going to begin work on mountingthe guns at once. I am going to use the Newton recoil check," he added."Ned, here, is responsible for that."

"Is that so?" asked the lieutenant, as Tom clapped his chum on the back.

"Yes, that's his invention."

"Oh, it isn't anything of the sort," Ned objected. "I just—"

"Yes, he just happened to solve the problem for me!" interrupted Tom,as he told the story of the door-spring.

"A good idea!" commented Lieutenant Marbury.

Tom then briefly described the principle on which his aerial warshipwould work, explaining how the lifting gas would raise it, with itsload of crew, guns and explosives, high into the air; how it could thenbe sent ahead, backward, to either side, or around in a circle, bymeans of the propellers and the rudders, and how it could be raised orlowered, either by rudders or by forcing more gas into the liftingbags, or by letting some of the vapor out.

And, while this was being done by the pilot or captain in charge, thecrew could be manning the guns with which hostile airships would beattacked, and bombs dropped on the forts or battleships of the enemy.

"It seems very complete," observed the lieutenant. "I shall be gladwhen I can give it an official test."

"Which ought to be in about a week," Tom said. "Meanwhile I shall beglad if you will be my guest here."

And so that was arranged.

Leaving Ned and the lieutenant to entertain each other, Tom went to seethe mechanics who had applied for places. He found them satisfactoryand engaged them. One of them had worked for him before. The other wasa stranger, but he had been employed in a large aeroplane factory, andbrought good recommendations.

There followed busy days at the Swift plant, and work was pushed on theaerial warship. The hardest task was the mounting of the guns, andequipping them with the recoil check, without which it would beimpossible to fire them with the craft sailing through the air.

But finally one of the big guns, and two of the smaller ones were inplace, with the apparatus designed to reduce the recoil shock, and thenTom decided to have a test of the Mars.

"Up in the air, do you mean?" asked Ned, who was spending all his sparetime with his chum.

"Well, a little way up in the air, at least," Tom answered. "I'll makea sort of captive balloon of my craft, and see how she behaves. I don'twant to take too many chances with that new recoil check, though itseems to work perfectly in theory."

The day came when, for the first time, the Mars was to come out of thebig shed where she had been constructed. The craft was not completedfor a flight as yet, but could be made so in a few days, with rushwork. The roof of the great shed slid back, and the big envelopecontaining the buoyant gas rose slowly upward. There was a cry ofsurprise from the many workmen in the yard, as they saw, most of themfor the first time, the wonderful new craft. It did not go up veryhigh, being held in place with anchor ropes.

The sun glistened on the bright brass and nickel parts, and glintedfrom the gleaming barrels of the quick-firing guns.

"That's enough!" Tom called to the men below, who were paying out theropes from the windlasses. "Hold her there."

Tom, Ned, Lieutenant Marbury and Mr. Damon were aboard the captive Mars.

Looking about, to see that all was in readiness, Tom gave orders toload the guns, blank charges being used, of course.

The recoil apparatus was in place, and it now remained to see if itwould do the work for which it was designed.

"All ready?" asked the young inventor.

"Bless my accident insurance policy!" exclaimed Mr. Damon. "I'm asready as ever I shall be, Tom. Let 'em go!"

"Hold fast!" cried Tom, as he prepared to press the electrical switchwhich would set off the guns. Ned and Lieutenant Marbury stood near theindicators to notice how much of the recoil would be neutralized by thecheck apparatus.

"Here we go!" cried the young inventor, and, at the same moment, fromdown below on the ground, came a warning cry:

"Don't shoot, Massa Tom. Don't shoot! Mah mule, Boomerang—"

But Eradicate had spoken too late. Tom pressed the switch; there was adeafening crash, a spurt of flame, and then followed wild cries andconfused shouts, while the echoes of the reports rolled about the hillssurrounding Shopton.

CHAPTER X

THE NEW MEN

"What was the matter down there?"

"Was anyone hurt?"

"Don't forget to look at those pressure gauges!"

"Bless my ham sandwich!"

Thus came the cries from those aboard the captive Mars. Ned, LieutenantMarbury and Tom had called out in the order named. And, of course, Ido not need to tell you what remark Mr. Damon made. Tom glanced towardwhere Ned and the government man stood, and saw that they had madenotes of the pressure recorded on the recoil checks directly after theguns were fired. Mr. Damon, blessing innumerable objects under hisbreath, was looking over the side of the rail to discover the cause ofthe commotion and cries of warning from below.

"I don't believe it was anything serious, Tom," said the odd man. "Noone seems to be hurt." "Look at Eradicate!" suddenly exclaimed Ned.

"And his mule! I guess that's what the trouble was, Tom!"

They looked to where the young bank employee pointed, and saw the oldcolored man, seated on the seat of his ramshackle wagon, doing his bestto pull down to a walk the big galloping mule, which was dragging thevehicle around in a circle.

"Whoa, dere!" Eradicate was shouting, as he pulled on the lines. "Whoa,dere! Dat's jest laik yo', Boomerang, t' run when dere ain't no callfo' it, nohow! Ef I done wanted yo' t' git a move on, yo'd lay down'side de road an' go to sleep. Whoa, now!"

But the noise of the shots had evidently frightened the long-earedanimal, and he was in no mood for stopping, now that he had oncestarted. It was not until some of the workmen ran out from the groupwhere they had gathered to watch Tom's test, and got in front ofBoomerang, that they succeeded in bringing him to a halt.

Eradicate climbed slowly down from the seat, and limped around until hestood in front of his pet.

"Yo'—yo're a nice one, ain't yo'?" he demanded in sarcastic tones."Yo' done enough runnin' in a few minutes fo' a week ob Sundays, an'now I won't be able t' git a move out ob ye! I'se ashamed ob yo', dat'swhat I is! Puffickly ashamed ob yo'. Go 'long, now, an' yo' won't gitno oats dish yeah day! No sah!" and, highly indignant, Eradicate ledthe now slowly-ambling mule off to the stable.

"I won't shoot again until you have him shut up, Rad!" laughed Tom. "Ididn't know you were so close when I set off those guns."

"Dat's all right, Massa Tom," was the reply. "I done called t' you t'wait, but yo' didn't heah me, I 'spects. But it doan't mattah, now.Shoot all yo' laik, Boomerang won't run any mo' dis week. He donerunned his laigs off now. Shoot away!"

But Tom was not quite ready to do this. He wanted to see what effectthe first shots had had on his aerial warship, and to learn whether ornot the newly devised recoil check had done what was expected of it.

"No more shooting right away," called the young inventor. "I want tosee how we made out with the first round. How did she check up, Ned?"

"Fine, as far as I can tell."

"Yes, indeed," added Lieutenant Marbury. "The recoil was hardlynoticeable, though, of course, with the full battery of guns in use, itmight be more so."

"I hope not," answered Tom. "I haven't used the full strength of therecoil check yet. I can tune it up more, and when I do, and when I haveit attached to all the guns, big and little, I think we'll do thetrick. But now for a harder test."

The rest of that day was spent in trying out the guns, firing them withpractice and service charges, though none of the shells used containedprojectiles. It would not have been possible to shoot these, with theMars held in place in the midst of Tom's factory buildings.

"Well, is she a success, Tom?" asked Ned, when the experimenting wasover for the time being.

"I think I can say so—yes," was the answer, with a questioning look atthe officer.

"Indeed it is—a great success! We must give the Newton shock absorberdue credit."

Ned blushed with pleasure.

"It was only my suggestion," he said. "Tom worked it all out."

"But I needed the suggestion to start with," the young inventor replied.

"Of course something may develop when you take your craft high in theair, and discharge the guns there," said the lieutenant. "In ararefied atmosphere the recoil check may not be as effective as at theearth's surface. But, in such case doubtless, you can increase thestrength of the springs and the hydrostatic valves."

"Yes, I counted on that," Tom explained. "I shall have to work out thatformula, though, and be ready for it. But, on the whole, I am prettywell satisfied."

"And indeed you may well feel that way," commented the governmentofficial.

The Mars was hauled back into the shed, and the roof slid shut over thecraft. Much yet remained to do on it, but now that Tom was sure theimportant item of armament was taken care of, he could devote hisentire time to the finishing touches.

As his plant was working on several other pieces of machinery, some ofit for the United States Government, and some designed for his own use,Tom found himself obliged to hire several new hands. An advertisementin a New York newspaper brought a large number of replies, and for aday or two Tom was kept busy sifting out the least desirable, andarranging to see those whose answers showed they knew something of thebusiness requirements.

Meanwhile Lieutenant Marbury remained as Tom's guest, and was helpfulin making suggestions that would enable the young inventor to meet thegovernment's requirements.

"I'd like, also, to get on the track of those spies who, I am sure,wish to do you harm," said the lieutenant, "but clues seem to be scarcearound here."

"They are, indeed," agreed Tom. "I guess the way in which we handledthat fire in the red shed sort of discouraged them."

Lieutenant Marbury shook his head.

"They're not so easily discouraged as that," he remarked. "And, withthe situation in Europe growing more acute every day, I am afraid someof those foreigners will take desperate measures to gain their ends."

"What particular ends do you mean?"

"Well, I think they will either try to so injure you that you will notbe able to finish this aerial warship, or they will damage the craftitself, steal your plans, or damage some of your other inventions."

"But what object would they have in doing such a thing?" Tom wanted toknow. "How would that help France, Germany or Russia, to do me aninjury?"

"They are seeking to strike at the United States through you," was theanswer. "They don't want Uncle Sam to have such formidable weapons asyour great searchlight, the giant cannon, or this new warship of theclouds."

"But why not, as long as the United States does not intend to go to warwith any of the foreign nations?" Tom inquired.

"No, it is true we do not intend to go to war with any of theconflicting European nations," admitted Lieutenant Marbury, "but youhave no idea how jealous each of those foreign nations is of all theothers. Each one fears that the United States will cease to be neutral,and will aid one or the other."

"Oh, so that's it?" exclaimed Tom.

"Yes, each nation, which may, at a moments notice, be drawn into a warwith one or more rival nations, fears that we may throw in our lot withits enemies."

"And, to prevent that, they want to destroy some of my inventions?"asked Tom.

"That's the way I believe it will work out. So you must be careful,especially since you have taken on so many new men."

"That's so," agreed the young inventor. "I have had to engage morestrangers than ever before, for I am anxious to get the Mars finishedand give it a good test. And, now that you have mentioned it, there aresome of those men of whom I am a bit suspicious."

"Have they done anything to make you feel that way?" asked thelieutenant.

"Well, not exactly; it is more their bearing, and the manner in whichthey go about the works. I must keep my eye on them, for it takes onlya few discontented men to spoil a whole shop full. I will be on myguard."

"And not only about your new airship and other inventions," said theofficer, "but about yourself, personally. Will you do that?"

"Yes, though I don't imagine anything like that will happen."

"Well, be on your guard, at all events," warned Lieutenant Marbury.

As Tom had said, he had been obliged to hire a number of new men. Someof these were machinists who had worked for him, or his father, onprevious occasions, and, when tasks were few, had been dismissed, to goto other shops. These men, Tom felt sure, could be relied upon.

But there were a number of others, from New York, and other largecities, of whom Tom was not so sure.

"You have more foreigners than I ever knew you to hire before, Tom,"his father said to him one day, coming back from a tour of the shops.

"Yes, I have quite a number," Tom admitted. "But they are all goodworkmen. They stood the test."

"Yes, some of them are too good," observed the older inventor. "I sawone of them making up a small motor the other day, and he was windingthe armature a new way. I spoke to him about it, and he tried to provethat his way was an improvement on yours. Why, he'd have had itshort-circuited in no time if I hadn't stopped him."

"Is that so?" asked Tom. "That is news to me. I must look into this."

"Are any of the new men employed on the Mars?" Mr. Swift asked.

"No, not yet, but I shall have to shift some there from other work Ithink, in order to get finished on time."

"Well, they will bear watching I think," his father said.

"Why, have you seen anything—do you—" began the young man, for Mr.Swift had not been told of the suspicions of the lieutenant.

"Oh, it isn't anything special," the older inventor went on. "Only Iwouldn't let a man I didn't know much about get too much knowledge ofmy latest invention."

"I won't, Dad. Thanks for telling me. This latest craft is sure goingto be a beauty."

"Then you think it will work, Tom?"

"I'm sure of it, Dad!"

Mr. Swift shook his head in doubt.

CHAPTER XI

A DAY OFF

Tom Swift pondered long and intently over what his father had said tohim. He sat for several minutes in his private office, after the agedinventor had passed out, reviewing in his mind the talk just finished.

"I wonder," said Tom slowly, "if any of the new men could have obtainedwork here for the purpose of furthering that plot the lieutenantsuspects? I wonder if that could be true?"

And the more Tom thought of it, the more he was convinced that such athing was at least possible.

"I must make a close inspection, and weed out any suspiciouscharacters," he decided, "though I need every man I have working now,to get the Mars finished in time. Yes, I must look into this."

Tom had reached a point in his work where he could leave much to hishelpers. He had several good foremen, and, with his father to takegeneral supervision over more important details, the young inventor hadmore time to himself. Of course he did not lay too many burdens on hisfather's shoulders since Mr. Swift's health was not of the best.

But Tom's latest idea, the aerial warship, was so well on towardcompletion that his presence was not needed in that shop more than twoor three times a day.

"When I'm not there I'll go about in the other shops, and sort of sizeup the situation," he decided. "I may be able to get a line on some ofthose plotters, if there are any here."

Lieutenant Marbury had departed for a time, to look after some personalmatters, but he was to return inside of a week, when it was hoped togive the aerial warship its first real test in flight, and under someof the conditions that it would meet with in actual warfare.

As Tom was about to leave his office, to put into effect his newresolution to make a casual inspection of the other shops, he met Koku,the giant, coming in. Koku's hands and face were black with oil andmachine filings.

"Well, what have you been doing?" Tom wanted to know. "Did you have anaccident?" For Koku had no knowledge of machinery, and could not evenbe trusted to tighten up a simple nut by himself. But if some onestood near him, and directed him how to apply his enormous strength,Koku could do more than several machines.

"No accident, Master," he replied. "I help man lift that hammer-hammerthing that pounds so. It get stuck!"

"What, the hammer of the drop forger?" cried Tom. "Was that out oforder again?"

"Him stuck," explained Koku simply.

There was an automatic trip-hammer in one of the shops, used forpounding out drop forgings, and this hammer seemed to take especialdelight in getting out of order. Very often it jammed, or "stuck," asKoku described it, and if the hammer could not be forced back on thechannel or upright guide-plates, it meant that it must be taken apart,and valuable time lost. Once Koku had been near when the hammer got outof order, and while the workmen were preparing to dismantle it, thegiant seized the big block of steel, and with a heave of his mightyshoulders forced it back on the guides.

"And is that what you did this time?" asked Tom.

"Yes, Master. Me fix hammer," Koku answered. "I get dirty, I no care.Man say I no can fix. I show him I can!"

"What man said that?"

"Man who run hammer. Ha! I lift him by one finger! He say he no like towork on hammer. He want to work on airship. I tell him I tell you,maybe you give him job—he baby! Koku can work hammer. Me fix it whenit get stuck."

"Well, maybe you know what you're talking about, but I don't," saidTom, with a pleasant smile at his big helper. "Come on, Koku, we'll gosee what it all means."

"Koku work hammer, maybe?" asked the giant hope fully.

"Well, I'll see," half promised Tom. "If it's going to get out of gearall the while it might pay me to keep you at it so you could get itback in place whenever it kicked up a fuss, and so save time. I'll seeabout it."

Koku led the way to the shop where the triphammer was installed. It wasworking perfectly now, as Tom could tell by the thundering blows itstruck. The man operating it looked up as Tom approached, and, at agesture from the young inventor, shut off the power.

"Been having trouble here?" asked Tom, noting that the workman was oneof the new hands he had hired.

"Yes, sir, a little," was the respectful answer. "This hammer goes on astrike every now and then, and gets jammed. Your giant there forced itback into place, which is more than I could do with a big bar for alever. He sure has some muscle."

"Yes," agreed Tom, "he's pretty strong. But what's this you said aboutwanting to give up this job, and go on the airship construction."

The man turned red under his coat of grime.

"I didn't intend him to repeat that to you, Mr. Swift," he said. "I wasa little put out at the way this hammer worked. I lose so much time atit that I said I'd like to be transferred to the airship department.I've worked in one before. But I'm not making a kick," he added quickly."Work is too scarce for that."

"I understand," said Tom. "I have been thinking of making a change.Koku seems to like this hammer, and knows how to get it in order onceit gets off the guides. You say you have had experience in airshipconstruction?"

"Yes, sir. I've worked on the engines, and on the planes."

"Know anything about dirigible balloons?"

"Yes, I've worked on them, too, but the engineering part is myspecialty. I'm a little out of my element on a trip-hammer."

"I see. Well, perhaps I'll give you a trial. Meanwhile you might breakKoku in on operating this machine. If I transfer you I'll put him onthis hammer."

"Thank you, Mr. Swift! I'll show him all I know about it. Oh, theregoes the hammer again!" he exclaimed, for, as he started it up, as Tomturned away, the big piece of steel once more jammed on thechannel-plates.

"Me fix!" exclaimed the giant eagerly, anxious for a chance to exhibithis great strength.

"Wait a minute!" exclaimed Tom. "I want to get a look at that machine."

He inspected it carefully before he signaled for Koku to force thehammer back into place. But, if Tom saw anything suspicious, he saidnothing. There was, however, a queer look on his face as he turnedaside, and he murmured to himself, as he walked away:

"So you want to be transferred to the airship department, do you? Well,we'll see about that. We'll see."

Tom had more problems to solve than those of making an aerial warshipthat would be acceptable to the United States Government.

Ned Newton called on his chum that evening. The two talked of manythings, gradually veering around to the subject uppermost in Tom'smind—his new aircraft.

"You're thinking too much of that." Ned warned him. "You're as bad asthe time you went for your first flight."

"I suppose I am," admitted Tom. "But the success of the Mars means awhole lot to me. And that's something I nearly forgot. I've got to goout to the shop now. Want to come along, Ned?"

"Sure, though I tell you that you're working too hard—burning theelectric light at both ends."

"This is just something simple," Tom said. "It won't take long."

He went out, followed by his chum.

"But this isn't the way to the airship shed," objected the young bankclerk, as he noted in which direction Tom was leading him.

"I know it isn't," Tom replied. "But I want to look at one of thetrip-hammers in the forge shop when none of the men is around. I'vebeen having a little trouble there."

"Trouble!" exclaimed his chum. "Has that plot Lieutenant Marbury spokeof developed?"

"Not exactly. This is something else," and Tom told of the trouble withthe big hammer.

"I had an idea," the young inventor said, "that the man at the machinelet it get out of order purposely, so I'd change him. I want to see ifmy suspicions are correct."

Tom carefully inspected the hammer by the light of a powerful portableelectric lamp Ned held.

"Ha! There it is!" Tom suddenly exclaimed.

"Something wrong?" Ned inquired.

"Yes. This is what's been throwing the hammer off the guides all thewhile," and Tom pulled out a small steel bolt that had been slippedinto an oil hole. A certain amount of vibration, he explained to Ned,would rattle the bolt out so that it would force the hammer to oneside, throwing it off the channel-plates, and rendering it useless forthe time being.

"A foxy trick," commented Tom. "No wonder the machine got out of kilterso easily."

"Do you think it was done purposely?"

"Well, I'm not going to say. But I'm going to watch that man. Hewants to be transferred to the airship department. He put this in thehammer, perhaps, to have an excuse for a change. Well, I'll give it tohim."

"You don't mean that you'd take a fellow like that and put him to workon your new aerial warship, do you, Tom?"

"Yes, I think I will, Ned. You see, I look at it this way: I haven'tany real proof against him now. He could only laugh at me if I accusedhim. But you've heard the proverb about giving a calf rope enough andhe'll hang himself, haven't you?"

"I think I have."

"Well, I'm going to give this fellow a little rope. I'll transfer him,as he asks, and I'll keep a close watch on him."

"But won't it be risky?"

"Perhaps, but no more so than leaving him in here to work mischief. Ifhe is hatching a plot, the sooner it's over with the better I shalllike it. I don't like a shot to hang fire. I'm warned now, and I'll beready for him. I have a line on whom to suspect. This is the firstclue," and Tom held up the incriminating bolt.

"I think you're taking too big a risk, Tom," his chum said. "Why notdischarge the man?"

"Because that might only smooth things over for a time. If this plot isbeing laid the sooner it comes to a head, and breaks, the better. Haveit done, short, sharp and quick, is my motto. Yes, I'll shift him inthe morning. Oh, but I wish it was all over, and the Mars was acceptedby Uncle Sam!" and Tom put his hand to his head with a tired gesture.

"Say, old man!" exclaimed Ned, "what you want is a day off, and I'mgoing to see that you get it. You need a little vacation."

"Perhaps I do," assented Tom wearily.

"Then you'll have it!" cried Ned. "There's going to be a little picnicto-morrow. Why can't you go with Mary Nestor? She'd like you to takeher, I'm sure. Her cousin, Helen Randall, is on from New York, and shewants to go, also."

"How do you know?" asked Tom quickly.

"Because she said so," laughed Ned. "I was over to the house to call. Ihave met Helen before, and I suggested that you and I would take thetwo girls, and have a day off. You'll come, won't you?"

"Well, I don't know," spoke Tom slowly. "I ought to—"

"Nonsense! Give up work for one day!" urged Ned. "Come along. It'll doyou good—get the cobwebs out of your head."

"All right, I'll go," assented Tom, after a moment's thought.

The next day, having instructed his father and the foremen to look wellto the various shops, and having seen that the work on the new aerialwarship was progressing favorably, Tom left for a day's outing with hischum and the two girls.

The picnic was held in a grove that surrounded a small lake, and afterluncheon the four friends went for a ride in a launch Tom hired. Theywent to the upper end of the lake, in rather a pretty but lonesomelocality.

"Tom, you look tired," said Mary. "I'm sure you've been working toohard!"

"Why, I'm not working any harder than usual," Tom insisted.

"Yes, he is, too!" declared Ned, "and he's running more chances, too."

"Chances?" repeated Mary.

"Oh, that's all bosh!" laughed Tom. "Come on, let's go ashore and walk."

"That suits me," spoke Ned. Helen and Mary assented, and soon the fouryoung persons were strolling through the shady wood.

After a bit the couples became separated, and Tom found himself walkingbeside Mary in a woodland path. The girl glanced at her companion'sface, and ventured:

"A penny for your thoughts, Tom."

"They're worth more than that," he replied gallantly. "I was thinkingof—you."

"Oh, how nicely you say it!" she laughed. "But I know better! You'repuzzling over some problem. Tell me, what did Ned mean when he hintedat danger? Is there any, Tom?"

"None at all," he assured her. "It's just a sort of notion—"

Mary made a sudden gesture of silence.

"Hark!" she whispered to Tom, "I heard someone mention your name then.Listen!"

CHAPTER XII

A NIGHT ALARM

Mary Nestor spoke with such earnestness, and her action in catchinghold of Tom's arm to enjoin silence was so pronounced that, though hehad at first regarded the matter in the light of a joke, he soonthought otherwise. He glanced from the girl's face to the denseunderbrush on either side of the woodland path.

"What is it, Mary?" he asked in a whisper.

"I don't just know. I heard whispering, and thought it was the rustlingof the leaves of the trees. Then someone spoke your name quite loudly.Didn't you hear it?"

Tom shook his head in negation.

"It may be Ned and his friend," he whispered, his lips close to Mary'sear.

"I think not," was her answer. "Listen; there it is again."

Distinctly then, Tom heard, from some opening in the screen of bushes,his own name spoken. "Did you hear it?" asked Mary, barely forming thewords with her lips. But Tom could read their motion.

"Yes," he nodded. Then, motioning to Mary to remain where she was, hestepped forward, taking care to tread only on grassy places where therewere no little twigs or branches to break and betray his presence. Hewas working his way toward the sound of the unseen voice.

There was a sudden movement in the bushes, just beyond the spot Tom wasmaking for. He halted quickly and peered ahead. Mary, too, was lookingon anxiously.

Tom saw the forms of two men, partially concealed by bushes, walkingaway from him. The men took no pains to conceal their movements, so Tomwas emboldened to advance with less caution. He hurried to where hecould get a good view, and, at the sight of one of the men, he utteredan exclamation.

"What is it?" asked Mary, who was now at his side. She had seen thatTom had thrown aside caution, and she had come up to join him.

"That man—I know him!" the young inventor exclaimed. "It isFeldman—the one who wanted to be changed from the trip-hammer to theairship department. But who is that with him?"

As Tom spoke the other turned, and at the sight of his face Mary Nestorsaid:

"He looks like a Frenchman, with that little mustache and imperial."

"So he is!" exclaimed Tom, in a hoarse whisper. "He must be theFrenchman that Eradicate spoke about. I wonder what this can mean? Ididn't know Feldman had left the shop."

"You may know what you're talking about, but I don't, Tom," said Mary,with a smile at her companion. "Are they friends of yours?"

"Hardly," spoke the young inventor dryly. "That one, Feldman, is one ofmy workmen. He had charge of a drop-forge press and trip-hammer that—"

"Spare me the details, Tom!" interrupted Mary. "You know I don'tunderstand a thing about machinery. The wireless you erected onEarthquake Island was as much as I could comprehend."

"Well, a trip-hammer isn't as complicated as that," spoke Tom, with alaugh, as he noticed that the two men were far enough away so theycould not hear him. "What I was going to say was, that one of those menworks in our shops. The other I don't know, but I agree with you thathe does look like a Frenchman, and old Eradicate had a meeting with aman whom he described as being of that nationality."

"And you say they are not friends of yours?"

"I have no reason to believe they are."

"Then they must be enemies!" exclaimed Mary with quick intuition. "Oh,Tom, you will be careful, won't you?"

"Of course I will, little girl," he said, a note of fondness creepinginto his voice, as he covered the small hand with his own large one."But there is no danger."

"Then why were these men discussing you?"

"I don't know that they were, Mary."

"They mentioned your name."

"Well, that may be. Probably one of them, Feldman, who works for me,was speaking to his companion about the chance for a position. Myfather and I employ a number of men, you know."

"Well, I suppose it is all right, Tom, and I surely hope it is. Butyou will be careful, won't you? And you look more worried than youused to. Has anything gone wrong?"

"Not a thing, little girl. Everything is going fine. My new aerialwarship will soon make a trial flight, and I'd be pleased to have youas a passenger."

"Would you really, Tom?"

"Of course. Consider that you have the first invitation."

"That's awfully nice of you. But you do look worried, Tom. Has anythingtroubled you?"

"No, not much. Everything is going all right now. We did have a littletrouble at a fire in one of my buildings—"

"A fire! Oh, Tom! You never told me!"

"Well, it didn't amount to much—the only suspicious fact about it wasthat it seemed to have been of incendiary origin."

Mary seemed much alarmed, and again begged Tom to be on his guard,which he promised to do. Had Mary known the warnings uttered byLieutenant Marbury she might have had more occasion for worry.

"Do you suppose that hammer man of yours came to these woods to meetthat Frenchman and talk about you, Tom?" asked his companion, when thetwo men had strolled out of sight, and the young people were on theirway back to the launch.

"Well, it's possible. I have been warned that foreign spies are tryingto get hold of some of my patents, and also to hamper the government inthe use of some others I have sold. But they'll have their own troublesto get away with anything. The works are pretty well guarded, and youforget I have the giant, Koku, who is almost a personal bodyguard."

"Yes, but he can't be everywhere at once. Oh, you will be careful,won't you, Tom?"

"Yes, Mary, I will," promised the young inventor. "But don't sayanything to Ned about what we just saw and heard."

"Why not?"

"Because he's been at me to hire a couple of detectives to watch overme, and this would give him another excuse. Just don't say anything,and I'll adopt all the precautions I think are needful."

"I will on condition that you do that."

"And I promise I will."

With that Mary had to be content. A little later they joined Ned andhis friend, and soon they were moving swiftly down the lake in thelaunch.

"Well, hasn't it done you good to take a day off?" Ned demanded of hischum, when they were on their homeward way.

"Yes, I think it has," agreed Tom.

"You swung your thoughts into a new channel, didn't you?"

"Oh, yes, I found something new to think about," admitted the younginventor, with a quick look at Mary.

But, though Tom thus passed off lightly the little incident of the day,he gave it serious thought when he was alone.

"Those fellows were certainly talking about me," he reasoned. "Iwonder what for? And Feldman left the shop without my knowledge. I'llhave to look into that. I wonder if that Frenchy looking chap I saw wasthe one who tried to pump Eradicate? Another point to settle."

The last was easily disposed of, for, on reaching his shops thatafternoon, Tom cross-questioned the colored man, and obtained a mostaccurate description of the odd foreigner. It tallied in every detailwith the man Tom had seen in the woods.

"And now about Feldman," mused Tom, as he went to the foreman of theshop where the suspected man had been employed.

"Yes, Feldman asked for a day off," the foreman said in response toTom's question. "He claimed his mother was sick, and he wanted to go tosee her. I knew you wouldn't object, as we were not rushed in hisdepartment."

"Oh, that's all right," said Tom quickly. "Did he say where his motherlived?"

"Over Lafayette way."

"Humph!" murmured Tom. To himself he added: "Queer that he should benear Lake Loraine, in an opposite direction from Lafayette. This willbear an investigation."

The next day Tom made it his business to pass near the hammer that wasso frequently out of order. He found Feldman busy instructing Koku inits operation. Tom resolved on a little strategy.

"How is it working, Feldman?" he asked.

"Very well, Mr. Swift. There doesn't seem to be any trouble at all, butit may happen any minute. Koku seems to take to it like a duck towater."

"Well, when he is ready to assume charge let me know."

"And then am I to go into the aeroplane shop?"

"I'll see. By the way, how is your mother?" he asked quickly, lookingFeldman full in the face.

"She is much better. I took a day off yesterday to go to see her," theman replied quietly enough, and without sign of embarrassment.

"That's good. Let me see, she lives over near Lake Loraine, doesn'tshe?"

This time Feldman could not repress a start. But he covered itadmirably by stooping over to pick up a tool that fell to the floor.

"No, my mother is in Lafayette," he said. "I don't know where LakeLoraine is."

"Oh," said Tom, as he turned aside to hide a smile. He was sure now heknew at least one of the plotters.

But Tom was not yet ready to show his hand. He wanted better evidencethan any he yet possessed. It would take a little more time.

Work on the aerial warship was rushed, and it seemed likely that atrial flight could be made before the date set. Lieutenant Marbury sentword that he would be on hand when needed, and in some of the shops,where fittings for the Mars were being made, night and day shifts wereworking.

"Well, if everything goes well, we'll take her for a trial flightto-morrow," said Tom, coming in from the shops one evening.

"Guns and all?" asked Ned, who had come over to pay his chum a visit.Mr. Damon was also on hand, invoking occasional blessings.

"Guns and all," replied Tom.

Ned had a little vacation from the bank, and was to stay all night, aswas Mr. Damon.

What time it was, save that it must be near midnight, Tom could nottell, but he was suddenly awakened by hearing yells from Eradicate:

"Massa Tom! Massa Tom!" yelled the excited colored man. "Git up! Gitup! Suffin' turrible am happenin' in de balloon shop. Hurry! An' yo'stan' still, Boomerang, or I'll twist yo' tail, dat's what I will!Hurry, Massa Tom!"

Tom leaped out of bed.

CHAPTER XIII

THE CAPTURE

Tom Swift was something like a fireman. He had lived so long in anatmosphere of constant alarms and danger, that he was always ready foralmost any emergency. His room was equipped with the end in view thathe could act promptly and effectively.

So, when he heard Eradicate's alarm, though he wondered what the oldcolored man was doing out of bed at that hour, Tom did not stop toreason out that puzzle. He acted quickly.

His first care was to throw on the main switch, connected with a bigstorage battery, and to which were attached the wires of the lightingsystem. This at once illuminated every shop in the plant, and also thegrounds themselves. Tom wanted to see what was going on. The use of astorage battery eliminated the running of the dynamo all night.

And once he had done this, Tom began pulling on some clothes and a pairof shoes. At the same time he reached out with one hand and pressed abutton that sounded an alarm in the sleeping quarters of Koku, thegiant, and in the rooms of some of the older and most trusted men.

All this while Eradicate was shouting away, down in the yard.

"Massa Tom! Massa Tom!" he called. "Hurry! Hurry! Dey is killin' Koku!"

"Killing Koku!" exclaimed Tom, as he finished his hasty dressing. "Thenmy giant must already be in the fracas. I wonder what it's all about,anyhow."

"What's up, Tom?" came Ned's voice from the adjoining room. "I thoughtI heard a noise."

"Your thoughts do you credit, Ned!" Tom answered. "If you listen rightclose, you'll hear several noises."

"By Jove! You're right, old man!"

Tom could hear his chum bound out of bed to the floor, and, at the sametime, from the big shed where Tom was building his aerial warship camea series of yells and shouts.

"That's Koku's voice!" Tom exclaimed, as he recognized the tones of thegiant.

"I'm coming, Tom!" Ned informed his chum. "Wait a minute."

"No time to wait," Tom replied, buttoning his coat as he sped down thehall.

"Oh, Tom, what is it?" asked Mrs. Baggert, the housekeeper, lookingfrom her room.

"I don't know. But don't let dad get excited, no matter what happens.Just put him off until I come back. I think it isn't anything serious."

Mr. Damon, who roomed next to Ned, came out of his own apartmentpartially dressed.

"Bless my suspenders!" he cried to Tom, those articles just thendangling over his hips. "What is it? What has happened? Bless my steamgauge, don't tell me it's a fire!"

"I think it isn't that," Tom answered. "No alarm has rung. Koku seemsto be in trouble."

"Well, he's big enough to look after himself, that's one consolation,"chuckled Mr. Damon. "I'll be right with you."

By this time Ned had run out into the hall, and, together, he and Tomsped down the corridor. They could not hear the shouts of Eradicate soplainly now, as he was on the other side of the house.

But when the two young men reached the front porch, they could hear theyells given with redoubled vigor. And, in the glare of the electriclights, Tom saw Eradicate leading along Boomerang, the old mule.

"What is it, Rad? What is it?" demanded the young inventor breathlessly.

"Trouble, Massa Tom! Dat's what it am! Trouble!"

"I know that—but what kind?"

"De worstest kind, I 'spects, Massa Tom. Listen to it!"

From the interior of the big shed, not far from the house, Tom and Nedheard a confused jumble of shouts, cries and pleadings, mingled withthe rattle of pieces of metal, and the banging of bits of wood. And,above all that, like the bellowing of a bull, was noted the rumblingvoice of Koku, the giant.

"Come on, Ned!" Tom cried.

"It's suah trouble, all right," went on Eradicate. "Mah mule,Boomerang, had a touch ob de colic, an' I got up t' gib him some hotdrops an' walk him around, when I heard de mostest terrificracket-sound, and den I 'spected trouble was comm."

"It isn't coming—it's here!" called Tom, as he sped toward the bigshop. Ned was but a step behind him. The big workshop where the aerialwarship was being built was, like the other buildings, brilliantlyilluminated by the lights Tom had switched on. The young inventor alsosaw several of his employees speeding toward the same point.

Tom was the first to reach the small door of the shed. This was builtin one of the two large main doors, which could be swung open when itwas desired to slide the Mars in from the ground, and not admit itthrough the roof.

"Look!" cried Tom, pointing.

Ned looked over his chum's shoulder and saw the giant, Koku, strugglingwith four men—powerful men they were, too, and they seemed bent onmischief.

For they came at Koku from four sides, seeking to hold his hands andfeet so that he could not fight them back. On the floor near where thestruggle was taking place was a coil of rope, and it was evident thatit had been the intention of the men to overcome Koku and truss him up,so that he would not interfere with what they intended to do. But Kokuwas a match for even the four men, powerful as they were.

"We're here, Koku!" cried Tom. "Watch for an opening, Ned!" he calledto his chum.

The sound of Tom's voice disconcerted at least two of the attackers,for they looked around quickly, and this was fatal to their chances.

Though such a big man, Koku was exceptionally quick, and no sooner didhe see his advantage, as two of the men turned their gaze away fromhim, than he seized it.

Suddenly tearing loose his hands from the grip of the two men who hadlooked around, Koku shot out his right and left fists, and secured goodhold on the necks of two of his enemies. The other two, at his back,were endeavoring to pull him over, but the giant's sturdy legs stillheld.

So big was Koku's hands that they almost encircled the necks of hisantagonists. Then happened a curious thing.

With a shout that might have done credit to some ancient cave-dwellerof the stone age, Koku spread out his mighty arms, and held apart thetwo men he had grasped. In vain they struggled to free themselves fromthat terrible grip. Their faces turned purple, and their eyes bulgedout.

"He's choking them to death!" shouted Ned.

But Koku was not needlessly cruel.

A moment later, with a quick and sudden motion he bent his arms,bringing toward each other the two men he held as captives. Theirheads came together with a dull thud, and a second later Koku allowedtwo limp bodies to slip from his grip to the floor.

"He's done for them!" Tom cried. "Knocked them unconscious. Good foryou, Koku!"

The giant grunted, and then, with a quick motion, slung himself around,hoping to bring the enemies at his back within reach of his powerfularms. But there was no need of this.

As soon as the other two ruffians had seen their companions fall to thefloor of the shop they turned and fled, leaping from an open window.

"There they go!" cried Ned.

"Some of the other men can chase them," said the young inventor. "We'lltie up the two Koku has captured."

As he approached nearer to the unconscious captives Tom uttered a cryof surprise, for he recognized them as two of the new men he hademployed.

"What can this mean?" he asked wonderingly.

He glanced toward the window through which the two men had jumped toescape, and he was just in time to see one of them run past the opendoor. The face of this one was under a powerful electric light, and Tomat once recognized the man as Feldman, the worker who had had so muchtrouble with the trip-hammer.

"This sure is a puzzle," marveled Tom. "My own men in the plot! Butwhy did they attack Koku?"

The giant, bending over the men he had knocked unconscious by beatingtheir heads together, seemed little worse for the attack.

"We tie 'em up," he said grimly, as he brought over the rope that hadbeen intended for himself.

CHAPTER XIV

THE FIRST FLIGHT

Little time was lost in securing the two men who had been soeffectively rendered helpless by Koku's ready, if rough, measures. Oneof them was showing signs of returning consciousness now, and Tom, notwilling to inflict needless pain, even on an enemy, told one of hismen, summoned by the alarm, to bring water. Soon the two men openedtheir eyes, and looked about them in dazed fashion.

"Did—did anything hit me?" asked one meekly.

"It must have been a thunderbolt," spoke the other dreamily. "But itdidn't look like a storm."

"Oh, dere was a storm, all right," chuckled Eradicate, who, having lefthis mule, Boomerang outside, came into the shed. "It was a giant stormall right."

The men put their hands to their heads, and seemed to comprehend. Theylooked at the rope that bound their feet. Their forearms had beenloosened to allow them to take a drink of water.

"What does this mean—Ransom—Kurdy?" asked Tom sternly, when the menseemed able to talk. "Did you attack Koku?"

"It looks as though he had the best of us, whether we did or not," saidthe man Tom knew as Kurdy. "Whew, how my head aches!"

"Me sorry," said Koku simply.

"Not half as sorry as we are," returned Ransom ruefully.

"What does it mean?" asked Tom sternly. "There were four of you.Feldman and one other got away."

"Oh, trust Feldman for getting away," sneered Kurdy. "He always leaveshis friends in the lurch."

"Was this a conspiracy?" demanded Tom.

The two captives looked at one another, sitting bound on the floor ofthe shop, their backs against some boxes.

"I guess it's all up, and we might as well make a clean breast of it,"admitted Kurdy.

"Perhaps it would be better," said Tom quietly. "Eradicate," he wenton, to the colored man, "go to the house and tell Mrs. Baggert thateverything is all right and no one hurt."

"No one hurt, Massa Tom? What about dem dere fellers?" and the coloredman pointed to the captives.

"Well, they're not hurt much," and Tom permitted himself a littlesmile. "I don't want my father to worry. Tell him everything is allright."

"All right, Massa Tom. I'se gwine right off. I'se got t' look after mahmule, Boomerang, too. I'se gwine," and he shuffled away.

"Who else besides Feldman got away?" asked Tom, looking alternately atthe prisoners.

They hesitated a moment about answering.

"We might as well give up, I tell you," spoke Kurdy to Ransom.

"All right, go ahead, we'll have to take our medicine. I might haveknown it would turn out this way—going in for this sort of thing. It'sthe first bit of crooked business I ever tried," the man saidearnestly, "and it will be the last—believe me!"

"Who was the fourth man?" Tom repeated.

"Harrison," answered Kurdy, naming one of the most efficient of the newmachinists Tom had hired during the rush.

"Harrison, who has been working on the motor?" cried the young inventor.

"Yes," said Ransom.

"I'm sorry to learn that," Tom went on in a low voice. "He was anexpert in his line. But what was your object, anyhow, in attackingKoku?"

"We didn't intend to attack him," explained Ransom, "but he came inwhen we were at work, and as he went for us we tried to stand him off.Then your colored man heard the racket, and—well, I guess you know therest."

"But I don't understand why you came into this shed at night," went onTom. "No one is allowed in here. You had no right, and Koku knew that.What did you want?"

"Look here!" exclaimed Kurdy, "I said we'd make a clean breast of it,and we will. We're only a couple of tools, and we were foolish ever togo in with those fellows; or rather, in with that Frenchman, whopromised us big money if we succeeded."

"Succeeded in what?" demanded the young inventor.

"In damaging your new aerial warship, or in getting certain parts of itso he could take them away with him."

Tom gave a surprised whistle.

"A frenchman!" he exclaimed. "Is he one of the—?"

"Yes, he's one of the foreign spies," interrupted Ransom. "You'd findit out, anyhow, if we didn't tell you. They are after you, Tom Swift,and after your machines. They had vowed to get them by fair means orfoul, for some of the European governments are desperate."

"But we were only tools in their hands. So were Feldman and Harrison,but they knew more about the details. We were only helping them."

"Then we must try to capture them," decided Tom. "Ned, see if the chasehad any results. I'll look after these chaps—Koku and I."

"Oh, we give in," admitted Kurdy. "We know when we've had enough," andhe rubbed his head gently where the giant had banged it against that ofhis fellow-conspirator.

"Do you mean that you four came into this shop, at midnight, to damagethe Mars?" asked Tom.

"That's about it, Mr. Swift," replied Kurdy rather shamefacedly. "Wewere to damage it beyond repair, set fire to the whole place, if needbe, and, at the same time, take away certain vital parts.

"Harrison, Feldman, Ransom and I came in, thinking the coast was clear.But Koku must have seen us enter, or he suspected we were here, for hecame in after us, and the fight began. We couldn't stop him, and he didfor us. I'm rather glad of it, too, for I never liked the work. It wasonly that they tempted me with a promise of big money."

"Who tempted you?" demanded Tom.

"That Frenchman—La Foy, he calls himself, and some other foreigners inyour shops."

"Are there foreigners here?" cried Tom.

"Bless my chest protector!" cried Mr. Damon, who had come in and hadbeen a silent listener to this. "Can it be possible?"

"That's the case," went on Kurdy. "A lot of the new men you took on areforeign spies from different European nations. They are trying to learnall they can about your plans, Mr. Swift!"

"Are they friendly among themselves?" asked Tom.

"No; each one is trying to get ahead of the other. So far the Frenchmanseems to have had the best of it. But to-night his plan failed."

"Tell me more about it," urged Tom.

"That's about all we know," spoke Ransom. "We were only hired to do therough work. Those higher up didn't appear. Feldman was only a stepabove us."

"Then my suspicions of him were justified," thought Tom. "He evidentlymet La Foy in the woods to make plans. But Koku and Eradicate spoiledthem."

The two captives seemed willing enough to make a confession, but theydid not know much. As they said, they were merely tools, acting forothers. And events had happened just as they had said.

The four conspirators had managed, by means of a false key, and bydisconnecting the burglar alarm, to enter the airship shed. They wereabout to proceed with their work of destruction when Koku came on thescene.

The giant's appearance was due to accident. He acted as a sort of nightwatchman, making a tour of the buildings, but he entered the shed wherethe Mars was because, that day, he had left his knife in there, andwanted to get it. Only for that he would not have gone in. When heentered he surprised the four men.

Of course he attacked them at once, and they sprang at him. Thenensued a terrific fight. Eradicate, arising to doctor his mule, as hehad said, heard the noise, and saw what was going on. He gave thealarm.

"Well, Ned, any luck?" asked Tom, as his chum came in.

"No, they got away, Tom. I had a lot of your men out helping me searchthe grounds, but it wasn't of much use."

"Particularly if you depended on some of my men," said Tom bitterly.

"What do you mean?"

"I mean that the place is filled with spies, Ned! But we will sift themout in the morning. This has been a lucky night for me. It was touchand go. Now, then, Koku, take these fellows and lock them up somewhereuntil morning. Ned, you and I will remain on guard here the rest of thenight."

"I'm with you, Tom."

"Will you be a bit easy on us, considering what we told you?" askedKurdy.

"I'll do the best I can," said Tom, gently, making no promises.

The two captives were put in secure quarters, and the rest of the nightpassed quietly. During the fight in the airship shed some machinery andtools had been broken, but no great amount of damage was done. Tom andNed passed the remaining hours of darkness there.

A further search was made in the morning for the two conspirators whohad escaped, but no trace of them was found. Tom then realized whyFeldman was so anxious to be placed in the aeroplane department—it wasin order that he might have easier access to the Mars.

A technical charge was made against the two prisoners, sufficient tohold them for some time. Then Tom devoted a day to weeding out thesuspected foreigners in his place. All the new men were discharged,though some protested against this action.

"Probably I am hitting some of the innocent in punishing those who, ifthey had the chance, would become guilty," Tom said to his chum, "butit cannot be helped—I can't afford to take any chances."

The Mars was being put in shape for her first flight. The guns, fittedwith the recoil shock absorbers, were mounted, and Lieutenant Marburyhad returned to go aloft in the big aerial warship. He congratulatedTom on discovering at least one plot in time.

"But there may be more," he warned the young inventor. "You are notdone with them yet."

The Mars was floated out of her hangar, and made ready for an ascent.Tom, Ned, Lieutenant Marbury, Mr. Damon, and several workmen were to bethe first passengers. Tom was busy going over the various parts to seethat nothing had been forgotten.

"Well, I guess we're ready," he finally announced. "All aboard!"

"Bless my insurance policy!" exclaimed Mr. Damon. "Now that the timecomes I almost wish I wasn't going."

"Nonsense!" exclaimed Tom. "You're not going to back out at the lastminute. All aboard! Cast off the ropes!" he cried to the assistants.

A moment later the Mars, the biggest airship Tom Swift had everconstructed, arose from the earth like some great bird, and soaredaloft.

CHAPTER XV

IN DANGER

"Well, Tom, we're moving!" cried Ned Newton, clapping his chum on theback, as he stood near him in the pilot-house. "We're going up, oldsport!"

"Of course we are," replied Tom. "You didn't think it wouldn't go up,did you?"

"Well, I wasn't quite sure," Ned confessed. "You know you were soworried about—"

"Not about the ship sailing," interrupted Tom. "It was only the effectthe firing of the guns might have. But I think we have that taken careof."

"Bless my pin cushion!" cried Mr. Damon, as he looked over the rail atthe earth below. "We're moving fast, Tom."

"Yes, we can make a quicker ascent in this than in most aeroplanes,"Tom said, "for they have to go up in a slanting direction. But we can'tquite equal their lateral speed."

"Just how fast do you think you can travel when you are in first-classshape?" asked Lieutenant Marbury, as he noted how the Mars wasbehaving on this, the first trip.

"Well, I set a limit of seventy-five miles an hour," the young inventorreplied, as he shifted various levers and handles, to change the speedof the mechanism. "But I'm afraid we won't quite equal that with allour guns on board. But I'm safe in saying sixty, I think."

"That will more than satisfy the government requirements," the officersaid. "But, of course, your craft will have to come up to expectationsand requirements in the matter of armament."

"I'll give you every test you want," declared Tom, with a smile. "Andnow we'll see what the Mars can do when put to it."

Up and up went the big dirigible aerial warship. Had you been fortunateenough to have seen her you would have observed a craft not unlike, inshape, the German Zeppelins. But it differed from those war balloons inseveral important particulars.

Tom's craft was about six hundred feet long, and the diameter of thegas bag, amidships, was sixty feet, slightly larger than the largestZeppelin. Below the bag, which, as I have explained, was made up of anumber of gas-tight compartments, hung from wire cables three cabins.The forward one was a sort of pilot-house, containing variousinstruments for navigating the ship of the air, observation rooms,gauges for calculating firing ranges, and the steering apparatus.

Amidships, suspended below the great bag, were the living and sleepingquarters, where food was cooked and served and where those who operatedthe craft could spend their leisure time. Extra supplies were alsostored there.

At the stern of the big bag was the motor-room, where gas was generatedto fill the balloon compartments when necessary, where the gasoline andelectrical apparatus were installed, and where the real motive power ofthe craft was located. Here, also, was carried the large quantity ofgasoline and oil needed for a long voyage. The Mars could carrysufficient fuel to last for over a week, provided no accidents occurred.

There was also an arrangement in the motor compartment, so that theship could be steered and operated from there. This was in case theforward pilot-house should be shot away by an enemy. And, also, in themotor compartment were the sleeping quarters for the crew.

All three suspended cabins were connected by a long covered runway, sothat one could pass from the pilot-house to the motor-room and backagain through the amidship cabin.

At the extreme end of the big bag were the various rudders and planes,designed to keep the craft on a level keel, automatically, and toenable it to make headway against a strong wind. The motive powerconsisted of three double-bladed wooden propellers, which could beoperated together or independently. A powerful gasoline engine was thechief motive power, though there was an auxiliary storage battery,which would operate an electrical motor and send the ship along formore than twenty-four hours in case of accident to the gasoline engine.

There were many other pieces of apparatus aboard, some not completelyinstalled, the uses of which I shall mention from time to time, as thestory progresses. The gas-generating machine was of importance, forthere would be a leakage and shrinking of the vapor from the big bag,and some means must be provided for replenishing it.

"You don't seem to have forgotten anything, Tom," said Ned admiringly,as they soared upward.

"We can tell better after we've flown about a bit," observed the younginventor, with a smile. "I expect we shall have to make quite a numberof changes."

"Are you going far?" asked Mr. Damon.

"Why, you're not frightened, are you?" inquired Tom. "You have been upin airships with me before."

"Oh, no, I'm not frightened!" exclaimed the odd man. "Bless mysuspenders, no! But I promised my wife I'd be back this evening, and..."

"We'll sail over toward Waterford," broke in Tom, "and I'll drop youdown in your front yard."

"No, don't do that! Don't! I beg of you!" cried Mr. Damon. "Yousee—er—Tom, my wife doesn't like me to make these trips. Of course, Iunderstand there is no danger, and I like them. But it's just as wellnot to make her worry-you understand!"

"Oh, all right," replied Tom, with a laugh. "Well, we're not going faron this trip. What I want to do, most of all, is to test the guns, andsee if the recoil check will work as well when we are aloft as it diddown on the ground. You know a balloon isn't a very stable base for agun, even one of light caliber."

"No, it certainly is not," agreed Lieutenant Marbury, "and I aminterested in seeing how you will overcome the recoil."

"We'll have a test soon," announced Tom.

Meanwhile the Mars, having reached a considerable height, being up sofar, in fact, that the village of Shopton could scarcely bedistinguished, Tom set the signal that told the engine-room force tostart the propellers. This would send them ahead.

Some of Tom's most trusted workmen formed the operating crew, the younginventor taking charge of the pilot-house himself.

"Well she seems to run all right," observed Lieutenant Marbury, as thebig craft surged ahead just below a stratum of white, fleecy clouds.

"Yes, but not as fast as I'd like to see her go," Tom replied. "Ofcourse the machinery is new, and it will take some little time for itto wear down smooth. I'll speed her up a little now."

They had been running for perhaps ten minutes when Tom shoved over thehand of an indicator that communicated with the engine-room from thepilot-house. At once the Mars increased her speed.

"She can do it!" cried Ned.

"Bless my-hat! I should say so!" cried Mr. Damon, for he was standingoutside the pilot-house just then, on the "bridge," and the suddenincrease of speed lifted his hat from his head.

"There you are—caught on the fly!" cried Ned, as he put up his handjust in time to catch the article in question.

"Thanks! Guess I'd better tie it fast," remarked the odd man, puttinghis hat on tightly.

The aerial warship was put through several evolutions to test herstability, and to each one she responded well, earning the praise ofthe government officer. Up and down, to one side and the other, aroundin big circles, and even reversing, Tom sent his craft with a true handand eye. In a speed test fifty-five miles was registered against aslight wind, and the young inventor said he knew he could do betterthan that as soon as some of the machinery was running more smoothly.

"And now suppose we get ready for the gun tests," suggested Tom, whenthey had been running for about an hour.

"That's what I'm mostly interested in," said Lieutenant Marbury. "It'seasy enough to get several good types of dirigible balloons, but few ofthem will stand having a gun fired from them, to say nothing of severalguns."

"Well, I'm not making any rash promises," Tom went on, "but I think wecan turn the trick."

The armament of the Mars was located around the center cabin. Therewere two large guns, fore and aft, throwing a four-inch projectile, andtwo smaller calibered quick-firers on either beam. The guns weremounted on pedestals that enabled the weapons to fire in almost anydirection, save straight up, and of course the balloon bag being abovethem prevented this. However, there was an arrangement whereby a smallautomatic quick-firer could be sent up to a platform built on top ofthe gas envelope itself, and a man stationed there could shoot at arival airship directly overhead.

But the main deck guns could be elevated to an angle of nearlyforty-five degrees, so they could take care of nearly any hostileaircraft that approached.

"But where are the bombs I heard you speaking of?" asked Ned, as theyfinished looking at the guns.

"Here they are," spoke Tom, as he pointed to a space in the middle ofthe main cabin floor. He lifted a brass plate, and disclosed threeholes, covered with a strong wire netting that could be removed. "Thebombs will be dropped through those holes," explained the younginventor, "being released by a magnetic control when the operatorthinks he has reached a spot over the enemy's city or fortificationwhere the most damage will be done. I'll show you how they work alittle later. Now we'll have a test of some of the guns."

Tom called for some of his men to take charge of the steering andrunning of the Mars while he and Lieutenant Marbury prepared to firethe two larger weapons. This was to be one of the most important tests.

Service charges had been put in, though, of course, no projectileswould be used, since they were then flying over a large city not farfrom Shopton.

"We'll have to wait until we get out over the ocean to give a completetest, with a bursting shell," Tom said.

He and Lieutenant Marbury were beside a gun, and were about to fire it,when suddenly, from the stern of the ship, came a ripping, tearingsound, and, at the same time, confused shouts came from the crew'squarters.

"What is it?" cried Tom.

"One of the propellers!" was the answer. "It's split, and has torn abig hole in the gas bag!"

"Bless my overshoes!" cried Mr. Damon. "We're going down!"

All on board the Mars became aware of a sudden sinking sensation.

CHAPTER XVI

TOM IS WORRIED

"Steady, all!" came in even tones from Tom Swift. Not for an instanthad he lost his composure. For it was an accident, that much wascertain, and one that might endanger the lives of all on board.

Above the noise of the machinery in the motor room could be heard thethrashing and banging of the broken or loose propeller-blade. Justwhat its condition was, could not be told, as a bulge of the gas baghid it from the view of those gathered about the gun, which was aboutto be fired when the alarm was given.

"We're sinking!" cried Mr. Damon. "We're going down, Tom!"

"That's nothing," was the cool answer. "It is only for a moment. Only afew of the gas compartments can be torn. There will soon enoughadditional gas in the others to lift us again."

And so it proved. The moment the pressure of the lifting gas in the bigoiled silk and aluminum container was lowered, it started thegenerating machine, and enough extra gas was pumped into the uninjuredcompartments to compensate for the loss.

"We're not falling so fast now," observed Ned.

"No, and we'll soon stop falling altogether," calmly declared Tom. "Toobad this accident had to happen, though."

"It might have been much worse, my boy!" exclaimed the lieutenant."That's a great arrangement of yours—the automatic gas machine."

"It's on the same principle as the air brakes of a trolley car,"explained Tom, when a look at the indicators showed that the Mars hadceased falling and remained stationary in the air. Tom had also sent asignal to the engine-room to shut off the power, so that the twoundamaged propellers, as well as the broken one, ceased revolving.

"In a trolley car, you see," Tom went on, when the excitement hadcalmed down, "as soon as the air pressure in the tanks gets below acertain point, caused by using the air for a number of applications ofthe brakes, it lets a magnetized bar fall, and this establishes anelectrical connection, starting the air pump. The pump forces more airinto the tanks until the pressure is enough to throw the pump switchout of connection, when the pump stops. I use the same thing here."

"And very clever it is," said Mr. Damon. "Do you suppose the danger isall over, Tom?"

"For the time being, yes. But we must unship that damaged propeller,and go on with the two."

The necessary orders were given, and several men from the engine-roomat once began the removal of the damaged blades.

As several spare ones were carried aboard one could be put on in placeof the broken one, had this been desired. But Tom thought the accidenta good chance to see how his craft would act with only two-thirds ofher motive force available, so he did not order the damaged propellerreplaced. When it was lowered to the deck it was carefully examined.

"What made it break?" Ned wanted to know.

"That's a question I can't answer," Tom replied. "There may have been adefect in the wood, but I had it all carefully examined before I usedit."

The propeller was one of the "built-up" type, with alternate layers ofash and mahogany, but some powerful force had torn and twisted theblades. The wood was splintered and split, and some jagged pieces,flying off at a tangent, so great was the centrifugal force, had tornholes in the strong gas bag.

"Did something hit it; or did it hit something?" asked Ned as he sawTom carefully examining the broken blades.

"Hard to say. I'll have a good look at this when we get back. Just nowI want to finish that gun test we didn't get a chance to start."

"You don't mean to say you're going to keep on, and with the balloondamaged; are you?" cried Mr. Damon, in surprise.

"Certainly—why not?" Tom replied. "In warfare accidents may happen,and if the Mars can't go on, after a little damage like this, what isgoing to happen when she's fired on by a hostile ship? Of course I'mgoing on!"

"Bless my necktie!" ejaculated the odd man.

"That's the way to talk!" exclaimed Lieutenant Marbury. "I'm with you."

There really was very little danger in proceeding. The Mars was just asbuoyant as before, for more gas had been automatically made, and forcedinto the uninjured compartments of the bag. At the same time enoughsand ballast had been allowed to run out to make the weight to belifted less in proportion to the power remaining.

True, the speed would be less, with two propellers instead of three,and the craft would not steer as well, with the torn ends of the gasbag floating out behind. But this made a nearer approach to warconditions, and Tom was always glad to give his inventions the mostsevere tests possible.

So, after a little while, during which it was seen that the Mars wasproceeding almost normally, the matter of discharging the guns wastaken up again.

The weapons were all ready to fire, and when Tom had attached thepressure gauges to note how much energy was expended in the recoil, hegave the word to fire.

The two big weapons were discharged together, and for a moment afterthe report echoed out among the cloud masses every soul on the shipfeared another accident had happened.

For the big craft rolled and twisted, and seemed about to turn turtle.Her forward progress was halted, momentarily, and a cry of fear camefrom several of the members of the crew, who had had only a littleexperience in aircraft.

"What's the matter?" cried Ned. "Something go wrong?"

"A little," admitted Tom, with a rueful look on his face. "Thoserecoil checks didn't work as well in practice as they did in theory."

"Are you sure they are strong enough?" asked Lieutenant Marbury.

"I thought so," spoke Tom. "I'll put more tension on the spring nexttime."

"Bless my watch chain!" cried Mr. Damon. "You aren't going to firethose guns again; are you, Tom?"

"Why not? We can't tell what's the matter, nor get things rightwithout experimenting. There's no danger."

"No danger! Don't you call nearly upsetting the ship danger?"

"Oh, well, if she turns over she'll right herself again," Tom said."The center of gravity is low, you see. She can't float in any positionbut right side up, though she may turn over once or twice."

"Excuse me!" said Mr. Damon firmly. "I'd rather go down, if it's allthe same to you. If my wife ever knew I was here I'd never hear thelast of it!"

"We'll go down soon," Tom promised. "But I must fire a couple of shotsmore. You wouldn't call the recoil checks a success, would you?" andthe young inventor appealed to the government inspector.

"No, I certainly would not," was the prompt answer. "I am sorry, too,for they seemed to be just what was needed. Of course I understand thisis not an official test, and I am not obliged to make a report of thistrial. But had it been, I should have had to score against you.

"I realize that, and I'm not asking any favors, but I'll try it againwith the recoil checks tightened up. I think the hydrostatic valveswere open too much, also."

Preparations were now made for firing the four-inch guns once more. Allthis while the Mars had been speeding around in space, being about twomiles up in the air. Tom's craft was not designed to reach as great anelevation as would be possible in an aeroplane, since to work havoc toan enemy's fortifications by means of aerial bombs they do not need tobe dropped from a great height.

In fact, experiments in Germany have shown that bombs falling from agreat height are less effective than those falling from an airshipnearer the earth. For a bomb, falling from a height of two miles,acquires enough momentum to penetrate far into the earth, so that muchof the resultant explosive force is expended in a downward direction,and little damage is done to the fortifications. A bomb dropped from alower altitude, expending its force on all sides, does much more damage.

On the other hand, in destroying buildings, it has been found desirableto drop a bomb from a good height so that it may penetrate even aprotected roof, and explode inside.

Once more Tom made ready to fire, this time having given the recoilchecks greater resistance. But though there was less motion imparted tothe airship when the guns were discharged, there was still too much forcomfort, or even safety.

"Well, something's wrong, that's sure," remarked Tom, in ratherdisappointed tones as he noted the effect of the second shots. "If weget as much recoil from the two guns, what would happen if we firedthem all at once?"

"Don't do it! Don't do it, I beg of you!" entreated Mr. Damon. "Blessmy toothbrush—don't do it!"

"I won't—just at present," Tom said, ruefully. "I'm afraid I'll haveto begin all over again, and proceed along new lines."

"Well, perhaps you will," said the lieutenant. "But you may inventsomething much better than anything you have now. There is no greatrush. Take your time, and do something good."

"Oh, I'll get busy on it right away," Tom declared. "We'll go down now,and start right to work. I'm afraid, Ned, that our idea of adoor-spring check isn't going to work."

"I might have known my idea wouldn't amount to anything," said theyoung bank clerk.

"Oh, the idea is all right," declared Tom, "but it wants modifying.There is more power to those recoils than I figured, though our firstexperiments seemed to warrant us in believing that we had solved theproblem."

"Are you going to try the bomb-dropping device?" asked the lieutenant.

"Yes, there can't be any recoil from that," Tom said. "I'll drop a fewblank ones, and see how accurate the range finders are."

While his men were getting ready for this test Tom bent over the brokenpropeller, looking from that to the recoil checks, which had not comeup to expectations. Then he shook his head in a worried and puzzledmanner.

CHAPTER XVII

AN OCEAN FLIGHT

Dropping bombs from an aeroplane, or a dirigible balloon, is acomparatively simple matter. Of course there are complications that mayensue, from the danger of carrying high explosives in the limitedquarters of an airship, with its inflammable gasoline fuel, andever-present electric spark, to the possible premature explosion of thebomb itself. But they seem to be considered minor details now.

On the other hand, while it is comparatively easy to drop a bomb from amoving aeroplane, or dirigible balloon, it is another matter to makethe bomb fall just where it will do the most damage to the enemy. It isnot easy to gauge distances, high up in the air, and then, too,allowance must be made for the speed of the aircraft, theever-increasing velocity of a falling body, and the deflection causedby air currents.

The law of velocity governing falling bodies is well known. It varies,of course, according to the height, but in general a body fallingfreely toward the earth, as all high-school boys know, is acceleratedat the rate of thirty-two feet per second. This law has been takenadvantage of by the French in the present European war. The French dropfrom balloons, or aeroplanes, a steel dart about the size of a leadpencil, and sharpened in about the same manner. Dropping from a heightof a mile or so, that dart will acquire enough velocity to penetrate aman from his head all the way through his body to his feet.

But in dropping bombs from an airship the damage intended does not somuch depend on velocity. It is necessary to know how fast the bombfalls in order to know when to set the time fuse that will explode it;though some bombs will explode on concussion.

At aeroplane meets there are often bomb-dropping contests, and ballsfilled with a white powder (that will make a dust-cloud on falling, andso show where they strike) are used to demonstrate the birdman'saccuracy.

"We'll see how our bomb-release works," Tom went on. "But we'll have todescend a bit in order to watch the effect."

"You're not going to use real bombs, are you, Tom?" asked Ned.

"Indeed not. Just chalk-dust ones for practice. Now here is where thebombs will be placed," and he pointed to the three openings in thefloor of the amidship cabin. The wire nettings were taken out and onecould look down through the holes to the earth below, the ground beingnearer now, as Tom had let out some of the lifting gas.

"Here is the range-finder and the speed calculator," the young inventorwent on as he indicated the various instruments. "The operator sitshere, where he can tell when is the most favorable moment for releasingthe bomb."

Tom took his place before a complicated set of instruments, and beganmanipulating them. One of his assistants, under the direction ofLieutenant Marbury, placed in the three openings bombs, made of lightcardboard, just the size of a regular bomb, but filled with a whitepowder that would, on breaking, make a dust-cloud which could beobserved from the airship.

"I have first to determine where I want to drop the bomb," Tomexplained, "and then I have to get my distance from it on therange-finder. Next I have to know how fast I am traveling, and how farup in the air I am, to tell what the velocity of the falling bomb willattain at a certain time. This I can do by means of these instruments,some of which I have adapted from those used by the government," hesaid, with a nod to the officer.

"That's right—take all the information you can get," was the smilingresponse.

"We will now assume that the bombs are in place in the holes in thefloor of the cabin," Tom went on. "As I sit here I have before me threebuttons. They control the magnets that hold the bombs in place. If Ipress one of the buttons it breaks the electrical current, the magnetno longer has any attraction, and it releases the explosive. Now lookdown. I am going to try and drop a chalk bomb near that stone fence."

The Mars was then flying over a large field and a stone fence was inplain view.

"Here she goes!" cried Tom, as he made some rapid calculations from hisgauge instruments. There was a little click and the chalk bomb dropped.There was a plate glass floor in part of the cabin, and through thisthe progress of the pasteboard bomb could be observed.

"She'll never go anywhere near the fence!" declared Ned. "You let itdrop too soon, Tom!"

"Did I? You just watch. I had to allow for the momentum that would begiven the bomb by the forward motion of the balloon."

Hardly had Tom spoken than a puff of white was seen on the very top ofthe fence.

"There it goes?" cried the lieutenant. "You did the trick, Swift!"

"Yes, I thought I would. Well, that shows my gauges are correct,anyhow. Now we'll try the other two bombs."

In succession they were released from the bottom of the cabin, at otherdesignated objects. The second one was near a tree. It struck withinfive feet, which was considered good.

"And I'll let the last one down near that scarecrow in the field," saidTom, pointing to a ragged figure in the middle of a patch of corn.

Down went the cardboard bomb, and so good was the aim of the younginventor that the white dust arose in a cloud directly back of thescarecrow.

And then a queer thing happened. For the figure seemed to come to life,and Ned, who was watching through a telescope, saw a very much excitedfarmer looking up with an expression of the greatest wonder on hisface. He saw the balloon over his head, and shook his fist at it,evidently thinking he had had a narrow escape. But the pasteboard bombwas so light that, had it hit him, he would not have been injured,though he might have been well dusted.

"Why, that was a man! Bless my pocketbook!" cried Mr. Damon.

"I guess it was," agreed Tom. "I took it for a scarecrow."

"Well, it proved the accuracy of your aim, at any rate," observedLieutenant Marbury. "The bomb dropping device of your aerial warship isperfect—I can testify to that."

"And I'll have the guns fixed soon, so there will be no danger of arecoil, too," added Tom Swift, with a determined look on his face.

"What's next?" asked Mr. Damon, looking at his watch. "I really oughtto be home, Tom."

"We're going back now, and down. Are you sure you don't want me to dropyou in your own front yard, or even on your roof? I think I couldmanage that."

"Bless my stovepipe, no, Tom! My wife would have hysterics. Just landme at Shopton and I'll take a car home."

The damaged airship seemed little the worse for the test to which shehad been subjected, and made her way at good speed in the direction ofTom's home. Several little experiments were tried on the way back. Theyall worked well, and the only two problems Tom had to solve were thetaking care of the recoil from the guns and finding out why thepropeller had broken.

A safe landing was made, and the Mars once more put away in her hangar.Mr. Damon departed for his home, and Lieutenant Marbury again took uphis residence in the Swift household.

"Well, Tom, how did it go?" asked his father.

"Not so very well. Too much recoil from the guns."

"I was afraid so. You had better drop this line of work, and go atsomething else."

"No, Dad!" Tom cried. "I'm going to make this work. I never hadanything stump me yet, and I'm not going to begin now!"

"Well, that's a good spirit to show," said the aged inventor, with ashake of his head, "but I don't believe you'll succeed, Tom."

"Yes I will, Dad! You just wait."

Tom decided to begin on the problem of the propeller first, as thatseemed more simple. He knew that the gun question would take longer.

"Just what are you trying to find out, Tom?" asked Ned, a few nightslater, when he found his chum looking at the broken parts of thepropeller.

"Trying to discover what made this blade break up and splinter thatway. It couldn't have been centrifugal force, for it wasn't strongenough."

Tom was "poking" away amid splinters, and bits of broken wood, when hesuddenly uttered an exclamation, and held up something. "Look!" hecried. "I believe I've found it."

"What?" asked Ned.

"The thing that weakened the propeller. Look at this, and smell!" Heheld out a piece of wood toward Ned. The bank employee saw where ahalf-round hole had been bored in what remained of the blade, and fromthat hole came a peculiar odor.

"It's some kind of acid," ventured Ned.

"That's it!" cried Tom. "Someone bored a hole in the propeller, and putin some sort of receptacle, or capsule, containing a corrosive acid. Indue time, which happened to be when we took our first flight, the acidate through whatever it was contained in, and then attacked the wood ofthe propeller blade. It weakened the wood so that the force used inwhirling it around broke it."

"Are you sure of that?" asked Ned.

"As sure as I am that I'm here! Now I know what caused the accident!"

"But who would play such a trick?" asked Ned. "We might all have beenkilled."

"Yes, I know we might," said Tom. "It must be the work of some of thoseforeign spies whose first plot we nipped in the bud. I must tellMarbury of this, but don't mention it to dad."

"I won't," promised Ned.

Lieutenant Marbury agreed with Tom that someone had surreptitiouslybored a small hole in the propeller blade, and had inserted a corrosiveacid that would take many hours to operate. The hole had been varnishedover, probably, so it would not show.

"And that means I've got to examine the other two blades," Tom said."They may be doctored too."

But they did not prove to be. A careful examination showed nothingwrong. An effort was made to find out who had tried to destroy the Marsin midair, but it came to nothing. The two men in custody declared theyknew nothing of it, and there was no way of proving that they did.

Meanwhile, the torn gas bag was repaired, and Tom began working on theproblem of doing away with the gun recoil. He tried several schemes,and almost was on the point of giving up when suddenly he received ahint by reading an account of how the recoil was taken care of on someof the German Zeppelins.

The guns there were made double, with the extra barrel filled withwater or sand, that could be shot out as was the regular charge. Asboth barrels were fired at the same time, and in opposite directions,with the same amount of powder, one neutralized the other, and therecoil was canceled, the ship remaining steady after fire.

"By Jove! I believe that will do the trick!" cried Tom. "I'm going totry it."

"Good luck to you!" cried Ned.

It was no easy matter to change all the guns of the Mars, and fit themwith double barrels. But by working day and night shifts Tom managedit. Meanwhile, a careful watch was kept over the shops. Several new menapplied for work, and some of them were suspicious enough in looks, butTom took on no new hands.

Finally the new guns were made, and tried with the Mars held on theground. They behaved perfectly, the shooting of sand or water from thedummy barrel neutralizing the shot from the service barrel.

"And now to see how it works in practice!" cried Tom one day. "Are youwith me for a long flight, Ned?"

"I sure am!"

The next evening the Mars, with a larger crew than before, and withTom, Ned, Mr. Damon and Lieutenant Marbury aboard, set sail.

"But why start at night?" asked Ned.

"You'll see in the morning," Tom answered.

The Mars flew slowly all night, life aboard her, at about the level ofthe clouds, going on almost as naturally as though the occupants of thecabins were on the earth. Excellent meals were served.

"But when are you going to try the guns?" asked Ned, as he got ready toturn in.

"Tell you in the morning," replied Tom, with a smile.

And, in the morning, when Ned looked down through the plate glass inthe cabin floor, he uttered a cry.

"Why, Tom! We're over the ocean!" he cried.

"I rather thought we'd be," was the calm reply. "I told George to headstraight for the Atlantic. Now we'll have a test with service chargesand projectiles!"

CHAPTER XVIII

IN A STORM

Surprise, for the moment, held Mr. Damon, Ned and Lieutenant Marburyspeechless. They looked from the heaving waters of the ocean below themto the young pilot of the Mars. He smiled at their astonishment.

"What—what does it mean, Tom?" asked Ned. "You never said you weregoing to take a trip as far as this."

"That's right," chimed in Mr. Damon. "Bless my nightcap! If I had knownI was going to be brought so far away from home I'd never have come."

"You're not so very far from Waterford," put in Tom. "We didn't makeany kind of speed coming from Shopton, and we could be back againinside of four hours if we had to."

"Then you didn't travel fast during the night?" asked the governmentman.

"No, we just drifted along," Tom answered. "I gave orders to run themachinery slowly, as I wanted to get it in good shape for the othertests that will come soon. But I told George, whom I left in chargewhen I turned in, to head for New York. I wanted to get out over theocean to try the guns with the new recoil arrangement."

"Well, we're over the ocean all right," spoke Ned, as he looked down atthe heaving waters.

"It isn't the first time," replied Tom cheerfully. "Koku, you may servebreakfast now," for the giant had been taken along as a sort of cookand waiter. Koku manifested no surprise or alarm when he found theairship floating over the sea. Whatever Tom did was right to him. Hehad great confidence in his master.

"No, it isn't the first time we've taken a water flight," spoke Ned. "Iwas only surprised at the suddenness of it, that's all."

"It's my first experience so far out above the water," observedLieutenant Marbury, "though of course I've sailed on many seas. Why,we're out of sight of land."

"About ten miles out, yes," admitted Tom. "Far enough to make it safeto test the guns with real projectiles. That is what I want to do."

"And we've been running all night?" asked Mr. Damon.

"Yes, but at slow speed. The engines are in better shape now than everbefore," Tom said. "Well, if you're ready we'll have breakfast."

The meal was served by Koku with as much unconcern as though they werein the Swift homestead back in Shopton, instead of floating near theclouds. And while it was being eaten in the main cabin, and while thecrew was having breakfast in their quarters, the aerial warship wasmoving along over the ocean in charge of George Watson, one of Tom'sengineers, who was stationed in the forward pilot-house.

"So you're going to give the guns a real test this time, is that it,Tom?" asked Ned, as he pushed back his plate, a signal that he hadeaten enough.

"That's about it."

"But don't you think it's a bit risky out over the water this way.Supposing something should—should happen?" Ned hesitated.

"You mean we might fall?" asked Tom, with a smile.

"Yes; or turn upside down."

"Nothing like that could happen. I'm so sure that I have solved theproblem of the recoil of the guns that I'm willing to take chances. Butif any of you want to get off the Mars while the test is being made, Ihave a small boat I can lower, and let you row about in that until—"

"No, thank you!" interrupted Mr. Damon, as he looked below. There wasquite a heavy swell on, and the ocean did not appear very attractive.They would be much more comfortable in the big Mars.

"I think you won't have any trouble," asserted Lieutenant Marbury. "Ibelieve Tom Swift has the right idea about the guns, and there will beso small a shock from the recoil that it will not be noticeable."

"We'll soon know," spoke Tom. "I'm going to get ready for the test now."

They were now well out from shore, over the Atlantic, but to makecertain no ships would be endangered by the projectiles, Tom and theothers searched the waters to the horizon with powerful glasses.Nothing was seen and the work of loading the guns was begun. The bombtubes, in the main cabin, were also to be given a test.

As service charges were to be used, and as the projectiles were filledwith explosives, great care was needed in handling them.

"We'll try dropping bombs first," Tom suggested. "We know they willwork, and that will be so much out of the way."

To make the test a severe one, small floating targets were firstdropped overboard from the Mars. Then the aerial warship, circlingabout, came on toward them. Tom, seated at the range-finders, pressedthe button that released the shells containing the explosives. Oneafter another they dropped into the sea, exploding as they fell, andsending up a great column of salt water.

"Every one a hit!" reported Lieutenant Marbury, who was keeping "score."

"That's good," responded Tom. "But the others won't be so easy. Wehave nothing to shoot at."

They had to fire the other guns without targets at which to aim. But,after all, it was the absence of recoil they wanted to establish, andthis could be done without shooting at any particular object.

One after another the guns were loaded. As has been explained, theywere now made double, one barrel carrying the projectile, and the othera charge of water.

"Are you ready?" asked Tom, when it was time to fire. LieutenantMarbury, Ned and Mr. Damon were helping, by being stationed at thepressure gauges to note the results.

"All ready," answered Ned.

"Do you think we'd better put on life preservers, Tom?" asked Mr. Damon.

"Nonsense! What for?"

"In case—in case anything happens."

"Nothing will happen. Look out now, I'm going to fire."

The guns were to be fired simultaneously by means of an electriccurrent, when Tom pressed a button.

"Here they go!" exclaimed the young inventor.

There was a moment of waiting, and then came a thundering roar. TheMars trembled, but she did not shift to either side from an even keel.From one barrel of the guns shot out the explosive projectiles, andfrom the other spurted a jet of water, sent out by a charge of powder,equal in weight to that which forced out the shot.

As the projectile was fired in one direction, and the water in onedirectly opposite, the two discharges neutralized one another.

Out flew the pointed steel shells, to fall harmlessly into the sea,where they exploded, sending up columns of water.

"Well!" cried Tom as the echoes died away. "How was it?"

"Couldn't have been better," declared Lieutenant Marbury. "Therewasn't the least shock of recoil. Tom Swift, you have solved theproblem, I do believe! Your aerial warship is a success!"

"I'm glad to hear you say so. There are one or two little things thatneed changing, but I really think I have about what the United StatesGovernment wants."

"I am, also, of that belief, Tom. If only—" The officer stoppedsuddenly.

"Well?" asked Tom suggestively.

"I was going to say if only those foreign spies don't make trouble."

"I think we've seen the last of them," Tom declared. "Now we'll go onwith the tests."

More guns were fired, singly and in batteries, and in each case theMars stood the test perfectly. The double barrel had solved the recoilproblem.

For some little time longer they remained out over the sea, goingthrough some evolutions to test the rudder control, and then as theirpresent object had been accomplished Tom gave orders to head back toShopton, which place was reached in due time.

"Well, Tom, how was it?" asked Mr. Swift, for though his son had saidnothing to his friends about the prospective test, the aged inventorknew about it.

"Successful, Dad, in every particular."

"That's good. I didn't think you could do it. But you did. I tell youit isn't much that can get the best of a Swift!" exclaimed the aged manproudly. "Oh, by the way, Tom, here's a telegram that came while youwere gone," and he handed his son the yellow envelope.

Tom ripped it open with a single gesture, and in a flash his eyes tookin the words. He read:

"Look out for spies during trial flights."

The message was signed with a name Tom did not recognize.

"Any bad news?" asked Mr. Swift.

"No—oh, no," replied Tom, as he crumpled up the paper and thrust itinto his pocket. "No bad news, Dad."

"Well, I'm glad to hear that," went on Mr. Swift. "I don't liketelegrams."

When Tom showed the message to Lieutenant Marbury, that official, afterone glance at the signature, said:

"Pierson, eh? Well, when he sends out a warning it generally meanssomething."

"Who's Pierson?" asked Tom.

"Head of the Secret Service department that has charge of this airshipmatter. There must be something in the wind, Tom."

Extra precautions were taken about the shops. Strangers were notpermitted to enter, and all future work on the Mars was kept secret.Nevertheless, Tom was worried. He did not want his work to be spoiledjust when it was about to be a success. For that it was a success,Lieutenant Marbury assured him. The government man said he would haveno hesitation in recommending the purchase of Tom's aerial warship.

"There's just one other test I want to see made," he said.

"What is that?" Tom inquired.

"In a storm. You know we can't always count on having good weather, andI'd like to see how she behaves in a gale."

"You shall!" declared the young inventor.

For the next week, during which finishing touches were put on the bigcraft, Tom anxiously waited for signs of a storm. At last they came.Danger signals were put up all along the coast, and warnings were sentout broadcast by the Weather Bureau at Washington.

One dull gray morning Tom roused his friends early and announced thatthe Mars was going up.

"A big storm is headed this way," Tom said, "and we'll have a chance tosee how she behaves in it."

And even as the flight began, the forerunning wind and rain came in agust of fury. Into the midst of it shot the big aerial warship, withher powerful propellers beating the moisture-laden air.

CHAPTER XIX

QUEER HAPPENINGS

"Say, Tom, are you sure you're all right?"

"Of course I am! What do you mean?"

It was Ned Newton who asked the question, and Tom Swift who answeredit. The chums were in the pilot-house of the dipping, swaying Mars,which was nosing her way into the storm, fighting on an upward slant,trying, if possible, to get above the area of atmospheric disturbance.

"Well, I mean are you sure your craft will stand all this straining,pulling and hauling?" went on Ned, as he clung to a brass hand rail,built in the side of the pilot-house wall for the very purpose to whichit was now being put.

"If she doesn't stand it she's no good!" cried Tom, as he clung to thesteering wheel, which was nearly torn from his hands by the deflectionsof the rudders.

"Well, it's taking a big chance, it seems to me," went on Ned, as hepeered through the rain-spotted bull's-eyes of the pilot-house.

"There's no danger," declared Tom. "I wanted to give the ship thehardest test possible before I formally offered her to the government.If she can't stand a blow like this she isn't what I thought her, andI'll have to build another. But I'm sure she will stand the racket,Ned. She's built strongly, and even if part of the gas bag is carriedaway, as it was when our propeller shattered, we can still sail. If youthink this is anything, wait until we turn about and begin to fight ourway against the wind."

"Are you going to do that, Tom?"

"I certainly am. We're going with the gale now, to see what is thehighest rate of speed we can attain. Pretty soon I'm going to turn heraround, and see if she can make any headway in the other direction. Ofcourse I know she won't make much, if any speed, against the gale; butI must give her that test."

"Well, Tom, you know best, of course," admitted Ned. "But to me itseems like taking a big risk."

And indeed it did seem, not only to Ned, but to some of the experiencedmen of Tom's crew, that the young inventor was taking more chances thanever before, and Tom, as my old readers well know, had, in his career,taken some big ones.

The storm grew worse as the day progressed, until it was a veritablehurricane of wind and rain. The warnings of the Weather Bureau had notbeen exaggerated. But through the fierce blow the Mars fought her way.As Tom had said, she was going with the wind. This was comparativelyeasy. But what would happen when she headed into the storm?

Mr. Damon, in the main cabin, sat and looked at Lieutenant Marbury, theeccentric man now and then blessing something as he happened to thinkof it.

"Do you—do you think we are in any danger?" he finally asked.

"Not at present," replied the government expert.

"You mean we will be—later?"

"It's hard to say. I guess Tom Swift knows his business, though."

"Bless my accident insurance policy!" murmured Mr. Damon. "I wish I hadstayed home. If my wife ever hears of this—" He did not seem able tofinish the sentence.

In the engine-room the crew were busy over the various machines. Someof the apparatus was being strained to keep the ship on her course inthe powerful wind, and would be under a worse stress when Tom turnedhis craft about. But, so far, nothing had given way, and everything wasworking smoothly.

As hour succeeded hour and nothing happened, the timid ones aboardbegan to take more courage. Tom never for a moment lost heart. He knewwhat his craft could do, and he had taken her up in a terrific stormwith a definite purpose in view. He was the calmest person aboard, withthe exception, perhaps, of Koku. The giant did not seem to know whatfear was. He depended entirely on Tom, and as long as his young masterhad charge of matters the giant was content to obey orders.

There was to be no test of the guns this time. They had workedsufficiently well, and, if need be, could have been fired in the gale.But Tom did not want his men to take unnecessary risks, nor was hefoolhardy himself.

"We'll have our hands full when we turn around and head into the wind,"he said to his chum. "That will be enough."

"Then you're really going to give the Mars that test?"

"I surely am. I don't want any comebacks from Uncle Sam after heaccepts my aerial warship. I've guaranteed that she'll stand up andmake headway against a gale, and I'm going to prove it."

Lieutenant Marbury was told of the coming trial, and he prepared totake official note of it. While matters were being gotten in readinessTom turned the wheel over to his assistant pilot and went to theengine-room to see that everything was in good shape to cope with anyemergency. The rudders had been carefully examined before the flightwas made, to make sure they would not fail, for on them depended theprogress of the ship against the powerful wind.

"I rather guess those foreign spies have given up trying to do Tom aninjury," remarked Ned to the lieutenant as they sat in the main cabin,listening to the howl of the wind, and the dash of the rain.

"Well, I certainly hope so," was the answer. "But I wouldn't be toosure. The folks in Washington evidently think something is likely tohappen, or they wouldn't have sent that warning telegram."

"But we haven't seen anything of the spies," Ned remarked.

"No, but that isn't any sign they are not getting ready to maketrouble. This may be the calm before the storm. Tom must still be onthe lookout. It isn't as though his inventions alone were in danger,for they would not hesitate to inflict serious personal injury if theirplans were thwarted."

"They must be desperate."

"They are. But here comes Tom now. He looks as though something new wasabout to happen."

"Take care of yourselves now," advised the young aero-inventor, as heentered the cabin, finding it hard work to close the door against theterrific wind pressure.

"Why?" asked Ned.

"Because we are going to turn around and fight our way back against thegale. We may be turned topsy-turvy for a second or two."

"Bless my shoe-horn!" cried Mr. Damon. "Do you mean upside down, Tom?"

"No, not that exactly. But watch out!"

Tom went forward to the pilot-house, followed by Ned and thelieutenant. The latter wanted to take official note of what happened.Tom relieved the man at the wheel, and gradually began to alter thedirection of the craft.

At first no change was noticeable. So strong was the force of the windthat it seemed as though the Mars was going in the same direction. ButNed, noticing a direction compass on the wall, saw that the needle wasgradually shifting.

"Hold fast!" cried Tom suddenly. Then with a quick shift of the ruddersomething happened. It seemed as though the Mars was trying to turnover, and slide along on her side, or as if she wanted to turn aboutand scud before the gale, instead of facing it. But Tom held her to thereverse course.

"Can you get her around?" cried the lieutenant above the roar of thegale.

"I—I'm going to!" muttered Tom through his set teeth.

Inch by inch he fought the big craft through the storm. Inch by inchthe indicator showed the turning, until at last the grip of the galewas overcome.

"Now she's headed right into it!" cried Tom in exultation. "She'snosing right into it!"

And the Mars was. There was no doubt of it. She had succeeded, underTom's direction, in changing squarely about, and was now going againstthe wind, instead of with it.

"But we can't expect to make much speed," Tom said, as he signaled formore power, for he had lowered it somewhat in making the turn.

But Tom himself scarcely had reckoned on the force of his craft, for asthe propellers whirled more rapidly the aerial warship did begin tomake headway, and that in the teeth of a terrific wind.

"She's doing it, Tom! She's doing it!" cried Ned exultingly.

"I believe she is," agreed the lieutenant.

"Well, so much the better," Tom said, trying to be calm. "If she cankeep this up a little while I'll give her a rest and we'll go up abovethe storm area, and beat back home."

The Mars, so far, had met every test. Tom had decided on ten minutesmore of gale-fighting, when from the tube that communicated with theengine-room came a shrill whistle.

"See what that is, Ned," Tom directed.

"Yes," called Ned into the mouthpiece. "What's the matter?"

"Short circuit in the big motor," was the reply. "We've got to run onstorage battery. Send Tom back here! Something queer has happened!"

CHAPTER XX

THE STOWAWAYS

Ned repeated the message breathlessly.

"Short circuit!" gasped Tom. "Run on storage battery! I'll have to seeto that. Take the wheel somebody!"

"Wouldn't it be better to turn about, and run before the wind, so asnot to put too great a strain on the machinery?" asked LieutenantMarbury.

"Perhaps," agreed Tom. "Hold her this way, though, until I see what'swrong!"

Ned and the government man took the wheel, while Tom hurried along therunway leading from the pilot-house to the machinery cabin. The galewas still blowing fiercely.

The young inventor cast a hasty look about the interior of the place ashe entered. He sniffed the air suspiciously, and was aware of the odorof burning insulation.

"What happened?" he asked, noting that already the principal motivepower was coming from the big storage battery. The shift had been madeautomatically, when the main motor gave out.

"It's hard to say," was the answer of the chief engineer. "We wererunning along all right, and we got your word to switch on more power,after the turn. We did that all right, and she was running as smooth asa sewing-machine, when, all of a sudden, she short-circuited, and thestorage battery cut in automatically."

"Think you put too heavy a load on the motor?" Tom asked.

"Couldn't have been that. The shunt box would have taken that up, andthe circuit-breaker would have worked, saving us a burn-out, and that'swhat happened—a burn-out. The motor will have to be rewound."

"Well, no use trying to fight this gale with the storage battery," Tomsaid, after a moment's thought. "We'll run before it. That's theeasiest way. Then we'll try to rise above the wind."

He sent the necessary message to the pilot-house. A moment later theshift was made, and once more the Mars was scudding before the storm.Then Tom gave his serious attention to what had happened in the engineroom.

As he bent over the burned-out motor, looking at the big shinyconnections, he saw something that startled him. With a quick motionTom Swift picked up a bar of copper. It was hot to the touch—so hotthat he dropped it with a cry of pain, though he had let go so quicklythat the burn was only momentary.

"What's the matter?" asked Jerry Mound, Tom's engineer.

"Matter!" cried Tom. "A whole lot is the matter! That copper bar iswhat made the short circuit. It's hot yet from the electric current.How did it fall on the motor connections?"

The engine room force gathered about the young inventor. No one couldexplain how the copper bar came to be where it was. Certainly no oneof Tom's employees had put it there, and it could not have fallen byaccident, for the motor connections were protected by a mesh of wire,and a hand would have to be thrust under them to put the bar in place.Tom gave a quick look at his men. He knew he could trust them—everyone. But this was a queer happening.

For a moment Tom did not know what to think, and then, as the memory ofthat warning telegram came to him, he had an idea.

"Were any strangers in this cabin before the start was made?" he askedMr. Mound.

"Not that I know of," was the answer.

"Well, there may be some here now," Tom said grimly. "Look about."

But a careful search revealed no one. Yet the young inventor was surethe bar of copper, which had done the mischief of short-circuiting themotor, had been put in place deliberately.

In reality there was no danger to the craft, since there was powerenough in the storage battery to run it for several hours. But thehappening showed Tom he had still to reckon with his enemies.

He looked at the height gauge on the wall of the motor-room, and notedthat the Mars was going up. In accordance with Tom's instructions theywere sending her above the storm area. Once there, with no gale tofight, they could easily beat their way back to a point above Shopton,and make the best descent possible.

And that was done while, under Tom's direction, his men took thedamaged motor apart, with a view to repairing it.

"What was it, Tom?" asked Ned, coming back to join his chum, afterGeorge Ventor, the assistant pilot, had taken charge of the wheel.

"I don't exactly know, Ned," was the answer. "But I feel certain thatsome of my enemies came aboard here and worked this mischief."

"Your enemies came aboard?"

"Yes, and they must be here now. The placing of that copper bar provesit."

"Then let's make a search and find them, Tom. It must be some of thoseforeign spies."

"Just what I think."

But a more careful search of the craft than the one Tom had casuallymade revealed the presence of no one. All the crew and helpers wereaccounted for, and, as they had been in Tom's service for some time,they were beyond suspicion. Yet the fact remained that a seeminglyhuman agency had acted to put the main motor out of commission. Tomcould not understand it.

"Well, it sure is queer," observed Ned, as the search came to nothing.

"It's worse than queer," declared Tom, "it's alarming! I don't knowwhen I'll be safe if we have ghosts aboard."

"Ghosts?" repeated Ned.

"Well, when we can't find out who put that bar in place I might as welladmit it was a ghost," spoke Tom. "Certainly, if it was done by a man,he didn't jump overboard after doing it, and he isn't here now. It sureis queer!"

Ned agreed with the last statement, at any rate.

In due time the Mars, having fought her way above the storm, came overShopton, and then, the wind having somewhat died out, she fought herway down, and, after no little trouble, was housed in the hangar.

Tom cautioned his friends and workmen to say nothing to his fatherabout the mysterious happening on board.

"I'll just tell him we had a slight accident, and let it go at that,"Tom decided. "No use in causing him worry."

"But what are you going to do about it?" asked Ned.

"I'm going to keep careful watch over the aerial warship, at any rate,"declared Tom. "If there's a hidden enemy aboard, I'll starve him out."

Accordingly, a guard, under the direction of Koku, was posted about thebig shed, but nothing came of it. No stranger was observed to sneak outof the ship, after it had been deserted by the crew. The mystery seemeddeeper than ever.

It took nearly a week to repair the big motor, and, during this time,Tom put some improvements on the airship, and added the finishingtouches.

He was getting it ready for the final government test, for theauthorities in Washington had sent word that they would have CaptainWarner, in addition to Lieutenant Marbury, make the final inspectionand write a report.

Meanwhile several little things occurred to annoy Tom. He was besiegedwith applications from new men who wanted to work, and many of thesemen seemed to be foreigners. Tom was sure they were either spies ofsome European nations, or the agents of spies, and they got no furtherthan the outer gate.

But some strangers did manage to sneak into the works, though they werequickly detected and sent about their business. Also, once or twice,small fires were discovered in outbuildings, but they were soonextinguished with little damage. Extra vigilance was the watchword.

"And yet, with all my precautions, they may get me, or damagesomething," declared Tom. "It is very annoying!"

"It is," agreed Ned, "and we must be doubly on the lookout."

So impressed was Ned with the necessity for caution that he arranged totake his vacation at this time, so as to be on hand to help his chum,if necessary.

The Mars was nearing completion. The repaired motor was better thanever, and everything was in shape for the final test. Mr. Damon waspersuaded to go along, and Koku was to be taken, as well as the twogovernment officials.

The night before the trip the guards about the airship shed weredoubled, and Tom made two visits to the place before midnight. Butthere was no alarm.

Consequently, when the Mars started off on her final test, it wasthought that all danger from the spies was over.

"She certainly is a beauty," said Captain Warner, as the big craft shotupward. "I shall be interested in seeing how she stands gun fire,though."

"Oh, she'll stand it," declared Lieutenant Marbury. The trip was toconsume several days of continuous flying, to test the engines. A largesupply of food and ammunition was aboard.

It was after supper of the first day out, and our friends were seatedin the main cabin laying out a program for the next day, when suddenyells came from a part of the motor cabin devoted to storage. Koku, whohad been sent to get out a barrel of oil, was heard to shout.

"What's up?" asked Tom, starting to his feet. He was answered almost atonce by more yells.

"Oh, Master! Come quickly!" cried the giant. "There are many men here.There are stowaways aboard!"

CHAPTER XXI

PRISONERS

For a moment, after hearing Koku's reply, neither Tom nor his friendsspoke. Then Ned, in a dazed sort of way, repeated:

"Stowaways!"

"Bless my—" began Mr. Damon, but that was as far as he got.

From the engine compartment, back of the amidship cabin, came a soundof cries and heavy blows. The yells of Koku could be heard above thoseof the others.

Then the door of the cabin where Tom Swift and his friends were wassuddenly burst open, and seven or eight men threw themselves within.They were led by a man with a small, dark mustache and a little tuft ofwhiskers on his chin—an imperial. He looked the typical Frenchman, andhis words, snapped out, bore out that belief.

What he said was in French, as Tom understood, though he knew little ofthat language. Also, what the Frenchman said produced an immediateresult, for the men following him sprang at our friends withoverwhelming fierceness.

Before Tom, Ned, Captain Warner, Mr. Damon or Lieutenant Marbury couldgrasp any weapon with which to defend themselves, had their intentionsbeen to do so, they were seized.

Against such odds little could be done, though our friends did not giveup without a struggle.

"What does this mean?" angrily demanded Tom Swift. "Who are you? Whatare you doing aboard my craft? Who are—"

His words were lost in smothered tones, for one of his assailants put aheavy cloth over his mouth, and tied it there, gagging him. Anotherman, with a quick motion, whipped a rope about Tom's hands and feet,and he was soon securely bound.

In like manner the others were treated, and, despite the struggles ofMr. Damon, the two government men and Ned, they were soon put in aposition where they could do nothing—helplessly bound, and laid on abench in the main cabin, staring blankly up at the ceiling. Each onewas gagged so effectively that he could not utter more than a faintmoan.

Of the riot of thoughts that ran through the heads of each one, I leaveyou to imagine.

What did it all mean? Where had the strange men come from? What didthey mean by thus assaulting Tom and his companions? And what hadhappened to the others of the crew—Koku, Jerry Mound, the engineer,and George Ventor, the assistant pilot?

These were only a few of the questions Tom asked himself, as he laythere, bound and helpless. Doubtless Mr. Damon and the others wereasking themselves similar questions.

One thing was certain—whatever the stowaways, as Koku had called them,had done, they had not neglected the Mars, for she was running along atabout the same speed, though in what direction Tom could not tell. Hestrained to get a view of the compass on the forward wall of the cabin,but he could not see it.

It had been a rough-and-tumble fight, by which our friends were madeprisoners, but no one seemed to have been seriously, or even slightly,hurt. The invaders, under the leadership of the Frenchman, were ratherruffled, but that was all.

Pantingly they stood in line, surveying their captives, while the manwith the mustache and imperial smiled in a rather superior fashion atthe row of bound ones. He spoke in his own tongue to the men, who, withthe exception of one, filed out, going, as Tom and the others couldnote, to the engine-room in the rear.

"I hope I have not had to hurt any of you," the Frenchman observed,with sarcastic politeness. "I regret the necessity that caused me to dothis, but, believe me, it was unavoidable."

He spoke with some accent, and Tom at once decided this was the sameman who had once approached Eradicate. He also recognized him as theman he had seen in the woods the day of the outing.

"He's one of the foreign spies," thought Tom "and he's got us and theship, too. They were too many for us!"

Tom's anxiety to speak, to hold some converse with the captor, was soobvious that the Frenchman said:

"I am going to treat you as well as I can under the circumstances. Youand your other friends, who are also made prisoners, will be allowed tobe together, and then you can talk to your hearts' content."

The other man, who had remained with the evident ringleader of thestowaways, asked a question, in French, and he used the name La Foy.

"Ah!" thought Tom. "This is the leader of the gang that attacked Kokuin the shop that night. They have been waiting their chance, and nowthey have made good. But where did they come from? Could they haveboarded us from some other airship?"

Yet, as Tom asked himself that question, he knew it could hardly havebeen possible. The men must have been in hiding on his own craft, theymust have been, as Koku had cried out—stowaways—and have come out ata preconcerted signal to overpower the aviators.

"If you will but have patience a little longer," went on La Foy, forthat was evidently the name of the leader, "you will all be together.We are just considering where best to put you so that you will notsuffer too much. It is quite a problem to deal with so many prisoners,but we have no choice."

The two Frenchmen conversed rapidly in their own language for a fewminutes, and then there came into the cabin another of the men who hadhelped overpower Tom and his friends. What he told La Foy seemed togive that individual satisfaction, for he smiled.

"We are going to put you all together in the largest storeroom, whichis partly empty," La Foy said. "There you will be given food and drink,and treated as well as possible under the circumstances. You will alsobe unbound, and may converse among yourselves. I need hardly pointout," he went on, "that calling for help will be useless. We are a mileor so in the air, and have no intention of descending," and he smiledmockingly.

"They must know how to navigate my aerial warship," thought Tom. "Iwonder what their game is, anyhow?"

Night had fallen, but the cabin was aglow with electric lights. Theforeigners in charge of the Mars seemed to know their way aboutperfectly, and how to manage the big craft. By the vibration Tom couldtell that the motor was running evenly and well.

"But what happened to the others—to Mound, Ventor and Koku?" wonderedTom.

A moment later several of the foreigners entered. Some of them did notlook at all like Frenchmen, and Tom was sure one was a German andanother a Russian.

"This will be your prison—for a while," said La Foy significantly, andTom wondered how long this would be the case. A sharp thought came tohim—how long would they be prisoners? Did not some other, and moreterrible, fate await them?

As La Foy spoke, he opened a storeroom door that led off from the main,or amidship, cabin. This room was intended to contain the supplies andstores that would be taken on a long voyage. It was one of two, beingthe larger, and now contained only a few odds and ends of littleimportance. It made a strong prison, as Tom well knew, having plannedit.

One by one, beginning with Tom, the prisoners were taken up and placedin a recumbent position on the floor of the storeroom. Then werebrought in the engineer and assistant pilot, as well as Koku and amachinist whom Tom had brought along to help him. Now the younginventor and all his friends were together. It took four men to carryKoku in, the giant being covered with a network of ropes.

"On second thought," said La Foy, as he saw Koku being placed with hisfriends, "I think we will keep the big man with us. We had troubleenough to subdue him. Carry him back to the engine-room."

So Koku, trussed up like some roped steer, was taken out again.

"Now then," said La Foy to his prisoners, as he stood in the door ofthe room, "I will unbind one of you, and he may loose the bonds of theothers."

As he spoke, he took the rope from Tom's hands, and then, quicklyslipping out, locked and barred the door.

CHAPTER XXII

APPREHENSIONS

For a moment or two, after the ropes binding his hands were loosed, TomSwift did nothing. He was not only stunned mentally, but the bonds hadbeen pulled so tightly about his wrists that the circulation wasimpeded, and his cramped muscles required a little time in which torespond.

But presently he felt the tingle of the coursing blood, and he found hecould move his arms. He raised them to his head, and then his firstcare was to remove the pad of cloth that formed a gag over his mouth.Now he could talk.

"I—I'll loosen you all in just a second," he said, as he bent over topick at the knot of the rope around his legs. His own voice soundedstrange to him.

"I don't know what it's all about, any more than you do," he went on,speaking to the others. "It's a fierce game we're up against, and we'vegot to make the best of it. As soon as we can move, and talk, we'lldecide what's best to do. Whoever these fellows are, and I believe theyare the foreign spies I've been warned about, they are in completepossession of the airship."

Tom found it no easy matter to loosen the bonds on his feet. The ropeswere well tied, and Tom's fingers were stiff from the lack ofcirculation of blood. But finally he managed to free himself. When hestood up in the dim storeroom, that was now a prison for all save Koku,he found that he could not walk. He almost toppled over, so weak werehis legs from the tightness of the ropes. He sat down and worked hismuscles until they felt normal again.

A few minutes later, weak and rather tottery, he managed to reach Mr.Damon, whom he first unbound. He realized that Mr. Damon was the oldestof his friends, and, consequently, would suffer most. And it wascharacteristic of the eccentric gentleman that, as soon as his gag wasremoved he burst out with:

"Bless my wristlets, Tom! What does it all mean?"

"That's more than I can say, Mr. Damon," replied Tom, with a mournfulshake of his head. "I'm very sorry it happened, for it looks as thoughI hadn't taken proper care. The idea of those men stowing themselvesaway on board here, and me not knowing it; and then coming outunexpectedly and getting possession of the craft! It doesn't speakvery well for my smartness."

"Oh, well, Tom, anyone might have been fooled by those plottingforeigners," said Mr. Damon. "Now, we'll try to turn matters about andget the best of them. Oh, but it feels good to be free once more!"

He stretched his benumbed and stiffened limbs and then helped Tom freethe others. They stood up, looking at each other in their dimly lightedprison.

"Well, if this isn't the limit I don't know what is!" cried Ned Newton.

"They got the best of you, Tom," spoke Lieutenant Marbury.

"Are they really foreign spies?" asked Captain Warner.

"Yes," replied his assistant. "They managed to carry out the plot wetried to frustrate. It was a good trick, too, hiding on board, andcoming out with a rush."

"Is that what they did?" asked Mr. Damon.

"It looks so," observed Tom. "The attack must have started in theengine-room," he went on, with a look at Mound and Ventor. "Whathappened there?" he asked.

"Well, that's about the way it was," answered the engineer. "We wereworking away, making some adjustments, oiling the parts and seeing thateverything was running smoothly, when, all at once, I heard Koku yell.He had gone in the oil room. At first I thought something had gonewrong with the ship, but, when I looked at the giant, I saw he wasbeing attacked by four strange men. And, before I, or any of the othermen, could do anything, they all swarmed down on us.

"There must have been a dozen of them, and they simply overwhelmed us.One of them hit Koku on the head with an iron bar, and that took allthe fight out of the giant, or the story might have been a differentone. As it was, we were overpowered, and that's all I know until wewere carried in here, and saw you folks all tied up as we were."

"They burst in on us in the same way," Tom explained. "But where didthey come from? Where were they hiding?"

"In the oil and gasoline storeroom that opens out of the motorcompartment," answered Mound, the engineer. "It isn't half full, youknow, and there's room for more than a dozen men in it. They must havegone in some time last night, when the airship was in the hangar, andremained hidden among the boxes and barrels until they got ready tocome out and overpower us."

"That's it," decided Tom. "But I don't understand how they got in. Thehangar was well guarded all night."

"Some of your men might have been bribed," suggested Ned.

"Yes, that is so," admitted Tom, and, later, he learned that such hadbeen the case. The foreign spies, for such they were, had managed tocorrupt one of Tom's trusted employees, who had looked the other waywhen La Foy and his fellow-conspirators sneaked into the airship shedand secreted themselves.

"Well, discussing how they got on board isn't going to do us any goodnow," Tom remarked ruefully. "The question is—what are we going to do?"

"Bless my fountain pen!" cried Mr. Damon. "There's only one thing todo!"

"What is that?" asked Ned.

"Why, get out of here, call a policeman, and have these scoundrelsarrested. I'll prosecute them! I'll have my lawyer on hand to see thatthey get the longest terms the statutes call for! Bless my pocketbook,but I will!" and Mr. Damon waxed quite indignant.

"That's easier said than done," observed Tom Swift, quietly. "In thefirst place, it isn't going to be an easy matter to get out of here."

He looked around the storeroom, which was then their prison. It wasilluminated by a single electric light, which showed some boxes andbarrels piled in the rear.

"Nothing in them to help us get out," Tom went on, for he knew what thecontents were.

"Oh, we'll get out," declared Ned confidently, "but I don't believewe'll find a policeman ready to take our complaint. The upper air isn'tvery well patrolled as yet."

"That's so," agreed Mr. Damon. "I forgot that we were in an airship.But what is to be done, Tom? We really are captives aboard our owncraft."

"Yes, worse luck," returned the young inventor. "I feel foolish when Ithink how we let them take us prisoners."

"We couldn't help it," Ned commented. "They came on us too suddenly. Wedidn't have a chance. And they outnumbered us two to one. If they couldtake care of big Koku, what chance did we have?"

"Very little," said Engineer Mound. "They were desperate fellows. Theyknow something about aircraft, too. For, as soon as Koku, Ventor and Iwere disposed of, some of them went at the machinery as if they hadbeen used to running it all their lives."

"Oh, the foreigners are experts when it comes to craft of the air,"said Captain Warner.

"Well, they seem to be running her, all right," admitted the younginventor, "and at good speed, too. They have increased our runningrate, if I am any judge."

"By several miles an hour," confirmed the assistant pilot. "Though inwhich direction they are heading, and what they are going to do with usis more than I can guess."

"That's so!" agreed Mr. Damon. "What is to become of us? They may heaveus overboard into the ocean!"

"Into the ocean!" cried Ned apprehensively. "Are we near the sea?"

"We must be, by this time," spoke Tom. "We were headed in thatdirection, and we have come almost far enough to put us somewhere overthe Atlantic, off the Jersey coast."

A look of apprehension was on the faces of all. But Tom's face did notremain clouded long.

"We won't try to swim until we have to," he said. "Now, let's take anaccount of stock, and see if we have any means of getting out of thisprison."

CHAPTER XXIII

ACROSS THE SEA

With one accord the hands of the captives sought their pockets.Probably the first thought of each one was a knife—a pocket knife. Butblank looks succeeded their first hopeful ones, for the hands came outempty.

"Not a thing!" exclaimed Mr. Damon. "Not a blessed thing! They haveeven taken my keys and—my fountain pen!"

"I guess they searched us all while they were struggling with us, tyingus up," suggested Ned. "I had a knife with a big, strong blade, butit's gone."

"So is mine," echoed Tom.

"And I haven't even a screwdriver, or a pocket-wrench," declared theengineer, "though I had both."

"They evidently knew what they were doing," said Lieutenant Marbury. "Idon't usually carry a revolver, but of late I have had a smallautomatic in my pocket. That's gone, too."

"And so are all my things," went on his naval friend. "That Frenchman,La Foy, was taking no chances."

"Well, if we haven't any weapons, or means of getting out of here, wemust make them," said Tom, as hopefully as he could under thecircumstances. "I don't know all the things that were put in thisstoreroom, and perhaps there may be something we can use."

"Shall we make the try now?" asked Ned. "I'm getting thirsty, at least.Lucky we had supper before they came out at us."

"Well, there isn't any water in here, or anything to eat, of so much Iam sure," went on Tom "So we will have to depend on our captors forthat."

"At least we can shout and ask for water," said Lieutenant Marbury."They have no excuse for being needlessly cruel."

They all agreed that this might not be a bad plan, and were preparingto raise a united shout, when there came a knock on the door of theirprison.

"Are you willing to listen to reason?" asked a voice they recognized asthat of La Foy.

"What do you mean by reason?" asked Tom bitterly. "You have no right toimpose any conditions on us."

"I have the right of might, and I intend exercising it," was the sharprejoinder. "If you will listen to reason—"

"Which kind—yours or ours?" asked Tom pointedly.

"Mine, in this case," snapped back the Frenchman. "What I was going tosay was that I do not intend to starve you, or cause you discomfort bythirst. I am going to open the door and put in food and water. But Iwarn you that any attempt to escape will be met with severe measures.

"We are in sufficient force to cope with you. I think you have seenthat." He spoke calmly and in perfect English, though with a markedaccent. "My men are armed, and will stand here ready to meet violencewith violence," he went on. "Is that understood?"

For a moment none of the captives replied.

"I think it will be better to give in to him at least for a while,"said Captain Warner in a low voice to Tom. "We need water, and willsoon need food. We can think and plan better if we are well nourished."

"Then you think I should promise not to raise a row?"

"For the time being—yes."

"Well, I am waiting!" came in sharp tones from the other side of theportal.

"Our answer is—yes," spoke Tom. "We will not try to get out—justyet," he added significantly.

A key was heard grating in the lock, and, a moment later, the door slidback. Through the opening could be seen La Foy and some of his menstanding armed. Others had packages of food and jugs of water. Aplentiful supply of the latter was carried aboard the Mars.

"Keep back from the door!" was the stern command of La Foy. "The foodand drink will be passed in only if you keep away from the entrance.Remember my men are armed!"

The warning was hardly needed, for the weapons could plainly be seen.Tom had half a notion that perhaps a concerted rush would carry the dayfor him and his friends, but he was forced to abandon that idea.

While the guards looked on, others of the "pirate crew," as Ned dubbedthem, passed in food and water. Then the door was locked again.

They all felt better after drinking the water, which was made cool byevaporation, for the airship was quite high above the earth when Tom'senemies captured it, and the young inventor felt sure it had notdescended any.

No one felt much like eating, however, so the food was put away for atime. And then, somewhat refreshed, they began looking about for somemeans of getting out of their prison.

"Of course we might batter down the door, in time, by using some ofthese boxes as rams," said Tom. "But the trouble is, that would make anoise, and they could stand outside and drive us back with guns andpistols, of which they seem to have plenty."

"Yes, and they could turn some of your own quick-firers on us," addedCaptain Warner. "No, we must work quietly, I think, and take themunawares, as they took us. That is our only plan."

"We will be better able to see what we have here by daylight," Tomsaid. "Suppose we wait until morning?"

That plan was deemed best, and preparations made for spending the nightin their prison.

It was a most uncomfortable night for all of them. The floor was theironly bed, and their only covering some empty bags that had containedsupplies. But even under these circumstances they managed to doze offfitfully.

Once they were all awakened by a violent plunging of the airship. Thecraft seemed to be trying to stand on her head, and then she rockedviolently from side to side, nearly turning turtle. "What is it?"gasped Ned, who was lying next to Tom.

"They must be trying some violent stunts," replied the young inventor,"or else we have run into a storm."

"I think the latter is the case," observed Lieutenant Marbury.

And, as the motion of the craft kept up, though less violently, thiswas accepted as the explanation. Through the night the Mars flew, butwhither the captives knew not.

The first gray streaks of dawn finally shone through the only window oftheir prison. Sore, lame and stiff, wearied in body and disturbed inmind, the captives awoke. Tom's first move was toward the window. Itwas high up, but, by standing on a box, he could look through it. Heuttered an exclamation.

"What is it?" asked Ned, swaying to and fro from the violent motion ofthe aerial warship.

"We are away out over the sea," spoke Tom, "and in the midst of a badstorm."

CHAPTER XXIV

THE LIGHTNING BOLT

Tom turned away from the window, to find his companions regarding himanxiously.

"A storm," repeated Ned. "What sort?"

"It might turn into any sort," replied Tom. "All I can see now is a lotof black clouds, and the wind must be blowing pretty hard, for there'squite a sea on."

"Bless my galvanometer!" cried Mr. Damon. "Then we are out over theocean again, Tom?"

"Yes, there's no doubt of it."

"What part?" asked the assistant pilot.

"That's more than I can tell," Tom answered.

"Suppose I take a look?" suggested Captain Warner. "I've done quite abit of sailing in my time."

But, when he had taken a look through the window at which Tom had beenstanding, the naval officer descended, shaking his head.

"There isn't a landmark in sight," he announced. "We might be over themiddle of the Atlantic, for all I could tell."

"Hardly as far as that," spoke Tom. "They haven't been pushing the Marsat that speed. But we may be across to the other side before we realizeit."

"How's that?" asked Ned.

"Well, the ship is in the possession of these foreign spies," went onTom. "All their interests are in Europe, though it would be hard to saywhat nationality is in command here. I think there are even someEnglishmen among those who attacked us, as well as French, Germans,Italians and Russians."

"Yes, it seems to be a combination of European nations against us,"admitted Captain Warner. "Probably, after they have made good theirseizure of Tom's aerial warship, they will portion her out amongthemselves, or use her as a model from which to make others."

"Do you think that is their object?" asked Mr. Damon.

"Undoubtedly," was the captain's answer. "It has been the object ofthese foreign spies, all along, not only to prevent the United Statesfrom enjoying the benefits of these progressive inventions, but to usethem for themselves. They would stop at nothing to gain their ends. Itseems we did not sufficiently appreciate their power and daring."

"Well, they've got us, at any rate," observed Tom, "and they may takeus and the ship to some far-off foreign country."

"If they don't heave us overboard half-way there," commented Ned, inrather gloomy tones.

"Well, of course, there's that possibility," admitted Tom. "They aredesperate characters."

"Well, we must do something," declared Lieutenant Marbury. "Come, it'sdaylight now, and we can see to work better. Let's see if we can't finda way to get out of this prison. Say, but this sure is a storm!" hecried, as the airship rolled and pitched violently.

"They are handling her well, though," observed Tom, as the craft camequickly to an even keel. "Either they have a number of expert birdmenon board, or they can easily adapt themselves to a new aircraft. She issailing splendidly."

"Well, let's eat something, and set to work," proposed Ned.

They brought out the food which had been given to them the nightbefore, but before they could eat this, there came a knock on the door,and more food and fresh water was handed in, under the same precautionsas before.

Tom and his companions indignantly demanded to be released, but theirprotests were only laughed at, and while the guards stood with readyweapons the door was again shut and locked.

But the prisoners were not the kind to sit idly down in the face ofthis. Under Tom's direction they set about looking through their placeof captivity for something by which they could release themselves. Atfirst they found nothing, and Ned even suggested trying to cut a waythrough the wooden walls with a fingernail file, which he found in oneof his pockets, when Tom, who had gone to the far end of the storeroom,uttered a cry.

"What is it—a way out?" asked Lieutenant Marbury anxiously.

"No, but means to that end," Tom replied. "Look, a file and a saw, lefthere by some of my workmen, perhaps," and he brought out the tools. Hehad found them behind a barrel in the far end of the compartment.

"Hurray!" cried Ned. "That's the ticket! Now we'll soon show thesefellows what's what!"

"Go easy!" cautioned Tom. "We must work carefully. It won't do to slamaround and try to break down the door with these. I think we had betterselect a place on the side wall, break through that, and make anopening where we can come out unnoticed. Then, when we are ready, wecan take them by surprise. We'll have to do something like that, forthey outnumber us, you know."

"That is so," agreed Captain Warner. "We must use strategy."

"Well, where would be a good place to begin to burrow out?" asked Ned.

"Here," said Tom, indicating a place far back in the room. "We can workthere in turns, sawing a hole through the wall. It will bring us out inthe passage between the aft and amidship cabins, and we can go eitherway."

"Then let's begin!" cried Ned enthusiastically, and they set to work.

While the aerial warship pitched and tossed in the storm, over somepart of the Atlantic, Tom and his friends took turns in working theirway to freedom. With the sharp end of the file a small hole was made,the work being done as slowly as a rat gnaws, so as to make no noisethat would be heard by their captors. In time the hole was large enoughto admit the end of the saw.

But this took many hours, and it was not until the second day of theircaptivity that they had the hole nearly large enough for the passage ofone person at a time. They had not been discovered, they thought.

Meanwhile they had been given food and water at intervals, but to alldemands that they be released, or at least told why they were heldprisoners, a deaf ear was turned.

They could only guess at the fate of Koku. Probably the giant was keptbound, for once he got the chance to use his enormous strength it mightgo hard with the foreigners.

The Mars continued to fly through the air. Sometimes, as Tom and hisfriends could tell by the motion, she was almost stationary in theupper regions, and again she seemed to be flying at top speed.Occasionally there came the sound of firing.

"They're trying my guns," observed Tom grimly.

"Do you suppose they are being attacked?" asked Ned, hopefully.

"Hardly," replied Captain Warner. "The United States possesses no craftable to cope with this one in aerial warfare, and they are hardlyengaging in part of the European war yet. I think they are just tryingTom's new guns."

Later our friends learned that such was the case.

The storm had either passed, or the Mars had run out of the path of it,for, after the first few hours of pitching and tossing, the atmosphereseemed reduced to a state of calm.

All the while they were secretly working to gain their freedom so theymight attack and overpower their enemies, they took occasionalobservations from the small window. But they could learn nothing oftheir whereabouts. They could only view the heaving ocean, far belowthem, or see a mass of cloud-mist, which hid the earth, if so be thatthe Mars was sailing over land.

"But how much longer can they keep it up?" asked Ned.

"Well, we have fuel and supplies aboard for nearly two weeks," Tomanswered.

"And by the end of that time we may all be dead," spoke the young bankclerk despondently.

"No, we'll be out of here before then!" declared Lieutenant Marbury.

Indeed the hole was now almost large enough to enable them to crawl outone at a time. They could not, of course, see how it looked from theoutside, but Tom had selected a place for its cutting so that thesawdust and the mark of the panel that was being removed, would notordinarily be noticeable.

They set night as the time for making the attempt—late at night, whenit was hoped that most of their captors would be asleep.

Finally the last cut was made, and a piece of wood hung over theopening only by a shred, all ready to knock out.

"We'll do it at midnight," announced Tom.

Anxious, indeed, were those last hours of waiting. The time had almostarrived for the attempt, when Tom, who had been nervously pacing to andfro, remarked:

"We must be running into another storm. Feel how she heaves and rolls!"

Indeed the Mars was most unsteady.

"It sure is a storm!" cried Ned, "and a heavy one, too," for there camea burst of thunder, that seemed like a report of Tom's giant cannon.

In another instant they were in the midst of a violent thunderstorm,the airship pitching and tossing in a manner to almost throw them fromtheir feet.

As Tom reached up to switch on the electric light again, there came aflash of lightning that well nigh blinded them. And so close after itas to seem simultaneous, there came such a crash of thunder as to stunthem all. There was a tingling, as of a thousand pins and needles inthe body of each of the captives, and a strong smell of sulphur. Then,as the echoes of the clap died away, Tom yelled:

"She's been struck! The airship has been struck!"

CHAPTER XXV

FREEDOM

For a moment there was silence, following Tom's wild cry and the noiseof the thunderclap. Then, as other, though less loud reverberations ofthe storm continued to sound, the captives awoke to a realization ofwhat had happened. They had been partially stunned, and were almost asin a dream.

"Are—are we all right?" stammered Ned.

"Bless my soul! What has happened?" cried Mr. Damon.

"We've been struck by lightning!" Tom repeated. "I don't know whetherwe're all right or not."

"We seem to be falling!" exclaimed Lieutenant Marbury.

"If the whole gas bag isn't ripped to pieces we're lucky," commentedJerry Mound.

Indeed, it was evident that the Mars was sinking rapidly. To all therecame the sensation of riding in an elevator in a skyscraper and beingdropped a score of stories.

Then, as they stood there in the darkness, illuminated only by flashesfrom the lightning outside the window, waiting for an unknown fate, TomSwift uttered a cry of delight.

"We've stopped falling!" he cried. "The automatic gas machine ispumping. Part of the gas bag was punctured, but the unbrokencompartments hold!"

"If part of the gas leaked out I don't see why it wasn't all set onfire and exploded," observed Captain Warner.

"It's a non-burnable gas," Tom quickly explained. "But come on. Thismay be our very chance. There seems to be something going on that maybe in our favor."

Indeed the captives could hear confused cries and the running to andfro of many feet.

He made for the sawed panel, and, in another instant, had burst out andwas through it, out into the passageway between the after and amidshipcabins. His companions followed him.

They looked into the rear cabin, or motor compartment, and a scene ofconfusion met their gaze. Two of the foreign men who had seized theship lay stretched out on the floor near the humming machinery, whichhad been left to run itself. A look in the other direction, toward themain cabin, showed a group of the foreign spies bending over the inertbody of La Foy, the Frenchman, stretched out on a couch.

"What has happened?" cried Ned. "What does it all mean?'

"The lightning!" exclaimed Tom. "The bolt that struck the ship hasknocked out some of our enemies! Now is the time to attack them!"

The Mars seemed to have passed completely through a narrow storm belt.She was now in a quiet atmosphere, though behind her could be seen thefitful play of lightning, and there could be heard the distant rumbleof thunder.

"Come on!" cried Tom. "We must act quickly, while they are demoralized!Come on!"

His friends needed no further urging. Jerry Mound and the machinistrushed to the engine-room, to look after any of the enemy that might bethere, while Tom, Ned and the others ran into the middle cabin.

"Grab 'em! Tie 'em up!" cried Tom, for they had no weapons with whichto make an attack.

But none were needed. So stunned were the foreigners by the lightningbolt, which had miraculously passed our friends, and so unnerved by thestriking down of La Foy, their leader, that they seemed like men halfasleep. Before they could offer any resistance they were bound with thesame ropes that had held our friends in bondage. That is, all but thebig Frenchman himself. He seemed beyond the need of binding.

Mound, the engineer, and his assistant, came hurrying in from themotor-room, followed by Koku.

"We found him chained up," Jerry explained, as the big giant, freedfrom his captivity, rubbed his chafed wrists.

"Are there any of the foreigners back there?'

"Only those two knocked out by the lightning," the engineer explained."We've made them secure. I see you've got things here in shape."

"Yes," replied Tom. "And now to see where we are, and to get back home.Whew! But this has been a time! Koku, what happened to you?"

"They no let anything happen. I be in chains all the while," the giantanswered. "Jump on me before I can do anything!"

"Well, you're out, now, and I think we'll have you stand guard overthese men. The tables are turned, Koku."

The bound ones were carried to the same prison whence our friends hadescaped, but their bonds were not taken off, and Koku was put in theplace with them. By this time La Foy and the two other stricken menshowed signs of returning life. They had only been stunned.

The young inventor and his friends, once more in possession of theirairship, lost little time in planning to return. They found that thespies were all expert aeronauts, and had kept a careful chart of theirlocation. They were then halfway across the Atlantic, and in a shorttime longer would probably have been in some foreign country. But Tomturned the Mars about.

The craft had only been slightly damaged by the lightning bolt, thoughthree of the gas bag compartments were torn, The others sufficed,however, to make the ship sufficiently buoyant.

When morning came Tom and his friends had matters running almost assmoothly as before their capture.

The prisoners had no chance to escape, and, indeed, they seemed to havebeen broken in spirit. La Foy was no longer the insolent, mockingFrenchman that he had been, and the two chief foreign engineers seemedto have lost some of their reason when the lightning struck them.

"But it was a mighty lucky and narrow escape for us," said Ned, as heand Tom sat in the pilot-house the second day of the return trip.

"That's right," agreed his chum.

Once again they were above the earth, and, desiring to get rid as soonas possible of the presence of the spies, a landing was made near NewYork City, and the government authorities communicated with. CaptainWarner and Lieutenant Marbury took charge of the prisoners, with someSecret Service men, and the foreigners were soon safely locked up.

"And now what are you going to do, Tom?" asked Ned, when, once more,they had the airship to themselves.

"I'm going back to Shopton, fix up the gas bag, and give her anothergovernment trial," was the answer.

And, in due time, this was done. Tom added some improvements to theaircraft, making it better than ever, and when she was given the testrequired by the government, she was an unqualified success, and therights to the Mars were purchased for a large sum. In sailing, and inthe matter of guns and bombs, Tom's craft answered every test.

"So you see I was right, after all, Dad," the young inventor said, wheninformed that he had succeeded. "We can shoot off even bigger guns thanI thought from the deck of the Mars."

"Yes, Tom," replied the aged inventor, "I admit I was wrong."

Tom's aerial warship was even a bigger success than he had dared tohope. Once the government men fully understood how to run it, in whichTom played a prominent part in giving instructions, they put the Marsto a severe test. She was taken out over the ocean, and her gunstrained on an obsolete battleship. Her bombs and projectiles blew thecraft to pieces.

"The Mars will be the naval terror of the seas in any future war,"predicted Captain Warner.

The Secret Service men succeeded in unearthing all the details of theplot against Tom. His life, at times, had been in danger, but at thelast minute the man detailed to harm him lost his nerve.

It was Tom's enemies who had set on fire the red shed, and who latertried to destroy the ship by putting a corrosive acid in one of thepropellers. That plot, though, was not wholly successful. Then came thetime when one of the spies hid on board, and dropped the copper bar onthe motor, short-circuiting it. But for the storage-battery that schememight have wrought fearful damage. The spy who had stowed himself awayon the craft escaped at night by the connivance of one of Tom's corruptemployees.

The foreign spies were tried and found guilty, receiving meritedpunishment. Of course the governments to which they belonged disclaimedany part in the seizure of Tom's aerial warship.

It came out at the trial that one of Tom's most trusted employees hadproved a traitor, and had the night before the test, allowed theforeign spies to secrete themselves on board, to rush out at anopportune time to overpower our hero and his friends. But luck was withTom at the end.

"Well, what are you going to tackle next, Tom?" asked Ned, one dayabout a month after these exciting experiences.

"I don't know," was the slow answer. "I think a self-swinging hammock,under an apple tree, with a never-emptying pitcher of ice-cold lemonadewould be about the thing."

"Good, Tom! And, if you'll invent that, I'll share it with you."

"Well, come on, let's begin now," laughed Tom. "I need a vacation,anyhow."

But it is very much to be doubted if Tom Swift, even on a vacation,could refrain from trying to invent something, either in the line ofairships, water, or land craft. And so, until he again comes to thefront with something new, we will take leave of him.

THE TOM SWIFT SERIES

By VICTOR APPLETON

These spirited tales convey in a realistic way, the wonderful advancesin land and sea locomotion. Stories like these are impressed upon thememory and their reading is productive only of good.

TOM SWIFT AND HIS MOTOR CYCLE
TOM SWIFT AND HIS MOTOR BOAT
TOM SWIFT AND HIS AIRSHIP
TOM SWIFT AND HIS SUBMARINE BOAT
TOM SWIFT AND HIS ELECTRIC RUNABOUT
TOM SWIFT AND HIS WIRELESS MESSAGE
TOM SWIFT AMONG THE DIAMOND MAKERS
TOM SWIFT IN THE CAVES OF ICE
TOM SWIFT AND HIS SKY RACER
TOM SWIFT AND HIS ELECTRIC RIFLE
TOM SWIFT IN THE CITY OF GOLD
TOM SWIFT AND HIS AIR GLIDER
TOM SWIFT IN CAPTIVITY
TOM SWIFT AND HIS WIZARD CAMERA
TOM SWIFT AND HIS GREAT SEARCHLIGHT
TOM SWIFT AND HIS GIANT CANNON
TOM SWIFT AND HIS PHOTO TELEPHONE
TOM SWIFT AND HIS AERIAL WARSHIP
TOM SWIFT AND HIS BIG TUNNEL
TOM SWIFT IN THE LAND OF WONDERS
TOM SWIFT AND HIS WAR TANK
TOM SWIFT AND HIS AIR SCOUT
TOM SWIFT AND HIS UNDERSEA SEARCH
TOM SWIFT AMONG THE FIRE FIGHTERS
TOM SWIFT AND HIS ELECTRIC LOCOMOTIVE

THE MOVING PICTURE BOYS SERIES

BY VICTOR APPLETON

Moving pictures and photo plays are famous the world over, and in thisline of books the reader is given a full description of how the filmsare made—the scenes of little dramas, indoors and out, trick picturesto satisfy the curious, soul-stirring pictures of city affairs, life inthe Wild West, among the cowboys and Indians, thrilling rescues alongthe seacoast, the daring of picture hunters in the jungle among savagebeasts, and the great risks run in picturing conditions in a land ofearthquakes. The volumes teem with adventures and will be foundinteresting from first chapter to last.

THE MOVING PICTURE BOYS
THE MOVING PICTURE BOYS IN THE WEST
THE MOVING PICTURE BOYS ON THE COAST
THE MOVING PICTURE BOYS IN THE JUNGLE
THE MOVING PICTURE BOYS IN EARTHQUAKE LAND
THE MOVING PICTURE BOYS AND THE FLOOD
THE MOVING PICTURE BOYS AT PANAMA
THE MOVING PICTURE BOYS UNDER THE SEA
THE MOVING PICTURE BOYS ON THE WAR FRONT
THE MOVING PICTURE BOYS ON FRENCH BATTLEFIELDS
MOVING PICTURE BOYS FIRST SHOWHOUSE
MOVING PICTURE BOYS AT SEASIDE PARK
MOVING PICTURE BOYS ON BROADWAY
THE MOVING PICTURE BOYS OUTDOOR EXHIBITION
THE MOVING PICTURE BOYS NEW IDEA