What Tom Swift held in his hand looked like a small cannon ball, but it could not have been solid or the young aviator would not so easily have held 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 the object over.
"Is that likely to go off?" the bank clerk asked, as he came to a halt a 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 damage itself. 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 fire in 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 things around where there are explosives, Tom."
"I didn't, Ned. I wouldn't have had this within a hundred miles of my shed, if I could have had my way. It's a fire bomb, and it was set to go off at a certain time. Only I think something went wrong, and the bomb started a fire ahead of time.
"If it had worked at night, when we were all asleep, we might not have put the fire out so easily. This sure is suspicious! I'm glad you found this, 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 had done all the harm it could, approached closer.
What he saw was merely a hollow shell of iron, with a small opening in it, as though intended for a place through which to put a charge of explosives and a fuse.
"But there was no explosion, Tom," explained Ned.
"I know it," said Tom quietly. "It wasn't an explosive bomb. Smell that!"
He held the object under Ned's nose so suddenly that the young bank clerk jumped back.
"Oh, don't get nervous," laughed Tom. "It can't hurt you now. But what does that smell like?"
Ned sniffed, sniffed again, thought for a moment, and then sniffed a third time.
"Why," he said slowly, "I don't just know the name of it, but it's that funny stuff you mix up sometimes to put in the oxygen tanks when we go up in the rarefied atmosphere in the balloon or airship."
"Manganese and potash," spoke Tom. "That and two or three other things that form a chemical combination which goes off by itself of spontaneous combustion after a certain time. Only the person who put this bomb together didn't get the chemical mixture just right, and it went 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 than seemed natural under the circumstances. "Only for the fact that the mixture went off before it was intended to, and found us all alert and ready—well, I don't like to think what might have happened," and Tom cast a look about at his group of buildings with their valuable contents.
"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 with chemicals which, of themselves, after a certain time, would send out a hot tongue of flame through this hole," and Tom pointed to the opening in 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 other explosives. Only it didn't."
"Why, Tom!" cried Ned. "It was just like one of those alarm-clock dynamite bombs—set to go off at a certain time."
"Exactly," Tom said, "only this was more delicate, and, if it had worked properly, there wouldn't have been a vestige left to give us a clue. But the fire, thanks to the ballast sand in the dirigible, was put out in time. The fuse burned itself out, but I can tell by the smell that chemicals were in it. That's all, Koku," he went on to the giant who had stood waiting, not understanding all the talk between Tom and Ned. "I'll take care of this now."
"Bad man put it there?" asked the giant, who at least comprehended that something 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 he clasped his two enormous hands together, as though they were already on the 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 successful inventor has persons who imagine he has stolen their ideas, whether he has ever seen them or not. It may have been one of those persons, or some 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? They wouldn't be trying to get back at you, would they?"
"Well, it's possible, Ned. But I don't imagine so. They seem to have been pretty well broken up. No, I don't believe it was the diamond-makers who put this fire bomb in the red shed. Their line of activities didn't include this branch. It takes a chemist to know just how to blend the things contained in the bomb, and even a good chemist is 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 that sort of thing," he went on; "fighting enemies whom I have never knowingly 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. "I wouldn'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 in which 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, and finding 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 to argue 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, as soon as we get straightened out from this fire, we'll tackle that problem again—you and I. But I sure would like to know who put this in my red shed," and Tom looked in a puzzled manner at the empty fire bomb he still held.
Tom paused, on his way to the house, to put the bomb in one of his offices.
"No use letting dad know about this," he went on. "It would only be something else for him to worry about."
"That's right," agreed Ned.
By this time nearly all evidences of the fire, except for the blackened ruins of the shed, had been cleared away. High in the air hung a cloud of black smoke, caused by some chemicals that had burned harmlessly save for that pall. Tom Swift had indeed had a lucky escape.
The young inventor, finding his father quieted down and conversing easily 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 our own account. We'll look for clues while they're fresh."
But, it must be confessed, after Tom and Ned had spent the rest of that day in and about the burned shed, they were little wiser than when they started. They found the place where the fire bomb had evidently been placed, right inside the main entrance to the shed. Tom knew it had been there because there were peculiar marks on the charred wood, and a certain queer smell of chemicals that confirmed his belief.
"They put the bomb there to prevent anyone going in at the first alarm and saving anything," Tom said. "They didn't count on the roof burning through first, giving me a chance to use the sand. I made the roof of the red shed flimsy just on that account, so the force of the explosion if one ever came, would be mostly upward. You know the expanding gases, caused by an explosion or by rapid combustion, always do just as electricity does, seek the shortest and easiest route. In this case I made the roof the easiest route."
"A lucky provision," observed Ned.
That night Tom had to confess himself beaten, as far as finding clues was concerned. The empty fire bomb was the only one, and that seemed valueless.
Close questioning of the workmen failed to disclose anything. Tom was particularly anxious to discover if any mysterious strangers had been seen about the works. There was a strict rule about admitting them to the plant, however, and it could not be learned that this had been violated.
"Well, we'll just have to lay that aside for a while," Tom said the next day, when Ned again came to pay a visit. "Now, what do you say to tackling, with me, that recoil problem on the aerial warship?"
"I'm ready, if you are," Ned agreed, "though I know about as much of those things as a snake does about dancing. But I'm game."
The two friends walked out toward the shed where Tom's new craft was housed. As yet Ned had not seen it. On the way they saw Eradicate walking 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 dem questions, 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 ashes away from Tom's various shops, and dump them in a certain swampy lot. With an old ramshackle cart, and his mule, Boomerang, Eradicate did this task to perfection.
"A man—what sort of a man?" asked Tom, always ready to be suspicious of anything unusual.
"He were a queer man," went on the aged colored helper. "First he stopped 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's he 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 de fire, an' how much damage it done, an' how we put it out. An' he end up by sayin' he'd laik t' buy mah mule, Boomerang, an' he wants t' come heah 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 seed him afo'. He suah was monstrous polite t' ole black Eradicate, an' he gib 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 whisper to his chum. "I don't like the idea of a mysterious stranger questioning Eradicate!"
Ned Newton looked at Tom questioningly. Then he glanced at the unsuspicious colored man, who was industriously polishing the half-dollar the mysterious stranger had given him.
"Rad, just exactly what sort of a man was this one you speak of?" asked Tom.
"Why, he were a gen'man—"
"Yes, I know that much. You've said it before. But was he an Englishman, an American—or—"
Tom paused and waited for an answer.
"I think he were a Frenchman," spoke Eradicate. "I done didn't see him eat 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's face was soon grave again.
"Tell us more about him, Rad," he suggested. "Did he seem especially interested in the fire?"
"No, sah, Massa Tom, he seemed laik he was more special interested in mah mule, Boomerang. He done asted how long I had him, an' how much I wanted fo' him, an' how old he was."
"But every once in a while he put in some question about the fire, or about our shops, didn't he, Rad?" Tom wanted to know.
The colored man scratched his kinky head, and glanced with a queer look at 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 were a-tellin' yo' all about him astin' fo' mah mule, done gets me—dat's what it suah does."
"Never mind, Rad. He asked questions about the plant, that's all I want to 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 man said. "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 want to 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 he is, 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. When he asted me about de fire I said it didn't do no damage at all—in fack dat we'd rather hab de fire dan not hab it, 'case it done gib us a chance 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 done start. 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 dat cigarette-smokin' Frenchman didn't hab no right t' asted me so many questions, did he?"
"No, indeed, Rad. And I'm glad you didn't give him straight answers. So he's coming here later on, is he?"
"T' see ef I wants t' sell mah mule, Boomerang, yais, sah. I sort ob thought 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' offer me a big lot ob money fo' him I'd take it."
"Of course," Tom answered. "Very well, Rad. You may go on now, and don't say anything to anyone about what you have told me."
"I won't, Massa Tom," promised the colored man, as he went off muttering to himself.
"Well, what do you make of it, Tom?" asked Ned of his chum, as they walked on toward the shed of the new, big aerial warship.
"I don't know just what to think, Ned. Of course things like this have happened before—persons trying to worm secrets out of Eradicate, or some of the other men."
"They never succeeded in getting much, I'm glad to say, but it always keeps 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 hear some of the men speaking about a foreigner—a stranger—being around town last week. It was just a casual reference, and I paid little attention 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?" Ned asked.
"Hardly. That was only talk to make Eradicate unsuspicious. The stranger, whoever he was, sized Rad up partly right. I surmised, when Rad said he asked a lot of questions about the mule, that was only to divert suspicion, and that he'd come back to the subject of the fire every chance he got."
"And you were right."
"Yes, so it seems. But I don't believe the fellow will come around here. 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 he does 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 do with the fire?"
"I don't know. He knew about it, of course. This isn't the first fire we'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 have known about it. And he might be in with those who set it, for I firmly believe 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 you alone, 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 it to you yet. Then we'll get ready for that mysterious Frenchman, if he comes—but I don't believe he will."
The young inventor unlocked the door of the shed where he kept his latest "pet," and at the sight which met his eyes Ned Newton uttered an exclamation 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 detailed examination of the wonderful craft he saw before him. That is, he made as detailed an examination as was possible under the circumstances, for it was a long time before the young bank clerk fully appreciated all Tom Swift had accomplished in building the Mars, which was the warlike name painted in red letters on the big gas container that tugged and swayed 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 longer time than you'd suppose, working on it at odd moments. I had a lot of help, too, or I never could have done it. And now it is nearly all finished, as far as the ship itself is concerned. The only thing that bothers 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 the affair works, and what I hope to accomplish with it."
In brief Tom's aerial warship was a sort of German Zeppelin type of dirigible balloon, rising in the air by means of a gas container, or, rather, several of them, for the section for holding the lifting gas element was divided by bulkheads.
The chief difference between dirigible balloons and ordinary aeroplanes, as you all know, is that the former are lifted from the earth by a gas, such as hydrogen, which is lighter than air, while the aeroplane 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 sail it through the air. The moment the stone, or aeroplane, loses its forward motion, it begins to fall.
This is not so with a dirigible balloon. It is held in the air by means of the lifting gas, and once so in the air can be sent in any direction by means of propellers and rudders.
Tom's aerial warship contained many new features. While it was as large as some of the war-type Zeppelins, it differed from them materially. But the details would be of more interest to a scientific builder of such things than to the ordinary reader, so I will not weary you with them.
Sufficient to say that Tom's craft consisted first of a great semi-rigid bag, or envelope, made of specially prepared oiled silk and aluminum, to hold the gas, which was manufactured on board. There were a number of gas-tight compartments, so that if one, or even if a number of them burst, or were shot by an enemy, the craft would still remain afloat.
Below the big gas bag was the ship proper, a light but strong and rigid framework about which were built enclosed cabins. These cabins, or compartments, housed the driving machinery, the gas-generating plant, living, sleeping and dining quarters, and a pilot-house, whence the ship 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 on either beam of the ship.
"They look just like places where you intend to mount guns," said Ned to Tom.
"And that's exactly what they are," the young inventor replied. "I have the guns nearly ready for mounting, but I can't seem to think of a way of providing for the recoil. And if I don't take care of that, I'm likely to find my ship coming apart under me, after we bombard the enemy 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 the United States Government. Foreign nations are getting ready large fleets of aerial warships, so why shouldn't we? Matters in Europe are mighty uncertain. There may be a great war there in which aerial craft will play a big part. I am conceited enough to think I can build one that will measure up to the foreign ones, and I'll soon be in a position to know."
"What do you mean?"
"I mean I have already communicated with our government experts, and they are soon to come and inspect this craft. I have sent them word that it is about finished. There is only the matter of the guns, and some of the ordnance officers may be able to help me out with a suggestion, for I admit I am stuck!" exclaimed Tom.
"Then you're going to do the same with this aerial warship as you did with your big lantern and that immense gun you perfected?" asked Ned.
"That's right," confirmed Tom. My former readers will know to what Ned Newton referred, and those of you who do not may learn the details of how Tom helped Uncle Sam, by reading the previous volumes, "Tom Swift and 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 I need to change. Well, now, let me explain about that gun recoil business. 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 was housed, to a small private office. As Ned entered, the door, pulled by a strong spring, swung after him. He held back his hand to prevent it from slamming, but there was no need, for a patent arrangement took up all the force, and the door closed gently. Ned looked around, not much surprised, for the same sort of door-check was in use at his bank. But a sudden idea came to him.
"There you are, Tom!" he cried. "Why not take up the recoil of the guns on your aerial warship by some such device as that?" and Ned pointed to the door-check.
For a moment or two Tom Swift did not seem to comprehend what Ned had said. He remained staring, first at his chum, who stood pointing, and from 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 that instant. But Ned said again:
"Wouldn't that do, Tom? Check the recoil of the gun with whatever stuff is in that arrangement!"
A sudden change came over Tom's face. It was lighted up with a gleam of understanding.
"By Jove, Ned, old man!" he cried. "I believe you've struck it! And to think that has been under my nose, or, rather, over my head, all this while, and I never thought of it. Hurray! That will solve the problem!"
"Do you think it will?" asked Ned, glad that he had contributed something, 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 to think 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 the world is a hydrostatic valve?"
"A valve through which liquids pass. In this door-check there may be a mixture of water, alcohol and glycerine, the alcohol to prevent freezing in cold weather, and the glycerine to give body to the mixture so it will not flow through the valves too freely."
"And do you think you can put something like that on your guns, so the recoil will be taken up?" Ned wanted to know.
"I think so," spoke Tom. "I'm going to work on it right away, and we'll soon 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 door closed."
"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 hydrostatic valve check like that will take up, and how much recoil my biggest gun will 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 guns at the bow and stern, to throw explosive shells in a horizontal or upward direction. For a downward direction we won't need any guns, we can 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 much chance of doing that to a rival aeroplane or dirigible. But in flying over cities or forts, explosive bombs can be dropped very nicely. For use in attacking other air craft I am going to depend on my lateral fire, 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 the air," said Ned.
"No, I don't believe I'll go that far," Tom replied. "Though, if the government wants my craft, I may have to go aloft and fire shots at targets for them to show them how things work.
"Please don't think that I am in favor of war, Ned," went on Tom earnestly. "I hate it, and I wish the time would come when all nations would disarm. But if the other countries are laying themselves out to have aerial battleships, it is time the United States did also. We must not be left behind, especially in view of what is taking place in Europe."
"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. I haven't dared do that yet, and fire them, until I provided some means of taking up the recoil. Now I'm going to get right to work on that problem."
There was considerable detailed figuring and computation work ahead of Tom Swift, and I will not weary you by going into the details of higher mathematics. Even Ned lost interest after the start of the problem, though he was interested when Tom took down the door-check and began measuring the amount of force it would take up, computing it on scales and spring balances.
Once this had been done, and Tom had figured just how much force could be expected to be taken up by a larger check, with stronger hydrostatic valves, 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'll fire them, and measure the recoil."
"Well, that may be some fun," spoke Ned, with a grin. "More fun than all 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 in the interim Tom had been busy making computations. These were about finished. Meanwhile further investigation had been made of clues leading to the origin of the blaze in the shed, but nothing had been learned.
A photo-telephone had been installed near Eradicate's quarters, in the hope that the mysterious stranger might keep his promise, and come to see about the mule. In that case something would have been learned about him. But, as Tom feared, the man did not appear.
Ned was much interested in the guns, and, a little later, he helped Tom and Koku mount them in a vacant lot. The giant's strength came in handy in handling the big parts.
Mr. Swift strolled past, as the guns were being mounted for the preliminary test, and inquired what his son was doing.
"It will never work, Tom, never!" declared the aged inventor, when informed. "You can't take up those guns in your air craft, and fire them with any degree of safety."
"You wait, Dad," laughed Tom. "You haven't yet seen how the Newton hydrostatic recoil operates."
Ned smiled with pleasure at this.
It took nearly a week to get all the guns mounted, for some of them required considerable work, and it was also necessary to attach gauges to them to register the recoil and pressure. In the meanwhile Tom had been in further communication with government experts who were soon to call 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 morning to make the first test of the guns.
"They will be here any day, now. They didn't set any definite date. I suppose 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 new powder is freakish stuff."
"Me no drop any, Master," spoke the giant, as he lifted the boxes of explosives in his strong arms.
The largest gun was loaded and aimed at a distant hill, for Tom knew that if the recoil apparatus would take care of the excess force of his largest gun, the problem of the smaller ones would be easy to solve.
"Here, Rad, where are you going?" Tom asked, as he noticed the colored man 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 man slowly. "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 carefully edged away from the big gun, "but I'se done changed de eatin' hours ob dat mule. He had a little touch ob indigestion de udder day, an' I'se feedin' him diff'rent now. So I guess as how yo'll hab t' 'scuse me now, Massa Tom."
"Oh, well, trot along," laughed the young inventor. "I guess we won't need you. Is everything all right there, Koku?"
"All right, Master."
"Now, Ned, if you'll stand here," went on Tom, "and note the extreme point to which the hand on the pressure gauge goes, I'll be obliged to you. Just jot it down on this pad."
"Here comes someone," remarked the bank clerk, as he saw that his pencil was sharpened. He pointed to the field back of them.
"It's Mr. Damon," observed Tom. "We'll wait until he arrives. He'll be interested in this."
"Bless my collar button, Tom! What's going on?" asked the eccentric man, as he came up. "Has war been declared?"
"Just practicing," replied the young inventor. "Getting ready to put the 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 been forgotten, and then the big gun was discharged. There was a loud report, not as heavy, though, as Ned had expected, but there was no puff of smoke, for Tom was using smokeless powder. Only a little flash of 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 hundred and twenty thousand pounds pressure. That ought to be margin of safety enough. Now we'll try another shot."
The echoes of the first had hardly died away before the second gun was ready for the test. That, too, was satisfactory, and then the smaller ones were operated. These were not quite so satisfactory, as the recoil developed 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, service charges being used, and explosive shells sent out so that dirt, stones and gravel flew in all directions. Danger signs and flags had been posted, and a cordon of Tom's men kept spectators away from the hill, so no one would be in the danger zone.
The young inventor was busy making some calculations after the last of the firing had been completed. Koku was packing up the unfired shells, and Mr. Damon was blessing his ear-drums, and the pieces of cotton he had stuffed in to protect them, when a tall, erect man was observed strolling 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 be Eradicate's Frenchman?"
But a look at the stranger's face disproved that surmise. He had a frank and pleasant countenance, obviously American.
"I beg your pardon," he began, addressing everyone in general, "but I am 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 some correspondence recently about—"
"Oh, yes, Lieutenant Marbury, of the United States Navy," interrupted Tom. "I'm glad to see you," he went on, holding out his hand. "We are just completing some tests with the guns. You called, I presume, in reference 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 have been delayed. There was a fire in the plant."
"A fire!" exclaimed the officer in surprise. "How was that? We heard nothing of it in Washington."
"No, I kept it rather quiet," Tom explained. "We had reason to suspect that it was a fire purposely set, in a shed where I kept a quantity of explosives."
"Ha!" exclaimed Lieutenant Marbury. "This fits in with what I have heard. And did you not receive warning?" he asked Tom.
"Warning? No. Of what?"
"Of foreign spies!" was the unexpected answer. "I am sorry. Some of our Secret Service men unearthed something of a plot against you, and I presumed you had been told to watch out. If you had, the fire might not have occurred. There must have been some error in Washington. But let me tell you now, Tom Swift—be on your guard!"
The officer's words were so filled with meaning that Tom started. Ned Newton, too, showed the effect he felt.
"Do you really mean that?" asked the young inventor, looking around to make sure his father was not present. On account of Professor Swift's weak heart, Tom wished to spare him all possible worry.
"I certainly do mean it," insisted Lieutenant Marbury. "And, while I am rather amazed at the news of the fire, for I did not think the plotters would be so bold as that, it is in line with what I expected, and what we 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. "As long as you have not received any official warning from Washington you had 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 working very hard, the last two days, making some intricate calculations. I have rather neglected my mail, to tell you the truth.
"And, come to think of it, there were several letters received with the Washington postmark. But, I supposed they had to do with some of my patents, 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 cipher or 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," the officer said. "Probably you overlooked that. The Secret Service men sent you the warning in code, so it would not be found out by the plotters, and, to make sure you could understand it, a method of translating the cipher was sent in a separate envelope. It is too bad you missed it."
"Yes, for I might have been on my guard," agreed Tom. "The red shed might 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 his dirigible!" cried Ned. "You should have seen it!"
"I should have liked to be here," the lieutenant spoke. "But, if I were you, Tom Swift, I would take means to prevent a repetition of such things."
"I shall," Tom decided. "But, if we want to talk, we had better go to my office, where we can be more private. I don't want the workmen to hear too much."
Now that the firing was over, a number of Tom's men from the shops had assembled 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 and the officer strolled toward the shed where was housed the aerial warship.
"Yes, and I spent the night in New York. I arrived in town a short time ago, and came right on out here. At your house I was told you were over in the fields conducting experiments, so I came on here."
"Glad you did," Tom said. "I'll soon have something to show you, I hope. But I am interested in hearing the details of this suspected plot. 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 Secret Service 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 are at 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 Uncle Sam—notably the giant cannon, which rivals anything foreign European powers have, and the great searchlight, which proved so effective against the border smugglers. The success of those two alone, to say nothing 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 destroy those inventions. More than once, of late, we have nipped a plot just in time."
"Have they really tried to damage the big gun?" asked Tom, referring to one he had built and set up at Panama.
"They have. And now this fire proves that they are taking other measures—they are working directly against you."
"Why, I wonder?"
"Either to prevent you from making further inventions, or to stop you from completing your latest—the aerial warship."
"But I didn't know the foreign governments knew about that," Tom exclaimed. "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 just beginning to appreciate it. But our own men have not been idle."
"Have they really learned anything?" Tom asked. "Nothing definite enough 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. He also brought out the fire-bomb, and explained his theory as to how it had operated to set the red shed ablaze.
"I think you are right," said Lieutenant Marbury. "And, as regards the French, I might say they are not the only nation banded to obtain our secrets—yours and the government's!"
"But I thought the French and the English were friendly toward us!" Ned exclaimed.
"So they are, in a certain measure," the officer went on. "And Russia is, too. But, in all foreign countries there are two parties, the war party, as it might be called, and the peace element.
"But I might add that it is neither France, England, nor Russia that we must fear. It is a certain other great nation, which at present I will not 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 go into details, I want to give you another warning. You must be very careful about—"
A sudden knock on the door interrupted the speaker.