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!"