For a moment Jackson thought Tom had discovered a clew to, or evidences of, some crime. He had an unpleasant suspicion, for an instant, that there was blood on the files, and that it might prove to be the blood of Mr. Nestor.

But the satisfaction that showed on Tom's face did not seem to indicate such dire possibilities as these.

"What is it?" asked Jackson, unable to guess at what Tom was looking through the powerful glass. "What do you see?"

"Metal filings on the grooves of these files," said the young inventor. "And, unless I'm greatly mistaken, the particles of filings are from the case of my aircraft silencer!"

"What!" cried the machinist. "Do you mean those are the files used in weakening the outer case of your new machine, so that it burst a little while ago?"

"That what I think," answered Tom. "I know it sounds pretty far-fetched," he went on. "But take a look for yourself. If those particles on, the files aren't exactly of the same color and texture as the material of which the silencer case is made, I'll never build another machine."

Jackson peered through the powerful glass moving out a little farther from the shack, so as to get the best light possible on the subject of his examination. It was fast getting dark, but there was enough glow in the western sky for his purpose.

"Am I right?" asked Tom.

"You're right!" declared his helper. "This is exactly the same metal as that of which your silencer case is made. It's a peculiar mixture of aluminum and vanadium steel. I never knew it used in any shop but yours, and these filings are certainly of that metal. It would seem, Tom, that these were the files used to cut a crease in the case of your silencer to weaken it so it would burst."

"My idea exactly!" cried Tom. "The spy, who got into my shop in some undiscovered manner, did his work and then fled here to hide. He left his files behind. Mr. Nestor must have been here, either before or after. No, I'll not say that, either. Finding his wallet here doesn't prove that he was here. It might have been brought here by one of the spies and dropped. But I'm sure we're on the track of the men who damaged my airship, as well as those who know something of the mystery of Mr. Nestor."

"I agree with you," said Jackson. "Of course there's a possibility that the same peculiar metal you used in your silencer case may have been used in some other machine shop, and these files may have come from there, and have been employed in perfectly regular work. But the chances are--"

"There's only one way to make sure," said Tom. "Let's take the files with us and see if they fit in the grooves where the break came. We'll take these back to where we left the Air Scout," and he clinked the files he held.

"We can just about make it before it gets black dark," returned Jackson. "But that won't give us any more time to look around here," and he indicated the hut.

"I fancy we've seen all there is to see here," said Tom. "Mr. Nestor isn't here, and whether he was or not is a question. Anyhow, some one was here who had something to do with him after his disappearance, I'm positive of that. And I'm sure some one was here who damaged my airship. Now we'll run down both those clews, find out who owns this place, who has been using it, and all we can along that line. So, if you're ready, let's travel."

The two set out to make their way back to where they had left the stranded airship. It was fast becoming dark, but they could hurry along with more speed now, as they did not have to stop to look for the marks of the peculiar automobile tires. They had noticed the path along which they had traveled, and in half the time they had spent coming they were back where the Air Scout rested undisturbed in the meadow amid the trees.

Making sure that, as far as they could tell, no one had visited the craft since they had left it, Tom and Jackson compared the file marks on what was left of the broken silencer case with the files they had found in the hut. They used a small, but powerful electric lamp to aid them in this examination, as it was too dark to see otherwise, and what they saw caused the young inventor to exclaim:

"That settles it! These were the files used!"

"That's right!" agreed his assistant. "You've called the turn, Tom. The next thing to do is to find who connects with the files."

"Yes. To do that and find Mr. Nestor," said Tom. "We have plenty of work ahead of us. But let's get nearer civilization and send some word to the folks at home. They'll be getting worried."

"It doesn't seem as if there was a way out of here without using an airship," remarked Jackson.

But he and Tom finally reached the seldom-used road which ran along the field that contained the lonely shack, and, following this, they reached a farmhouse about a mile farther on. Greatly to their relief, there was a telephone in the place. True it was only a party line, set up by some neighboring farmers for their own private use, but one of the subscribers, to whose home the private line ran, had a long distance instrument, and after a talk with him, this man promised Tom to call up Mr. Swift and acquaint him with the fact that his son and Jackson were all right, and would be home later.

"And now," said Tom, after thanking their temporary host, a farmer named Bloise, "can you tell us anything about an old cabin that stands back there?" and he indicated the location of the mysterious shack.

"Well, yes, I can tell you a little about it, but not very much," said Mr. Bloise. "It was built, some years ago, by a rich New Yorker, who bought up a lot of land around here for a game preserve. But it didn't pan out. This cabin was only the start of what he was going to call a 'hunting lodge,' I believe it was. There was to be a big building on the same order, but it never was built.

"Some say the fellow lost all his money in Wall Street, and others say the state wouldn't let him make a game preserve here. However it was, the thing petered out, and the old shack hasn't been used since."

"Oh, yes, it has!" exclaimed Tom. "We just came from there, and there are signs which show some one has been sleeping there and eating there."

"There has!" exclaimed the farmer. "Well, I didn't know that."

"I did," said his son, a young man about Tom's age. "I meant to speak of it the other day. I saw an automobile turn into the old road that the men used when they built the shack. I thought it was kind of queer to see a touring car turn in there, and I meant to speak of it, but I forgot. Yes, some one has been at the old cabin lately."

"Do you know who they are?" asked Tom eagerly. "We are looking for a Mr. Nestor, who disappeared mysteriously about two weeks ago, and I just found his wallet there in the shack!"

"You did!" exclaimed Mr. Bloise. "That's queer! You relatives of this Mr. Nestor?" he asked.

"Not exactly," Tom answered. "Just very close friends."

"Well, it's too bad about his being missing in that way," went on the farmer. "I read about it in the paper, but I never suspected he was around here."

"Oh, we're not sure that he was," said Tom quickly. "Finding his wallet doesn't prove that," and he told the story of his own and Jackson's appearance on the scene, to the no small wonder of the farmer and his family. Tom said nothing about the finding of the files, nor the evidence he deduced from them. That was another matter to be taken up later.

"Who were in the auto you saw?" asked Tom of the farmer's son. "Was Mr. Nestor in the car?"

"I couldn't be sure of that. There were two men in the machine, and they were both strangers to me. They were talking together, pretty earnestly, it seemed to me."

"One did not appear as if he was being taken away against his will, did he?" asked Tom.

"No, I can't say that he did," was the answers "They looked to me, and acted like, business men looking over land, or something like that. They just turned in on the road that leads to the old hunting cabin, as we call it around here, and didn't pay any attention to me. Then I forgot all about them."

"Neither of them could have been Mr. Nestor," decided Tom. "At least it doesn't seem as if he'd talk at all companionably to a man who had treated him as we think Mr. Nestor has been treated. I guess that clew isn't going to amount to much."

"It may!" insisted Jackson. "They may have had Mr. Nestor in the car all the while--concealed in the back you know. We've got to find out more about these men and their auto, Tom."

"Well, yes, perhaps we have. But how?"

"Station some one at the shack, or at the beginning of the private road. The men may come back."

"That's so--they may. We'll do that!" cried the young inventor. "We must tell the police and Mr. Nestor's folks what we have learned. How can we get back to Shopton in a hurry?" he asked the farmer.

"Well, I can drive you to the railroad station" was the answer.

"Thank you," remarked Tom. "We'll accept your offer. And as soon as we get back we must send some one from the shop to stand guard over the airship," he added in an aside to Jackson. "Those file fellows may come back."

"That's so, we can't take any chances."

The farmer soon had his team at the door, and, after they had had a hasty but satisfying supper at the farmhouse, the son drove Tom and Jackson several miles to a railroad station, where they could catch a train for Shopton.

In due season Tom's home was reached. He intended to stop but a minute, to assure his father that everything was all right, and then get out his speedy runabout to go to see Mary, to tell her the news.

But when Tom sought his father in the library, he was told that there was a visitor in the house.

"Tom," said his father, "this gentleman is from Washington. He wants to arrange for a government test of your silent airship. I told him I thought you were about ready for it."

"A government test !" cried Tom. "Why, I didn't think the government even knew I was working on such an idea!" Tom was greatly surprised.