For several seconds the young inventor remained bending over the queer marks in that little sandy path of the lonely field in the midst of the silent woods. Jackson watched him curiously, and then Tom straightened up, exclaiming as he did so:

"I have it! Now I know where it was! I saw marks like these the night Mr. Nestor disappeared. Mr. Damon and I noticed the marks in the dust on the road the time we made the forced landing the first night we tried out the silent motor. That's it! They are the same marks! I'm sure of it!"

"I wouldn't go so far as to say that," said Jackson slowly. He was more deliberate than Tom Swift, a fact for which the young inventor was often glad, as it saved him from impulsive mistakes.

"This may not be the same auto," went on the mechanician. "I'll admit I never saw square tire marks like those before. Most of the usual ones are circular, diamond-shape or oblong. Some tire manufacturer must have tried a new stunt. But as for saying these marks were made by the same machine you saw evidences of the night Mr. Nestor disappeared, why, that's going a little too far, Tom."

"Yes, I suppose it is," admitted the young inventor. "But it's a clew worth following. Maybe Mr. Nestor has been brought to some lonely place like this, and is being held."

"Why would any one want to do that?" asked Jackson. "He had no enemies.

"Well, perhaps those who ran him down and injured him are afraid to let him go for fear he will prosecute them and ask for heavy damages," suggested Tom. "They may be holding him a captive until he gets well, and aim on treating him so nicely that he won't bring suit."

"That's a pretty far-fetched theory," said the mechanician as he carefully looked at the tracks. "But of course it may be true. Anyhow, these tire marks are rather recent, I should say, and they are made by a new tire. Do you think we can follow them?"

"I'm going to try !" declared Tom. "The only trouble is we can't tell whether it was going or coming--that is we don't know which way to go."

"That's so," agreed his companion. "And so the only thing to do is to travel a bit both ways. The path, or road, or whatever you call it, is plainly enough marked here, though you can't always pick out the tire marks. They show only on bare ground. The grass doesn't leave any tracks that we can see, though doubtless they are there.

"But as for thinking this car is the same one the marks of which you saw on the lonely moor, the night you heard the call for help--that's going too far, Tom Swift."

"Yes, I realize that. Of course there must be more than one car with tires which have square protuberances. But it's worth taking a chance on--following this clew."

"Oh, sure!" agreed Jackson.

"The only question is, then, which way to go," returned Tom.

They settled that, arbitrarily enough, by going on in the direction they had started after leaving the stranded airship. They followed a half-defined path, and were rewarded by getting occasional glimpses on bare ground of the odd tire marks.

Through a devious winding way, now hidden amid a lane of trees, and again cutting across an open space, the path led. They saw the marks often enough to make sure they were on the right trail, and in one place they saw several different patches of the odd marks.

They went on perhaps half a mile more. when they came to a lonely road and saw where the car had turned from that into the wood-lot, as Tom called the place where his craft had settled down.

"Look!" cried the young inventor to Jackson. "They've been here more than once, and have gone along the road in both directions. They seem to have used this turning into the lot as a sort of stopping place."

This was plain enough from an examination of the marks in the sandy soil of the road, which was one not often used. The automobile with the queer, square marks on the tires had turned into the lot, coming and going in both directions.

"This settles it!" cried Tom, when he finished making an examination. "There's something farther back in this lot that we've got to see. This auto has been coming and going, and we should have followed the tracks the other way from the point where we first saw them, instead of coming this way."

"Except that we've learned the place of departure," suggested Jackson. "Evidently the wood-lot is a blind alley. The car goes in, but it can come out only just at this point, or, at least, it does."

"That's right!" agreed Tom. "Now the thing to do is to follow our track back to where we started. There must be some place where the car went to--some headquarters, or meeting place with some one, farther back in the lot. If we can only follow the trail back as well as we did coming, we may find out something."

"Well, let's try, anyhow," suggested Jackson.

They had no difficulty in making their way back to the spot where they had first seen the queer marks. But from then on their task was not so easy. For sandy or bare patches of earth were not frequent, and they had to depend on these to give them direction, for the road was overgrown and not well defined.

Often they would search about for some time after leaving one patch of the marks before they found another that would justify them in keeping on.

"They have headquarters, or a rendezvous, somewhere back in this lot!" declared Tom, as they hurried on. "I think we're on the track of a mystery."

"Unless it turns out that some farmer has treated himself to an auto with new tires of square tread, and is hauling wood," said Jackson. "It may turn out that way."

"Yes, it may," agreed Tom. "But, taking everything into consideration, I think we're on the verge of finding out something. Even if we do discover that the owner of this auto is only hauling wood, he may be able to help us to a clew as to the whereabouts of Mr. Nestor."

"How?"

"Well, maybe he was in his machine on the moor the night the call for help came. He may even have aided to carry Mr. Nestor away. And if he doesn't know a thing about it--which, of course, is possible--the man who bought these queer tires can tell us who makes them, or who deals in them, and we can find out what autoists around here have their cars equipped with this odd tread."

"Yes," agreed Jackson, "that can be done."

And so they kept on, scouting here and there to either side of the half-defined path, until they were far back from the spot where they had left the Air Scout.

"We don't appear to be getting any warmer, as the children say," remarked Jackson, as he straightened up and looked about, for his back ached from so much stooping over to look for the odd marks.

"We haven't seen anything yet, I'll admit," said Tom. "But it won't be dark for another hour or so, and I vote that we keep on."

"Oh, I wasn't thinking of giving up!" exclaimed Jackson. "If there's anything here--at the end of the route, as you might say --we'll find it. Only I hope it doesn't turn out to be just a wood pile, from which some farmer has been hauling logs."

"That would be a disappointment," assented Tom.

The day was waning, and they realized that they ought not to spend too much time on what might turn out to be a wild goose chase. They were in a lonely neighborhood, and while they were not at all apprehensive of danger, they felt it would be best to get to shelter before dark.

"We'll want to send word to Mr. Swift that we're all right."

"Yes," said Tom, "I'd like to get to a place where I can telephone to him or Mrs. Baggert. Well, if we don't find something pretty soon we'll have to turn back. I must complete work on the new motor, for if I'm to offer it to Uncle Sam for air scout purposes, the sooner I can do so the better. Things are getting pretty hot over in Europe, and if ever the United States needed aircraft on the western front they need them now. I want to help all I can, and I also want to help Mary--you understand-- Miss Nestor."

"I understand," said Jackson simply. "I only hope you can help her. But I'm afraid--this may turn out to be nothing--following these marks, you know."

"And yet," said Tom slowly, "it would be strange if it was only a coincidence--the two tire marks being the same--the night Mr. Nestor disappeared and now."

And so they kept on, hoping.

The half-defined path through the wood-lot led them in a series of turns and twists, and it extended through a dense patch of woods, growing thickly, where it was so dark that it seemed as if night had fallen.

"We can't spend much more time here," said Tom. "If we don't find something in the next half mile we'll go back and take up the search to-morrow. I'm going to find out what's at the end of this road--even if it's only a wood pile."

For ten minutes more the two went on, making sure, by occasional glimpses at the marks, that they were on the right track. Then, suddenly, they saw something which made them feel sure they had reached their goal.

In a clearing among the trees was a little cabin --a shack of logs--and from the appearance it was deserted. There was not a sign of life around