"Who's there?" suddenly called Tom, and in such a sharp voice that Mr. Damon started, ready as he was for something unusual.
There was no answer and Tom suddenly switched on all the lights in the shed. Up to then there had been only a few glowing—just enough for him to show the new Air Scout to his friend.
"Who's there?" asked Tom again, sharply.
"Bless my opera glasses, Tom!" cried Mr. Damon, "but are you seeing things?"
"No; but I'm hearing them," answered Tom with a short laugh. "Did you think you heard some one moving around near the rudders of Silent Sam, Mr. Damon?"
"No, I can't say that I did. Everything seems to me to be all right."
"Well, it doesn't to me," went on Tom grimly. "I think there is an intruder in this shed, though how any one could get in when the doors have been locked all day, is more than I can figure out. But I'm going to have a look."
"I'll help you," offered Mr. Damon, and, in the bright glare from many electric lights, the two began a search of the big hangar where the new craft was kept.
But though the young inventor and his friend went around to the rear of the aeroplane, walking in opposite directions, they saw no one, nor did any one try to escape past them.
"And yet I was sure I heard some one in here," declared Tom, when a search had revealed nothing. "It sounded as if some one were scuffling softly about in rubber-soled shoes, trying to hide."
"Bless my suspenders!" cried Mr. Damon, "who do you think it could have been, Tom?"
"Who else but some spy trying to get possession of my secrets?" was the answer. "But I guess I was too quick for them. They couldn't learn much from looking at the outside of my muffler, and it hasn't been disturbed, as far as I can see."
"Who would want to gain a knowledge of it in that unlawful way?" asked Mr. Damon.
"Perhaps some of the Universal crowd. They may have been disappointed in perfecting a silent motor themselves, and think stealing my idea would be the easiest way out of it."
"Do they know you are working on such a model as this Silent Sam of yours, Tom?"
"Yes, I imagine they do. One of the firm members, as you recall, overheard something, I think, that gave them a hint as to what my plans were, though, thanks to the time I fooled the spy, they haven't any real data to go by, I believe."
"Let us hope not," said Mr. Damon.
Tom and he made a thorough search of the big shed, but found no one, nor was there any trace of an intruder. Tom notified Jackson, who, in turn, told the guards and watchmen to be on the lookout for any suspicious strangers, but none was seen in the vicinity of the Swift works.
"Well, everything seems to be all right, so we'll have the test," remarked Tom, after a further search of the premises. "Now, Mr. Damon, if all goes as I hope you will see what my new machine can do. Strain your ears for a sound, and let me know how much you hear."
His men helping him, Tom started the new motor which was tried for the first time attached to the new craft. No flight was to be made yet, the motor being tested as though on the block, though, in reality, the craft was ready for instant flight if need be.
Slowly the great propellers began to revolve, and then Tom, taking his place in the cockpit, turned on more power. The new craft—Silent Sam—was made fast so it could not progress even though the propellers revolved at high speed.
"I'm not sending her to the limit," said Tom to his friend, as the young inventor throttled down the motor. "If I did I'd tear her loose from the holding blocks."
"Her!" cried Mr. Damon. "Bless my typewriter, Tom! but I thought Silent Sam was a gentleman aeroplane.
"So he is!" laughed the young man, frankly. "I forgot about 'Silent Sam.' Guess I'll have to say 'him' instead of 'her,' though the latter sounds more natural. Anyhow what do you think?"
"I think it's wonderful!" exclaimed Mr. Damon. "There the motor is, going at almost full speed, and I can hardly hear a thing. You can the easier believe that when I say that I can hear you talk perfectly well. And I guess you hear me, don't you?"
"Yes," replied Tom. "And we don't have to shout, either. This is the best test ever! I think everything is a success."
"Are you going to take her aloft, Tom?" the eccentric man went on.
"Yes, now that I'm sure the engine is all right. Will you go for a flight with me?"
"I certainly will! I only wish we could find him, though. I'd go with a better heart."
"Oh! Mr. Nestor?"
"Yes, I can't imagine what has become of him. It is almost as if the earth had opened and swallowed him. His disappearance is a great mystery."
"It surely is," agreed Tom. "Can't seem to get any trace of him. But if we hear another cry for help, when we have to land, you can make up your mind I'll investigate more quickly than I did at first."
"I agree with you," said Mr. Damon.
It was nearly evening then, and until it was dark enough for his flight Tom spent the time tuning up the engine and seeing that all was in readiness for the latest test. He had decided not to go aloft while it was light enough for curiosity seekers to note the flight.
Tom rather wished Mary Nestor might have a sail with him in his latest improved silent Air Scout, but the girl was too much occupied at home and in trying to find some trace of her father.
Tom, his father, and Mr. Damon had helped all they could, but there were no results. A private detective had been engaged, but he had no more of a clew than the regular police.
At last it was dark enough for the flight, and Tom and Mr. Damon took their places in the machine. Once more the propellers were turned around, and when the compression had been made, and the spark switched on, around spun the big wooden blades, and the great craft moved over the grass.
On and on and up and up sailed Tom and Mr. Damon, and as they left behind them the shops and the Swift homestead, the two passengers were aware of their almost silent flight. The big aeroplane, the exhaust of which, ordinarily, would have nearly deafened them, was now as silent as a bird.
"Silent Sam for Uncle Sam!" cried Tom in delight, as he went on faster. "I'm sure the government ought to be glad to get this plane for air scout work. It's a success! A great success!"
"Yes, so it is!" agreed Mr. Damon. "You do well to speak of it so, Tom."
For, modest as the young inventor was, he felt, in justice to himself, that he must acknowledge the fact that his craft was a success. For it rose and sailed almost as silently as a bat, and a few hundred feet away no one, not seeing it, would have believed a big aeroplane was in motion.
Tom and Mr. Damon flew about twenty miles at a swift pace, and all the fault Tom had to find was that the machine was not as steady in flight as she should have been.
"But I can remedy that with the use of some of dad's gyroscope stabilizers," he told Mr. Damon.
They returned to the hangar safely, and the first trip of the new Silent Sam was an assured success.
It was the following day, when Tom was busy in the machine shop installing the gyroscopes spoken of, that Jackson came to tell him there was a visitor to see him.
"Who is it?" asked the young inventor.
"Mr. Gale of the Universal Company," was the answer.
"I don't want to see him!" declared Tom quickly. "I have nothing to say to him after his clumsy threats."
"He seems very much in earnest," said Jackson. "Better see him, if only for a minute or so."
"All right, I will," assented Tom. "Show him in."
Mr. Gale, as blusteringly bluff as ever, entered the shop. Tom had carefully put away all papers and models, as well as the finished machines, so he had no fear that his visitor might discover some secret.
"Oh, Mr. Swift!" began the president of the Universal Company, when he met the young inventor, "I wish to assure you that what has been done was entirely without our knowledge. And, though this man may have acted as our agent at one time, we repudiate any acts of his that might—"
"What are you talking about?" asked Tom in surprise. "Have I been so impolite as to sleep during part of your talk? I don't understand what you are driving at."
"Oh, I thought you did," said Gale, and he showed surprise. "I understood that the man who—"
"Do you mean there was some one here in the shed last night?" cried the young inventor suddenly, all his suspicions aroused.
"Some one here last night?" repeated Mr. Gale. "No, I don't refer to last night. But perhaps I am making a mistake. I—er—I—"
"Some one is making a mistake!" said Tom significantly.
For perhaps a quarter of a minute Tom Swift and the president of the Universal Flying Machine Company of New York sat staring at one another. Mr. Gale's face wore a puzzled expression, and so did Tom's. And, after the last remark of the young inventor, the man who had called to see him said:
"Well, perhaps we are talking at cross purposes. I don't blame you for not feeling very friendly toward us, and if I had had my way that last correspondence with you would never have left our office."
"It wasn't very business-like," said Tom dryly, referring to the veiled threats when he had refused to sell his services to the rival company.
"I realize that," said Mr. Gale. "But we have some peculiar men working for us, and sometimes there is so much to do, so many possibilities of which to take advantage, that we may get a little off our balance. But what I called for was not to renew our offer to you. I understand that is definitely settled."
"As far as I am concerned, it is," said Tom, as his caller seemed to want an answer.
"Yes. Well, then, what I called to say was that if you are thinking of taking any legal action against us because of the action of that man Lydane, I wish to state that he had absolutely no authority to—"
"Excuse me!" broke in Tom, "but by Lydane do you mean the man who also posed as Bower, the spy?"
"No, I do not. Though I regret to say that Bower once worked for us. He, too, had no authority to come here and get a position. He was still in our service when he did that."
"So I have suspected," said Tom. "I realize now that he was a spy, who came here to try to find out for you some of my secrets."
"Not with my permission!" exclaimed Mr. Gale. "I was against that from the first and I came to tell you so. But Bower really did you no harm."
"No, he didn't get the chance!" chuckled Tom. "Nor did that other spy—the one with the gold tooth. I wonder how he liked our mud hole?"
"He was Lydane," said Mr. Gale. "It is about him I came."
"You might have saved yourself the trouble," returned Tom. "I don't wish to discuss him."
"But I wish to make sure," said Mr. Gale, "that what he has done will not come back on us. We repudiate him entirely. His methods we can not countenance. He is too daring—"
"Oh, don't worry!" interrupted Tom. "He hasn't done anything to me—he didn't get the chance, as I guess he's told you. You needn't apologize on his account. He did me no harm, and—"
"But I understood from him that—"
"Now I don't want to seem impolite!" broke in Tom, "nor do I want to take pattern after some of your company's acts, if not your own. But I am very busy. I have an important test to make for the government, and my time is fully occupied. I am afraid I shall have to bid you good-morning and—"
"But won't you give me a chance to—" began the president.
"Now, the less we discuss this matter the better!" interrupted Tom. "Lydane, as you call the man with the gold tooth didn't really do anything to me nor any great harm to any of my possessions, as far as I can learn. His career is a closed book—a book with muddy covers!" and the young inventor laughed.
"Oh, well, if you look at it that way, there is nothing further for me to say," said Mr. Gale stiffly. "I understood— But hasn't my partner, Mr. Ware, seen you?" he asked Tom quickly.
"No. And I don't care to see him."
"Oh, then that accounts for it," was the quick answer. "Well, if you regard the matter as closed I suppose we should also. We are not to blame for what Lydane does when he is no longer in our employ, and we repudiate anything he may do, or may have done."
This struck Tom, afterward, as being rather a queer remark, but he did not think so at the time.
The truth was that the young inventor wished very much to try out a new device on his noiseless aeroplane and wanted to get rid of Mr. Gale before doing so. So he did not pay as much attention to the remarks of the president as, otherwise, he might have done.
It was not until after Mr. Gale had taken his leave and Tom had finished the particular work on which he was engaged when the president of the rival company came in, that the young man did some hard thinking. And this thinking was done after he had received a telephone call from Mary Nestor, asking, if by any chance, he had heard anything like a clew as to the whereabouts of her father.
Tom had been obliged to tell her that he had not. Everything possible was being done to find the missing man but he had disappeared as completely as though he had ridden on his bicycle into the crater of some extinct volcano on the meadow, and had fallen to the bottom.
An effort was made to trace him through an automobile association which had a large membership. That is, the members were asked to make inquiries to ascertain, if possible, whether any one had heard of an unreported accident—one in which Mr. Nestor might have been carried away by persons who accidently ran him down.
But this came to naught, and the police and other authorities were at a loss how farther to proceed. It was a theory in some quarters that Mr. Nestor was perfectly safe, but that he was out of his mind, and was either wandering around, not knowing who he was, or was, in this condition, detained somewhere, the persons having him in charge not realizing that he was the missing man so widely sought.
This belief was a relief to Mrs. Nestor and Mary in many ways for it prevented them from giving way to the fear that Mr. Nestor was dead. That he was alive was Tom Swift's firm opinion, and he was doing all he could to prove it.
It was not until the day after the visit of Mr. Gale that Tom, having concluded some intricate calculations about the strength of cylinder valves, uttered an exclamation.
"I wonder if he could have meant that?" cried the young inventor. "I wonder if he could have meant that? I must find out at once! Queer I didn't think of that before!"
He put in a long distance call to New York, asking to speak to Mr. Gale. But when, eventually, he was connected with the office of the Universal Flying Machine Company he was told that Mr. Gale and Mr. Ware had sailed for France that day, going over as government representatives to investigate aeroplane motors. Gale's visit to Tom had been just previous to taking the boat, it was said.
"This is tough luck!" mused Tom, his suspicions doubly aroused now. "I can't let this rest here! I've got to get after it! As soon as I make this final test, and invite Uncle Sam's experts out to see how my noiseless motor works, I'll get after Gale and Ware if I have to follow them to the battlefields of France! I wonder if it was that he was hinting at all the while! I begin to believe it was!"
Tom Swift had decided on another flight for his new craft before he would let the government experts see it.
"Silent Sam must do his very best work for Uncle Sam before I turn him over," said the young inventor.
"And after this flight I'll offer the machine to the government, and then devote all my time to finding Mr. Nestor," said Tom. "I'd do it now, but private matters, however deeply they affect us, must be put aside to help win the war. But this will end my inventive work until after Mr. Nestor is found—if he's alive."
Preparations for the test flight went on apace, and one afternoon Tom and Jackson took their places in the big, new aeroplane. He no longer feared daylight crowds in case of an accident. They made a good start, and the motor was so quiet that as Tom passed over his own plant the men working in the yard, who did not know of the flight, did not look up to see what was going on. They could not hear the engine.
"I think we've got everything just as we want it, Jackson," said Tom, much pleased.
"I believe you," answered the mechanician. "It couldn't be better. Now if—"
And at that moment there came a loud explosion, and Silent Sam began drifting rapidly toward the earth, as falls a bird with a broken wing.
"What happened?" cried Jackson to Tom, as he leaned forward in his seat which was in the rear of the young inventor's.
"Don't know, exactly," was the answer, as Tom quickly shifted the rudders to correct the slanting fall of his craft. "Sounded as though there was a tremendous back-fire, or else the muffler blew up. The engine is dead."
"Can you take her down safely?"
"Oh, yes, I guess so. She's a bit out of control, but the stabilizer will keep her on a level keel. Good thing we installed it."
"You're right!" said Jackson.
Now they were falling earthward with great rapidity, but, thanks to the gyroscope stabilizer, the "side-slipping," than which there is no motion more dreaded by an aviator, had nearly ceased. The craft was volplaning down as it ought, and Tom had it under as perfect control as was possible under the circumstances.
"We'll get down all right if something else doesn't happen," he said to Jackson, with grim humor.
"Well, let's hope that it won't," said the mechanic. "We're a good distance up yet."
They were, as a matter of fact, for the explosion, or whatever had happened to the craft, had occurred at a height of over two miles, and they at once began falling. As yet Tom Swift was unaware of the exact nature of the accident or its cause. All he knew was that there had been a big noise and that the engine had stopped working. He could not see the silencer from where he sat, as it was constructed on the underside of the motor, but he had an idea that the same sort of mishap had occurred as on the occasion when the test machine had sailed through the roof of his workshop.
"But, luckily, this wasn't as bad," mused Tom. "Anyhow the motor is out of business."
And this was very evident. The young inventor had tried to start the apparatus after its stoppage by the explosion, but it had not responded to his efforts, and then he had desisted, fearing to cause some further damage, or, perhaps, endanger his own life and that of Jackson.
Down, down swept Silent Sam—doubly silent now, and Tom began looking about for a good place to make a landing. This was nothing new for either him or his mechanician, and they accepted the outcome as a matter of course.
"Not a very lively place down there," remarked Jackson, as he looked over the side of the cockpit.
"If we have to depend for help on any one down there, I guess we'll be a long time waiting," agreed Tom. They were about to land in a very lonely spot. It was one he had never before visited, though he knew it could not be much more than twenty miles from his own home, as they had not flown much farther than that distance.
But, somehow or other, Tom had not visited this particular section, and knew nothing of it. He saw below him, as Jackson had seen, a lonely stretch of country—a big field, once a wood-lot, evidently, as scattered about were some stumps and some second growth trees. There were also a number of evergreens—Christmas trees Jackson called them. And this was the only open place for miles, the surrounding country being a densely wooded one. There did not appear to be a house or other building in sight where they might seek help.
"But maybe we can make the repairs ourselves and keep on," the lad thought.
With practiced eye he picked out a smooth, grassy, level spot, in the midst of scattered evergreen trees, and there Tom Swift skillfully brought his Air Scout to rest. With a gentle thud the rubber-tired wheels struck the Earth, rolled along a little distance, and then called to a stop.
Hardly had the aeroplane ceased moving when Tom and his companion jumped out and began eagerly to examine the machinery to see the extent of damage.
"I thought so!" Tom exclaimed. "The silencer cracked under the strain. Those exhaust gases have more pressure that I believed possible. I increased the margin of safety on this muffler, too. But she's cracked, and I can't use the machine until I put on a new one. Good thing I didn't ask for a government inspection until after this trial flight."
"That's so," agreed Jackson. "But can't you patch it up, or go on without a muffler, so we can get back home?"
"I'm afraid not," Tom answered. "You see I removed all the old exhaust pipe fittings when I put on my new silencer. Now if I took off my attachment there wouldn't be anything to carry off the discharged gases, and they'd form a regular cloud about us. We couldn't stand it without gas masks, such as they use in the trenches, and we haven't any of those with us."
"That's right," agreed Jackson. "Well, what do you want to do? Have me stay here and guard the machine while you go for help? Or shall I go?"
"I don't know why we both can't go," said Tom. "There is no use trying to patch up this machine here. I'll have to send a truck after it, and dismantle it before I can get it home.
"As for either of us staying here on guard, I don't quite see the need of that. This looks like the jumping-off place to me. I don't believe there's a native within miles. I didn't see any houses as we came down, and I think Silent Sam will be perfectly safe here. No one can run off with him, anyhow. He'd be as hard to start as an automobile with all four wheels gone. Let's leave it here and both walk back."
"All right," agreed Jackson. "That suits me. Might as well leave our togs here, too. It will be easier walking without them," and he began taking off the fur-lined suit, his cap, and his goggles, such as he and Tom wore against the piercing cold of the upper regions.
"We can stuff them in the cockpit and leave them," went on the mechanician, as he divested himself of his garments. As he stowed them away in his seat he gave one more look at the broken muffler. As Tom Swift said, his new silencer had literally blown up, a large piece having been torn from the gas chamber.
Something that Jackson saw caused him to utter an exclamation that brought Tom Swift to his side.
"What is it?" asked the young inventor.
"Look!" was the answer. "See! Just at the edge of that break! It's been filed to make the metal thinner there than anywhere else. You didn't do that, did you?"
"I should say not!" cried Tom. "Why, to file there would mean to weaken the whole structure."
"And that's exactly what's happened!" declared Jackson, as he gave another look. "Some one has filed this nearly through—leaving only a thin metal skin, and when the gas pressure became too much it blew out. That's what happened!"
Tom Swift made a quick but thorough examination.
"You're right, Jackson!" he exclaimed. "That was filed deliberately to cause the accident. And it must have been done lately, for I carefully inspected the silencer when I put it on, and it was in perfect order. There's been spy work here. Some one got into the hangar and filed that casing. Then the accumulated pressure of the gases did the rest."
"As sure as you're alive!" agreed Jackson. "Maybe that's what Gale did when he called."
"No," returned Tom, shaking his head, "he didn't get a chance to do anything like that. I watched him all the while. But perhaps this is what he referred to when he said he and his company would repudiate any act of that spy with the gold tooth—Lydane, so Gale said his name was. Maybe that's what Lydane did."
"He was capable of it," agreed the mechanic, "but he couldn't have done it that time you tripped him into the mud puddle. This silencer wasn't built then."
"No, you're right," assented Tom. "Then he must have been around since, doing some of his tricky work!"
"I don't see how that could have been," said Jackson slowly. "We've kept a very careful watch, and your shop has been specially guarded."
"I know it has," said Tom. "There couldn't much get past Koku; but some one seems to have done it, or else how could that filing have been done?"
Jackson shook his head. The problem was too much for him. He looked carefully at the exploded and broken silencer, and Tom, too, gave it a critical eye. There was no doubt but that it had been filed in several places to weaken the structure of the metal.
"When did you last see that it was in perfect condition?" asked Jackson.
Tom named a certain date.
"That was just before Gale called," observed the mechanician. "He might have known of it."
"I wish I'd known of it at the time," said Tom savagely. "He wouldn't have gotten away as easily as he did. Well, there's no use standing here talking about it. Let's get back to civilization and we'll send back one of the trucks. Luckily I have another silencer I can put on for the government test. This one will never be of any more use, though I may be able to save some of the valves and baffle plates."
Slowly they turned from the disabled aeroplane and started to look for a path that would lead them out of the lonely place. Tom as the first to strike what seemed to be a cow path, or perhaps what had been a road into the wood lot in the early days.
As he tramped along it, followed by Jackson, the young inventor suddenly stopped, as he came to a sandy place, and, stooping over, looked intently at some queer marks in the soil.
"What is it?" asked the mechanician.
"Looks like the marks of an automobile," said Tom slowly. "And I was just trying to remember where I'd seen marks like these before."
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.
For a moment, at sight of the deserted cabin, staring at Tom and his friend, as it were, from its hiding place amid the trees, the young inventor and his companion did not move. They just stood looking at the place.
"Well," said Tom, at length, "we found it, didn't we?"
"We found something anyhow," agreed Jackson. "Whether it amounts to anything or not, we've got to see."
"Come on!" cried Tom, impulsively. "I'm going to see what's there."
"There doesn't appear to be much of anything," said Jackson, as he looked toward the lonely cabin with critical eyes. "I should say that place hadn't been used, even as a chicken coop, in a long while."
"We can soon tell!" exclaimed Tom, striding forward.
"Wait just a minute!" cried his companion, catching him by the coat. "Don't be in such a hurry."
"Why not?" asked Tom. "There isn't any danger, is there?"
"I don't know about that. There's no telling who may be hidden in that cabin, in spite of its deserted appearance. And though there aren't any 'No Trespass' signs up, it may be that we wouldn't be welcome. If there are some tramps there, which is possible, they might take a notion to shoot at us first and ask questions as to our peaceable intentions afterward—when it would be too late."
"Nonsense!" exclaimed Tom. "There aren't any tramps there and, if there were, they wouldn't dare shoot. I'm going to see what the mystery is—if there is one."
But there was no sign of life, and, taking this as an indication that their advance would not be disputed, Jackson followed Tom. The latter advanced until he could take in all the details of the shack. It was made of logs, and once had been chinked with mud or clay. Some of this had fallen out, leaving spaces between the tree trunks.
"It wasn't a bad little shack at one time," decided Tom. "Maybe it was a place where some one camped out during the summer. But it hasn't been used of late. I never knew there was such a place around here, and I thought I knew this locality pretty well."
"I never heard of it, either," said Jackson. "Let's give a shout and see if there's any one around. They may be asleep. Hello, there!" he called in sufficiently vigorous tones to have awakened an ordinary sleeper.
Put there was no answer, and as the shadows of the night began to fall, the place took on a most lonely aspect.
"Let's go up and knock—or go in if the door's open," suggested Tom. "We can't lose any more time, if we're to get out of here before night."
"Go ahead," said Jackson, and together they went to the cabin door.
"Locked!" exclaimed Tom, as he saw a padlock attached to a chain. It appeared to be fastened through two staples, driven one into the door and the other into the jamb, at right angles to one another and overlapping.
"Knock!" suggested Jackson. But when Tom had done so, and there was no answer, the machinist took hold of the lock. To his own surprise and that of Tom, one of the staples pulled out and the door swung open. The place had evidently been forced before, and the lock had not been opened by a key. The staple had been pulled out and replaced loosely in the holes.
For a moment nothing could be made out in the dark interior of the shack. But as their eyes became used to the gloom, Tom and his companion were able to see that the shack consisted of two rooms.
In the first one there was a rusty stove, a table, and some chairs, and it was evident, from pans and skillets hanging on the wall, as well as from a small cupboard built on one side, that this was the kitchen and living room combined.
"Anybody here?" cried Tom, as he stepped inside.
Only a dull echo answered.
The two could now see where a door gave entrance to an inner room, and this, a quick glance showed, was the sleeping apartment, two bunks being built on the side walls.
"Well, somebody had it pretty comfortable here," decided Tom, as he looked around. "They've been cooking and sleeping here, and not so very long ago, either. It wouldn't be such a bad place if it was cleaned out."
"That's right," agreed Jackson. "Wouldn't mind camping here myself, if there was any fishing near."
"The river can't be far away," suggested Tom. "And now let's see what we can find, and see if we can get a line on who has been here. But first we'll let in a little light."
He opened a window in the sleeping room, and pushed back the heavy plank shutter that had been closed. When the light entered it was seen that both bunks bore evidence of having been lately slept in. The blankets were tossed back, as if the occupants had risen, and in the outer room, on the stove, were signs that indicated a meal had been served not many days gone by.
"Now," observed Tom musingly, as he wandered about the place, "if we could only find out who owns this, and who has been here lately—"
Jackson stooped over, and, thrusting aside an end of the blankets that trailed on the floor from one of the bunks, picked up something.
"What is it?" asked Tom.
"Looks like a leather pocketbook," was the answer. "That's what it is," the mechanic went on, as he held the object to the light. "It's a wallet."
"Let me see it!" exclaimed Tom quickly. He took the wallet from the hands of Jackson. Then the young inventor uttered a cry. "A clew at last!" he exclaimed. "A clew at last! Mr. Nestor has been in this cabin!"
"How do you know?" asked Jackson quickly.
"This is his wallet," said Tom excitedly. "I've often seen him have it. In fact he had it with him on Earthquake Island, the time I sent the wireless message for help. I saw it several times then. He kept in it what few papers he had saved from the wreck. And I've seen it often enough since. That's Mr. Nestor's wallet all right. Besides, if you want any other evidence—look!" He opened the leather flaps and showed Jackson on one, stamped in gold letters, the name of Mary's father.
"Well, what do you make of it, Tom?" asked the mechanician, as he finished his examination of the wallet. "What does it mean? The pocket-book is empty and that—"
"Might mean almost anything," completed Tom. "But it's a clew all right! He's been here, and I'm pretty certain he was brought here in the auto with the odd tires—the one Mr. Damon and I saw traces of the night we heard the cries for help."
"But that doesn't help us now," said Jackson. "The point is to find out how lately Mr. Nestor was here, and what has happened to him since. There isn't anything in the wallet, is there?"
"Nothing," answered Tom, making a careful examination so as to be sure. "It's as empty as a last year's bird nest. He's been robbed—that's what has happened to Mr. Nestor. He was waylaid that night, instead of being run down as I thought—waylaid and robbed and then his body was brought here."
"There you go again, Tom! Jumping to conclusions!" said Jackson, with a friendly smile, and with the familiarity of an old and valued helper. "Maybe he's in perfectly good health. Just because you found his empty wallet doesn't argue that your friend is in serious trouble. He may have dropped this on the road and some one picked it up. I'll admit they may have taken whatever was in it, but that doesn't prove anything. The thing for us to do is to find out who knows about this shack; who owns it, on whose land it is, and whether any one has been seen here lately."
"They've been here lately whether they've been seen or not," said Tom positively. "There are the auto tracks. It rained two days ago, and the tracks were made since. Mr. Nestor must have been here within two days."
"He may or may not," said Jackson. "Say, rather, that some one was here and left his wallet after him. Now see if we can find other clews!"
They looked about in the fast fading light, but at first could discover nothing more than evidences that three or four persons had been living in the shack and at some recent date—probably within a day or two.
They had had their meals there and had slept there. But this seemed to be all that could be established, other than that Mr. Nestor's wallet was there, stripped of its contents.
Tom was looking through the closet, from which a frightened chipmunk sprang as he opened the door. There were the remains of some food, which accounted for the presence of the little striped animal. And, as Tom poked about, his hand came in contact with something wrapped in paper on an upper shelf. It was something that clinked metallicly.
"What's that?" asked Jackson. "Knives, or some other weapons?"
"Neither," answered Tom. "It's a couple of files, and they've been used lately. I can see something in the grooves yet and—"
Suddenly Tom ceased speaking and drew from his pocket a small but powerful magnifying glass. Through this he looked at one of the files, taking it out in front of the shack where the light was better.
"I thought so!" he cried. "Look here, Jackson!"
"What is it?"
"Another clew!" answered Tom.