Eagerly Mr. Damon and the government agent leaned over and looked down. In the moonlight they saw the same sight that had attracted Tom Swift. The touring car, the two men in front, and the huddled, bound figure in the back.

"Can you go down, Tom, without letting them hear you?" asked Mr. Damon, using a low voice, as if fearful the men in the automobile would hear him.

"I guess so," answered the young inventor. "I can land nearer to the cabin than Jackson and I did, and then we can see what these fellows are up to. It looks suspicious to me. That is, unless they're some of the Secret Service men, and have made a capture," he added to Mr. Terrill.

"Those aren't any of Uncle Sam's men," declared the agent. "That is, unless the bound one is. I can't see him very well. Better go down, and we'll see if we can surprise them."

"My plan," voiced Tom.

Quickly he shifted the rudder, and then, shutting off the motor, as he wanted to volplane down, he headed his craft for an open spot that showed in the bright moonlight. By this time the automobile and its occupants were out of sight behind a clump of trees, but Tom and his companions felt sure of the destination of the men--the deserted cabin in the wood.

As silently as a wisp of grass falling, the big craft came down on a level spot, and then, leaping out, the young inventor and his two companions crept along the path toward the cabin. Mr. Terrill was armed, Tom carried a flashlight, while Mr. Damon picked up a heavy club.

As soon as he came near a place where he thought the marks of the automobile wheels would show, Tom flashed his light.

"I thought so!" he exclaimed, as he saw the square, knobby tread marks left by the tires. "It's the same gang, or some of them in the same car. If we can only capture them!"

"The Secret Service men ought to do that," returned Mr. Terrill, but, as it developed later, they were not on hand, though through no fault of theirs.

On and on crept Tom and the two men, until they came within sight of the cabin. They saw a light gleaming in it, and Tom whispered:

"Now we have them! Work our way up quietly and make them surrender, if we find they're what we think."

"Is there a rear door?" asked Mr. Terrill in a whisper.

Tom answered in the negative, and then all three, in fan shape, crept up to the front portal. It was open, and silently reaching a place where they could make an observation, Tom and his companions looked in.

What they saw filled them with wild and righteous rage, and brought to an end the mystery of the disappearance of Mr. Nestor. For there he sat, bound in a chair, and at a table in front of him were two forbidding-looking men.

"What do you intend to do now?" asked Mr. Nestor in a faint voice. "I cannot stand this captivity much longer. You admit that you don't want me--that you never wanted me--so why do you keep me a prisoner? It cannot do the least good."

"There's no use going over that again !" exclaimed the harsh voice of one of the men. told you that if you will promise to keep still about what happened to you, and not to give the police any information about us, we'll let you go gladly. We don't want you. It was all a mistake, capturing you. You were the wrong man. But we re not going to let you go and have you set the police on us as soon as you get a chance. Give us your promise to say nothing, and we'll let you join your friends. If you don't--"

"Make no promises, Mr. Nestor!" cried Tom Swift in a ringing voice, as he leaped from his hiding place, followed by his companions. "Your friends are here, and you can tell them everything!"

"Up with 'em!" called Mr. Terrill to the two conspirators as he confronted them with his automatic pistol ready for firing. He had no need to mention hands--they knew what he meant and took the characteristic attitude.

"Tom! Tom Swift!" cried Mr. Nestor, struggling ineffectually at his bonds. "Is it really you?"

"Well, I hope it isn't any imitation," was the grim answer. "We'll tell you all about it later. Jove, but I'm glad we found you! If it hadn't been for Silent Sam we might never have been able to."

"Well, I don't know who Silent Sam is," said Mr. Nestor faintly. "But I'm sure I'm much obliged to him and your other friends. It has been very hard. Tell me, are my wife and Mary all right?"

"In good health, yes, but, of course, worrying," said Tom. "We saw them in the garden a little while ago. Now don't talk until I set you free."

And as Tom cut the ropes from Mr. Nestor, Mr. Damon used them to bind the two conspirators, while Mr. Terrill stood guard over them. And when they were safely bound, and Mr. Nestor had somewhat recovered from the shock, Tom had a chance to examine the prisoners.

"What does it all mean? Who are you fellows, anyhow, and what's your game?" he demanded.

"Guess it--since you're so smart!" snapped one.

And no sooner had he opened his mouth and Tom had a glance of something gleaming brightly yellow, than the young inventor cried:

"The gold tooth! So it's you again, is it, you spy?"

The man shrugged his shoulders with an assumption of indifference. And, as Tom took a closer look, he became aware that the man was surely none other than Lydane, the spy he had chased into the mud puddle some weeks before. His companion was a stranger to Tom.

"What does it all mean, Mr. Nestor ?" asked Tom. "Have these men held you a prisoner ever since you called for help on the moor that night?"

"Yes, Tom, they have. And I did call for help after they attacked me as I was riding my wheel, but I didn't know any one heard me. I began to be afraid no one would ever help me."

"We've been trying to, a long time," said Mr. Damon, "but we couldn't find you. Where did they keep you?"

"Here, part of the time," was Mr. Nestor's answer. "And in other lonely houses. They bound and gagged me when they took me from place to place."

"But what was their object?" asked Tom, concluding it was useless to question the two captives. "Why did they make you a prisoner, Mr. Nestor?"

"Because they took me for you, Tom."

"For me?"

"Yes. The night I called at your house, and found you were not at home, I put back in my pocket a bundle of papers I had brought over to show you. They were plans of a little kitchen appliance a friend of mine had invented, and I wanted to ask your opinion of it."

"These scoundrels must have followed me, or have seen the bundle of papers, and, mistaking me for you, they followed, attacked me in a lonely spot and, bundling me and my wrecked wheel into an auto, carried me off. They first demanded that I gave up the 'plans,' and when I wouldn't they choked off my cries for help and knocked me into unconsciousness. Then they brought me here, and kept me here for several days.

"They soon learned that the plans I had weren't those they wanted, though what they were thin after I couldn't imagine. Only, from what I laser overheard, I knew they mistook me for you and that they were bitterly disappointed in not getting plans of some new airship you were working on. They have kept me a prisoner ever since, and though they offered to let me go if I would keep silent, I refused. I did not think, to secure my own comfort, I should let such men go unpunished if I could bring about their arrest."

"I should say not!" cried Tom.

"Did they treat you brutally, Mr. Nestor?" asked Mr. Damon.

"Not after they found out who I was, by looking through my wallet. Of course they didn't behave very decently, but they weren't actually cruel, except that they bound and gagged me. Oh, but I'm glad you came, Tom! How did it happen?"

Then they told Mr. Nestor their story, and how the test of the new Air Scout had led to his rescue.

"But where are the Secret Service men?" asked Mr. Terrill, when it became evident that none them was on guard at the cabin.

Later it developed that, by following a false clew, the Secret Service men had been drawn miles away from the cabin. And only that Tom and his companions in the silent airship saw the men. Mr. Nestor might not have been rescued for some further time.

His version of what had happened was correct. He had been mistaken for Tom, and the spy with the gold tooth and his accomplice had waylaid Mary's father, under the belief that it was Tom Swift with the plans of the new silent motor. Mr. Nestor had been attacked while riding his wheel in a lonely place, and had been carried off and kept in hiding, a prisoner even after his identity became known.

"Well, this is a good night's work!" exclaimed Tom, when the two rogues had been sent to jail and Mr. Nestor taken to the Bloise farmhouse, to be refreshed before he went home. Word of his rescue was telephoned to Mary and her mother, and it can be imagined how they regarded Tom Swift for his part in the affair.

Little the worse for his experience, save that he was very nervous, Mr. Nestor was taken home. He gave the details of his being waylaid, and told how the men, for many days, were at their wits' ends to keep him concealed when they found what a stir his disappearance had created. The conspirators were well supplied with money, and in the automobile they took their prisoner from one place to another. They had usurped the use of the cabin and had lived there nearly a week in hiding, leaving just before the first visit of Tom and Jackson. The rifled wallet had been dropped by accident.

And it did not take much delving to disclose the fact that, Lydane, "Gold Tooth," as he was called, and his crony, were spies in the pay of the Universal Flying Machine Company. As the men went under several aliases there is no need of giving their names. It is to be doubted if they ever used their real ones--or if they had any.

Of course, there was quite a sensation when Mr. Nestor was found, and a greater one when it became known the part the Universal Flying Machine people had in his disappearance in mistake for Tom. The officials of the company were indicted, and several of the minor ones sent to jail but Gale and Ware escaped by remaining abroad.

It came out that they both knew of the acts of Lydane and his companion in crime, and that the two officials realized the mistake that had been made by their clumsy operatives. It was believed that this knowledge led to the visit of Gale to Tom, the time the latter's suspicions were first aroused. Gale made a clumsy attempt to clear his own skirts of the conspiracy, but in vain, though he did escape his just punishment.

What had happened, in brief, was this. Gale and Ware, unable to secure Tom's services, even by the offer of a large sum of money, had stooped to the sending of spies to his shop, to get possession of information about his silent motor. This was after Gale had, by accident, heard Tom speaking of it to Mr. Damon.

But, thanks to Tom's vigilance, Bower was discovered. The man tripped into the mud hole lost in the muck the plans Bower passed to him. They were never recovered. Then Lydane tried again. He managed, through bribery, to gain access to the hangar where the new silent machine was kept, and, unable to get the silencer apart, tried to file it. In doing so he weakened it so that it burst.

The attempt to waylay Tom, and so get the plans from him, had been tried before this, only a mistake had been made, and Mr. Nestor was caught instead. Finding out their error, Lydane and his companions did not tell the Universal people of their mistake, though Gale and Ware knew the attempt was to be made against Tom Swift.

Later, hearing that the young inventor was still at work on his invention, Gale was much surprised, and paid his queer visit, in an attempt to repudiate the actions of Lydane. At this time it was assumed that Gale and his partner did not know that it was Mr. Nestor who had been kidnapped by mistake or they might have insisted on his release. As it was, Lydane had Mary's father, and was afraid to let him go, though really their prisoner became a white elephant on the hands of the conspirators and kidnappers.

And it was after all this was cleared up, and Mr. Nestor restored to his family and friends, that one day, Tom Swift received another visit from Mr. Terrill, the government agent.

"Well, Mr. Swift," was the genial greeting, "I have come to tell you that the favorable report made by my friends and myself as to the performance of your noiseless motor, has been accepted by the War Department, and I have come to ask what your terms are. For how much will you sell your patent to the United States?"

Tom Swift arose.

"The United States hasn't money enough to buy my patent of a noiseless motor," he said.

"Wha--what!" faltered Mr. Terrill. "Why, I understood--you don't mean--they told me you were rather patriotic, and--"

"I hope I am patriotic!" interrupted Tom with a smile. "And when I say that the United States hasn't money enough to buy my latest invention I mean just that."

"My Air Scout is not for sale!"

"You mean," faltered the government agent. "You say--"

"I mean," went on Tom, "that Silent Sam is for Uncle Sam without one cent of cost! My father and I take great pleasure in presenting such machines as are already manufactured, those in process of making, and the entire patents, and all other rights, to the government for the winning of the war!"

"Oh!" said Mr. Terrill in rather a strange voice. "Oh!"

And that was all he could say for a little while.

But Tom Swift reckoned without a knowledge of a peculiar law which prohibits the United States from accepting gifts totally without compensation, and so, in due season, the young inventor received a check for the sum of one dollar in full payment for his silent motor, and the patent rights thereto. And Tom has that check framed, and hanging over his desk.

And so the silent motor became an accomplished fact and a great success. Those of you who have read of its work against the Boches, and how it helped Uncle Sam to gain the mastery of the sky, need not be reminded of this. By it many surprise attacks were made, and much valuable information was obtained that otherwise could not have been brought in.

One day, after the rogues had been sent to prison for long terms, and Tom had turned over to his government his silent aircraft--except one which he was induced to keep for his own personal use--the young inventor went to call on Mary Nestor. The object of his call, as I believe he stated it, was to see how Mr. Nestor was, but that, of course, was camouflage.

"Would you like to come for a ride, Mary, in the silent airship?" asked Tom, after he had paid his respects to Mr. Nestor and his wife. "We can talk very easily on board Silent Sam without the use of a speaking tube. Come on--we'll go for a moonlight sky ride."

"It sounds enticing," said Mary, with a shy look at Tom. "But wouldn't you just as soon sit on a bench in the garden? It's moonlight there, and we can talk, and--and--"

"I'd just as soon!" said Tom quickly.

And out they went into the beautiful moonlight; and here we will leave them and say good-bye.