With a reassuring smile the visitor from Washington looked at Tom Swift.

"The government officials," he said, "know more than some people give them credit for--especially in these war times. Our intelligence bureau and secret service has been much enlarged of late. But don't be alarmed, Mr. Swift," went on the caller, whose name was Mr. Blair Terrill. "Your secret is safe with the government, but I think the time is ripe to use it now--that is, if you have perfected it to a point where we can use it."

"Yes," answered Tom slowly, "the invention is practically finished and it is a success, except for a few minor matters that will not take long to complete.

"Our accident this afternoon had nothing to do with the efficiency of the silencer," Tom went on. "It was deliberately damaged by some spy. I'll take that up later. That I am interested to know how you heard of my Air Scout, as I call it."

"Well, we have agents, you know, watching all the inventors who have helped us in times past, and we haven't forgotten your giant cannon or big searchlight. I might say, to end your curiosity and lull your suspicions, that your friend, Ned Newton, who has been doing such good Liberty Bond work, informed us of your progress on the silent motor."

"Oh, so it was Ned!" exclaimed Tom.

"Yes. He told us the time was about ripe for us to make you an offer for your machine. I think we can use it to great advantage in scout work on the western front," went on the agent, and he soon convinced Tom that when it came to a knowledge of airships, he had some very pertinent facts at his disposal.

"When can you give me a test?" Mr. Terrill asked Tom.

"As soon as I can get my craft back to the shop and fit on a new outer case. That won't take long, as I have some spare ones. But I must help the Nestors," he went on, speaking to his father. "I didn't mention it over the wire," he added, "but we've found in the cabin a clew to the missing man. I must tell Mary and her mother, and help them all I can."

"And allow me to help, too," begged Mr. Terrill. "Since this affects you, Mr. Swift, and since you are, in a way, working for Uncle Sam, you must let him help you. This is the first I have heard of the missing gentleman, of whom your father just told me something, but you must allow me to help search for him. I will get the United States Secret Service at work."

"That will be fine!" cried Tom. "I wanted to get their aid, but I didn't see how I could, as I knew they were too busy with army matters and tracing seditious alien enemies, to bother with private cases. I'm sure the Secret Service men can get trace of the persons responsible for the detention of Mr. Nestor, wherever he is."

"They'll do their best," said Mr. Terrill. "I'm a member of that body," he went on, "and I'll give my personal attention to the matter."

Then followed a busy time. Tom did not get to bed until nearly morning. For he had to arrange to send some of his men to guard the stranded airship, and then he went to see Mary and her mother, taking them the good news that the search for Mr. Nestor would be prosecuted with unprecedented vigor.

"If it isn't too late!" sadly said the missing man's wife.

"Oh, I'm sure it isn't !" declared Tom.

In addition to sending a guard to the airship, other men, some of them hastily summoned from the nearest federal agency, were sent to keep watch in the vicinity of the lonely cabin. They had orders to arrest whoever approached, and a relay of the men was provided, so that watch could be kept up night and day. Besides this, other men from the Secret Service began scouring the country around the locality of the cabin, seeking a trace of the two persons the farmer's son had seen in the automobile.

"If Mr. Nestor is to be found, they'll find him!" declared Tom Swift.

Mr. Damon, as might be expected, was very much excited and wrought up over all these happenings.

"Bless my watch chain, Tom Swift!" cried the eccentric man, "but something is always happening to you. And to think I wasn't along when this latest happened!"

"Well, you can be in at the finish," promised Tom, and it was strange how his promise was fulfilled.

Meanwhile there was much to do. During the time the Secret Service men were busy looking up clews which might lead to the finding of Mr. Nestor and keeping watch in the vicinity of the hut, Tom had his airship brought back to the hangar, and a new silencer was attached. While this work was going on the place was guarded night and day by responsible men, so there was no chance for an enemy spy to get in and do further damage.

An investigation was made of the Universal Flying Machine Company, but nothing could be proved to link them with the outrage. Gale and Ware were in Europe--ostensibly on government business, but it was said that if anything could be proved connecting them with the attempt made on Tom Swift's craft, they would be deprived of all official contracts and punished.

All this took time, and the waits were wearisome, particularly in the case of Mr. Nestor. No further trace of him was found, though every effort was made. Tom began to feel that his boast of his enemies having to get up early in the morning to get ahead of him, had been premature, to say the least.

Tom Swift worked hard on his new Air Scout. He determined there would be nothing lacking when it came to the government test, and not only did he make sure that no enemy could tamper with his machine, but he took pains to see that no inherent defect would mar the test.

Jackson and the other men helped to the best of their ability, and Mr. Swift suggested some improvements which were incorporated in the new machine.

One of the puzzles the Secret Service men had to solve was that of the connection, if any, between the men who had to do with the missing Mr. Nestor and those who had damaged Tom's airship by filing the muffler case so it was weakened and burst. That there was some connection Tom was certain, but he could not work it out, nor, so far, had the government men.

At last the day came when the big government test was to be made. Tom had completed his Air Scout and had refined it to a point where even his critical judgment was satisfied. All that remained now was to give Mr. Terrill a chance to see how silently the big craft could fly, and to this end a flight was arranged.

Tom had put the silencer on a larger machine than the one he and Jackson had used. It held three easily, and, on a pinch, four could be carried. Tom's plan was to take Mr. Damon and Mr. Terrill, fly with them for some time in the air, and demonstrate how quiet his new craft was. Then, by contrast, a machine without the muffler and the new motor with its improved propellers would be flown, making as much noise as the usual craft did.

"I only wish," said Tom, as the time arrived for the official government test, "that Mary could be here to see it. She was the one who really started me on this idea, so to speak, as it was because I couldn't talk to her that I decided to get up a silent motor."

But Mary Nestor was too grief-stricken over her missing father to come to the test, which was to take place late one afternoon, starting from the aerodrome of the Swift plant.

"First," said Tom, to Mr. Terrill, "I'll show you how the machine works on the ground. I'll run the motor while the plane is held down by means of ropes and blocks. Then we'll go up in it."

"That suits me," said the agent. "If it does all you say it will do, and as much as I believe it will do, Uncle Sam will be your debtor, Mr. Swift."

"Well, we'll see," said Tom with a smile.

Preparations were made with the greatest care, and Tom went over every detail of the machine twice to make certain that, in spite of the precautions, no spy had done any hidden damage, that might be manifested at an inopportune moment. But everything seemed all right, and, finally, the motor was started, while Mr. Terrill, and some of his colleagues from the Army Aviation department looked on.

"Contact!" cried Tom, as Jackson indicated that the compression had been made.

The mechanic nodded, gave the big propeller blades a quarter turn and jumped back. In an instant the motor was operating, and the craft would have leaped forward and cleaved the air but for the holding ropes and blocks. Tom speeded the machinery up to almost the last notch, but those in the aerodrome hardly heard a sound. It was as though some great, silent dynamo were working.

"Fine!"

"Wonderful!"

"Wouldn't have believed it possible!"

These were some of the comments of the government inspectors.

"And now for the final test--that in the air," said Mr. Terrill.

Previous to this he and his colleagues had made a minute examination of the machinery, and had been shown the interior construction of the silencer by means of one built so that a sectional view could be had. Tom's principles were pronounced fundamental and simple.

"So simple, in fact, that it is a wonder no one thought of it before," said a navy aviation expert. "It is the last word in aircraft construction--a silent motor that will not apprise the enemy of its approach! You have done wonders, Mr. Swift!"

"I'd rather hear you say that after the air test," replied Tom, with a laugh. "Are you ready, Mr. Terrill?"

"Whenever you are."

"How about you, Mr. Damon?"

"Oh, I'm always ready to go with you, Tom Swift. Bless my trench helmet, but you can't sail any too soon for me!"

There was a genial laugh at his impetuosity, and the three took their seats in the big craft. Once more the engine was started. It operated as silently as before, and the first good impressions were confirmed. Even as the machine moved along the ground, just previous to taking flight into the air, there was no noise, save the slight crunch made by the wheels. This, of course, would be obviated when Silent Sam was aloft.

Up and up soared the great craft, with Tom at the engine and guide controls, while Mr. Terrill and Mr. Damon sat behind him, both eagerly watching. Mr. Terrill was there to find fault if he could, but he was glad he did not have to.

"The machine works perfectly, Mr. Swift," he said. "My report cannot be otherwise than favorable."

"We mustn't be in too much of a hurry," said Tom, who had learned caution some time ago. "I want to sail around for several hours. Sometimes a machine will work well at first, but defects will develop when it is overheated. I'm going to do my best to make a noise with this new motor."

But it seemed impossible. The machinery worked perfectly, and though Silent Sam took his passengers high and low, in big circles and small ones, there was no appreciable noise from the motor. The passengers could converse as easily, and with as little effort, as in a balloon.

"Of course that isn't the prime requisite," said Mr. Terrill, "but it is a good one. What we want is a machine that can sail over the enemy's lines at night without being heard, and I think this one will do it--in fact, I'm sure it will. Of course the ability of the passengers to converse and not have to use the uncertain tube is a great advantage."

As Tom Swift sailed on and on, it became evident that the test was going to be a success. The afternoon passed, and it began to grow dark, but a glorious full moon came up.

"Shall I take you down?" the young inventor asked Mr. Terrill.

"Not quite yet. I thoroughly enjoy this, and it isn't often I get a chance for a moonlight airship ride. Go a little lower, if you please, and we'll see if we attract any attention from the inhabitants of the earth. We'll see if they can possibly hear the machine, though I don't see how they can."

And they did not. Tom piloted the machine over Shopton, sailing directly over the center of the town, where there was a big crowd walking about. Though the airship sailed only a few hundred feet above their heads, not a person was aware of it, since the craft's lights were put out for this test.

"That settles it," said Mr. Terrill. "You have succeeded, Tom Swift!"

But Tom was not yet satisfied. He wanted a longer test. Hardly knowing why he did it he sent the craft in the direction of Mary Nestor's home. As he sailed across her lawn he saw, in the moonlight, that she and her mother were walking in the garden. They did not look up as the aircraft passed over their heads, and were totally unaware of its presence, unless they caught a glimpse of it as it flitted silently along, like some great bird of the night.

"It is perfectly wonderful!" declared Mr. Terrill, and he spoke in ordinary tones, that carried perfectly to the ears of Tom and Mr. Damon.

"Wonderful!" cried the eccentric man. "Bless my chimney, but it's the greatest invention in the world! Yes, it is! Don't tell me it 'isn't!"

And no one did.

Passing the Nestor home, the saddened occupants of which were unaware of the passage, Tom sent the Air Scout about in a circle, intending to proceed to the hangar. And then, some whim, perhaps, caused him to guide Silent Sam out toward the lonely hut. Mr. Damon and Mr. Tenrill seemed perfectly content to sail on and on indefinitely in the moonlight. Tom thought he would take them over a lonely neighborhood, and then bring them back.

In a little while the craft was directly over the stretch of country where the aeroplane accident bad occurred, and where Tom and Jackson had found the deserted hut.

Rather idly Tom looked down, wondering if the Secret Service men were on the watch and if they had discovered anything.

Suddenly Tom was aware of an automobile moving along the field path toward the cabin. There were two men in the car, both on the front seat, and as Tom looked down the brilliant moonlight showed him the figure of another man, behind, and huddled in the tonneau of the car. The aeroplane was low enough for all these details to be seen by the moon's gleam, but the men in the car, not hearing any noise, did not look up, so they were unconscious of this aerial espionage.

"Look! Look!" exclaimed Tom in a low voice to his companions. "Doesn't that seem suspicious?"