Taking a lesson from what had happened, Tom was very much more careful in the following experiments on his new, silent motor. He made some changes in his shop, and took Jackson in to help on the new machine, thus insuring perfect secrecy as the apparatus developed.

Tom also changed the safe in which he kept his plans, for the one he had used previous to the episode in which Bower and the stranger who took the mud bath figured, was one the combination of which could easily be ascertained by an expert. The new safe was more complicated, and Tom felt that his plans, specifications, and formulae which he had worked out were in less danger.

"I can just about figure out what happened," said Ned Newton to Tom, when told of the circumstances. "These Universal people were provoked because you wouldn't give them the benefit of your experience on their flying machines, and so they sent a spy to get work with you. They, perhaps, hoped to secure some of your ideas for their own, or they may have had a deeper motive."

"What deeper motive could they have, Ned?" "They might have hoped to disable you, or some of your machines, so that you couldn't compete with them. They're unscrupulous, I hear, and will do anything to succeed and make money. So be on your guard against them."

"I will," Tom promised. "But I don't believe there's any more danger now. Anyhow, I have to take some chances."

"Yes, but be as careful as you can. How is the silent motor coming on?"

"Pretty good. I've had a lot of failures, and the thing isn't so easy as I at first imagined it would be. Noise is a funny thing, and I'm just beginning to understand some of the laws of acoustics we learned at high school. But I think I'm on the right track with the muffler and the cutting down of the noise of the explosions in the cylinders. I'm working both ends, you see-- making a motor that doesn't cause as much racket as those now in use, and also providing means to take care of the noise that is made. It isn't possible to make a completely silent motor of an explosive gas type. The only thing that can be done is to kill the noise after it is made."

"What about the propeller blades?"

"Oh, they aren't giving me any trouble. The noise they make can't be heard a hundred feet in the air, but I am also working on improvements to the blades. Take it altogether, I'll have an almost silent aeroplane if my plans come out all right."

"Have you said anything to the government yet?"

"No; I want to have it pretty well perfected before I do. Besides, I don't want any publicity about it until I'm ready. If these Universal people are after me I'll fool 'em."

"That's right, Tom! Well, I must go. Another week of this Liberty Bond campaign!"

"I suppose you'll be glad when it's over."

"Well, I don't know," said Ned slowly. "It's part of my small contribution to Uncle Sam. I'm not like you--I can't invent things."

"But you have an awful smooth line of talk, Ned!" laughed his chum. "I believe you could sell chloride of sodium to some of the fishes in the Great Salt Lake--that is if it has fishes."

"I don't know that it has, Tom. And, anyhow, I'm not posing as a salt salesman," and Ned grinned. "But I must really go. Our bank hasn't reached its quota in the sale of Liberty Bonds yet, and it's up to me to see that it doesn't fall down."

"Go to it, Ned! And I'll get busy on my silent motor."

"Getting busy" was Tom Swift's favorite occupation, and when he was working on a new idea, as was the case now, he was seldom idle, night or day.

"I have hardly seen you for two weeks," Mary Nestor wrote him one day. "Aren't you ever coming to see me any more, or take me for a ride?"

"Yes," Tom wrote back. "I'll be over soon. And perhaps on the next ride we take I won't have to shout at you through a speaking tube because the motor makes so much noise."

From this it may be gathered that Tom was on the verge of success. While not altogether satisfied with his progress, the young inventor felt that he was on the right track. There were certain changes that needed to be made in the apparatus he was building--certain refinements that must be added, and when this should be done Tom was pretty certain that he would have what would prove to be a very quiet aeroplane, if not an absolutely silent one.

The young inventor was engaged one day with some of the last details of the experiment. The new motor, with the silencer and the changed cylinders, had been attached to one of Tom's speedy aeroplanes, and he was making some intricate calculations in relation to a new cylinder block, to be used when he started to make a completely new machine of the improved type.

Tom had set down on paper some computations regarding the cross-section of one of the cylinders, and was working out the amount of stress to which he could subject a shoulder strut, when a shadow was cast across the drawing board he had propped up in his lap.

In an instant Tom pulled a blank sheet over his mass of figures and looked up, a sudden fear coming over him that another spy was at hand. But a hearty voice reassured him.

"Bless my rice pudding!" cried Mr. Damon, "you shut yourself up here, Tom, like a hermit in the mountains. Why don't you come out and enjoy life?"

"Hello! Glad to see you!" cried Tom, joyfully. "You're just in time!"

"Time for what--dinner?" asked the eccentric man, with a chuckle. "If so, my reference to rice pudding was very proper."

"Why, yes, I imagine there must be a dinner in prospect somewhere, Mr. Damon," said Tom with a smile. "We'll have to see Mrs. Baggert about that. But what I meant was that you're just in time to have a ride with me, if you want to go."

"Go where?"

"Oh, up in cloudland. I have just finished my first sample of a silent motor, and I'm going to try it this evening. Would you like to come along?"

"I would!" exclaimed Mr. Damon. "Bless my onion soup, Tom, but I would! But why fly at night? Isn't it safer by daylight?"

"Oh, that doesn't make much difference. It's safe enough at any time. The reason I'm going to make my first flight after dark is that I don't want any spies about."

"Oh, I see! Are they camping on your trail?"

"Not exactly. But I can't tell where they may be. If I should start out in daylight and be forced to make a landing-- Well, you know what a crowd always collects to see a stranded airship."

"That's right, Tom."

"That decided me to start off after dark. Then if we have to come down because of some sort of engine trouble or because my new attachment doesn't work right, we sha'n't have any prying eyes."

"I see! Well, Tom, I'll go with you. Fortunately I didn't tell my wife where I was going when I started out this afternoon, so she won't worry until after it's over, and then it won't hurt her. I'm ready any time you are."

"Good! Stay to dinner and I'll show you what I've made. Then we'll take a flight after dark."

This suited the eccentric man, and a little later, after he had eaten one of Mrs. Baggert's best meals, including rice pudding, of which he was very fond, Mr. Damon accompanied Tom to one of the big hangars where the new aeroplane had been set up.

"So that's the Air Scout, is it, Tom?" asked Mr. Damon, as he viewed the machine.

"Yes, that's the girl. 'Air Scout' is as good a name as any, until I see what she'll do."

"It doesn't look different from one of your regular craft of the skies, Tom."

"No, she isn't. The main difference is here," and Tom showed his friend where a peculiar apparatus had been attached to the motor. This was the silencer--the whole secret of the invention, so to speak.

To Mr. Damon it seemed to consist of an amazing collection of pipes, valves, baffle-plates, chambers, cylinders and reducers, which took the hot exhaust gases as they came from the motor and "ate them up," as he expressed it.

"The cylinders, too, and the spark plugs are differently arranged in the motor itself, if you could see them," said Tom to his friend. "But the main work of cutting down the noise is done right here," and he put his hand on the steel case attached to the motor, the case containing the apparatus already briefly described.

"Well, I'm ready when you are, Tom," said Mr. Damon.

"We'll go as soon as it's dark," was the reply. "But first I'll give you a demonstration. Start the motor, Jackson!" Tom called to his chief helper.

Mr. Damon had ridden in aeroplanes before, and had stood near when Tom started them; so he was prepared for a great rush of air as the propellers whirled about, and for deafening explosions from the engine.

The big blades, of new construction, were turned until the gas in the cylinders was sufficiently compressed. Then Jackson stepped back out of danger while Tom threw over the switch.

"Contact!" cried the young inventor.

Jackson gave the blades a quarter pull, and, a moment later, as he leaped back out of the way, they began to revolve with the swiftness of light. There was the familiar rush of air as the wooden wings cut through the atmosphere, but there was scarcely any noise. Mr. Damon could hardly believe his ears.

"I'm not running her at full speed," said Tom. "If I did she'd tear loose from the holding blocks. But you can see what little racket she makes."

"Bless my fountain pen!" cried Mr. Damon. "You are right, Tom Swift! Why, I can hear you talk almost as easily as if no engine were going. And I don't have to shout my head off, either."

This was perfectly true. Tom could converse with Mr. Damon in almost ordinary tones. The exhaust from the motor was nearly completely muffled.

"Out in the air it will seem even more quiet," said Tom. "I'll soon give you a chance to verify that statement."

He ran the engine a little longer, the aeroplane quivering with the vibrations, but remaining almost silent.

"I'm anxious to see what she'll do when in motion," said Tom, as he shut off the gas and spark.

Soon after supper, when the shades of evening were falling, he and Mr. Damon took their places in the first of the Air Scouts, to give it the preliminary test in actual flying.

Would Tom's hopes be justified or would he be disappointed?