"All ready, Mr. Damon?" asked Tom, as he looked to see that all the levers, wheels, valves, and other controls were in working order on his Air Scout.

"As ready as I ever shall be, Tom," was the answer. "I don't know why it is, but somehow I feel that something is going to happen on this trip."

"Nonsense!" laughed Tom. "You're nervous; that's all."

"I suppose so. Don't think I'm going to back out, or anything like that, but I wish it were successfully over with, Tom Swift, I most certainly do."

"It will be in a little while," returned Tom, as he settled himself comfortably in his seat and pulled the safety strap tight. "You've gone up in this same plane before, when it didn't have the silent motor aboard."

"Yes, I know I have. Oh, I dare say it will be all right, Tom. And yet, somehow, I can't help feeling--"

But Tom Swift felt that the best way to set Mr. Damon's premonitions to rest was to start the motor, and this he gave orders to have done, Jackson and some others of the men from the shops congregating about the craft to see the beginning of the night flight. Mr. Swift was there also, and Eradicate. Mary Nestor had been invited, but her Red Cross work engaged her that evening, she said. Ned Newton was away from town on Liberty Bond business, and he could not be present at the test.

However, as Tom expected to have other trials when his motor was in even better shape, he was not exactly sorry for the absence of his friends.

"Contact!" called the young inventor, when Jackson had stepped back, indicating it was time to throw over the switch.

"Let her go!" cried Tom, and the next moment the motor was in operation, but so silently that his voice and that of Mr. Damon's could easily be heard above the machinery.

"Good, Tom! That's good!" cried Mr. Swift, and Tom easily heard his father's voice, though under other, and ordinary, circumstances this would have been impossible.

True, the hearing of Tom and Mr. Damon was muffled to a certain extent by the heavy leather and fur-lined caps they wore. But Tom had several small eyelet holes set into the flaps just over the opening of the ears, and these holes were sufficient to admit sounds, while keeping out most of the cold that obtains in the upper regions.

The aeroplane moved swiftly along the level starting ground, and away from the lighted hangars. Faster and faster it swung along as Tom headed it into the wind, and then, as the speed of the motor increased, the Air Scout suddenly left the earth and went soaring aloft as she had done before.

But there was this difference. She moved almost as silently as a great owl which swoops down out of the darkness--a bit of the velvety blackness itself. Up and up, and onward and onward, went the Air Scout. Tom Swift's improved, silent motor urged it onward, and as the young inventor listened to catch the noise of the machinery, his heart gave a bound of hope. For he could detect only very slight sounds.

"She's a success!" exulted Tom to himself. "She's a success, but she isn't perfect yet," he added. "I've got to make the muffler bigger and put in more baffle-plates. Then I think I can turn the trick."

He swung the machine out over the open country, and then, when they were up at a height and sailing along easily, he called back to Mr. Damon in the seat behind him:

"How do you like it?"

"Great!" exclaimed the eccentric man. "Bless my postage stamp, but it's great! Why, there's hardly a sound, Tom, and I can hear you quite easily."

"And I can hear you," added Tom. "I don't believe, down below there," and he nodded toward the earth, though Mr. Damon could not see this, as the airship, save for a tiny light over the instrument board, was in darkness, "they know that we're flying over their heads."

"I agree with you," was the answer. "Tom, my boy, I believe you've solved the trick! You have produced a silent aeroplane, and now it's up to the government to make use of it."

"I'm not quite ready for that yet," replied the young inventor. "I have several improvements to make. But, when they are finished, I'll let Uncle Sam know what I have. Then it's up to him."

"And you must be careful, Tom, that some of your rivals don't hear of your success and get it away from you," warned Mr. Damon, as Tom guided the Air Scout along the aerial way--an unlighted and limitless path in the silent darkness.

"Oh, they'll have to get up pretty early in the morning to do that!" boasted Tom, and afterward he was to recall those words with a bit of chagrin.

On and on they sailed, and as Tom increased the speed of the motor, and noted how silently it ran, he began to have high hopes that he had builded better than he knew. For even with the motor running at almost full speed there was not noise enough to hinder talk between himself and Mr. Damon.

Of course there was some little sound. Even the most perfect electric motor has a sort of hum which can be detected when one is close to it. But at a little distance a great dynamo in operation appears to be silence itself.

"I can go this one better, though," said Tom as he sailed along in the night. "I see where I've made a few mistakes in the baffle plate of the silencer. I'll correct that and--"

As he spoke the machine gave a lurch, and the motor, instead of remaining silent, began to cough and splutter as in the former days.

"Bless my rubber boots, Tom! what's the matter?" cried Mr. Damon.

"Something's gone wrong," Tom answered, barely able to hear and make himself heard above the sudden noise. "I'll have to shut off the power and glide down. We can make a landing in this big field," for just then the moon came out from behind a cloud, and Tom saw, below them, a great meadow, not far from the home of Mary Nestor. He had often landed in this same place.

"Something has broken in the muffler, I think, letting out some of the exhaust," he said to Mr. Damon, for, now that the motor was shut off, Tom could speak in his ordinary tones. "I'll soon have it fixed, or, if I can't, we can go back in the old style-- with the machine making as much racket as it pleases."

So Tom guided the machine down. It went silently now, of course, making, with the motor shut off, no more sound than a falling leaf. Down to the soft, springy turf in the green meadow Tom guided the machine. As it came to a stop, and he and Mr. Damon got out, there was borne to their ears a wild cry:

"Help! Help!"