Tom rushed from his private office, and when he reached the outer door he heard with more distinctness the sounds that had alarmed him. They seemed to come from a small building given over to electrical apparatus, and which, at the time, was not supposed to be in use. It had been Tom's workroom, so to speak, when he was developing his electric runabout and rifle, but of late he had not spent much time in it.

"Somebody's in there !" reflected the young inventor, as he heard yells coming from the open door of the place. "And if it isn't Koku and Eradicate I miss my guess! Wonder what they can be doing there."

He crossed the yard between his private office and the electrical shop in a few rapid strides, and, as he entered the latter place, he was greeted with a series of wild yells.

"Good volume of sound here, at all events," mused Tom. "Almost as much as my motor made when I was trying to talk to Mary. Hello there! What's going on? Is any one hurt? What's the matter?" he cried, for, at first, he could see no one in the dim light of the place. The interior was a maze of electrical apparatus.

"Who's here?" demanded Tom, as he advanced.

"Oh, Master! Come quick! Koku 'most dead an' no can let go!" was the cry.

"Yo' jest bet yo' cain't let go!" chimed in the voice of Eradicate. "I done knowed yo would git into trouble ef yo' come heah, an' I'se glad ob it! So I is!"

"What is it, Rad? What has happened to Koku?" cried Tom, running forward, for though no very powerful current could be turned on in the electrical shop at this period of unuse, there was enough to be very painful. "What is it, Rad?"

"Oh, dat big foolish giant, Koku, done got his se'f into trouble!" chuckled the colored man. "He done got holt ob one ob dem air contraptions, Massa Tom, an' he cain't let go! Ha! Ha! Golly! Look at him squirm!" and Rad laughed shrilly, which accounted for some of the sounds Tom had heard.

Then came yells of rage and pain from the giant, and they were so loud and vigorous, mingling with Eradicate's as they did, that it was no wonder Tom was startled. The sounds were heard in the other shops, and men came running out. But before then Tom had put an end to the trouble.

One look showed him what had happened. Just how or why Koku and Eradicate had entered the electrical shop Tom did not then stop to inquire. But he saw that the giant had grasped the handles of one of the electric machines, designed for charging Leyden jars used in Tom's experiments, and the powerful, though not dangerous, current had so paralyzed, temporarily, the muscles of the giant's hands and arms that he could not let go, and there he was, squirming, and not knowing how to turn off the current, and unable to ease himself, while Eradicate stood and laughed at him, fairly howling with delight.

"Ha! Guess yo' won't do no mo' spadin' in' Massa Tom's garden right away, big man!" taunted Eradicate.

"Be quiet, Rad!" ordered Tom, as he reached up and pulled out the switch, thus shutting off the current. "This isn't anything to laugh at."

"But he done look so funny, Massa Tom!" pleaded the colored man. "He done squirm laik--"

But Eradicate did not finish what he intended to say. Once free from the powerful current, the giant looked at his numb hands, and then, seeming to think that Eradicate was the cause of it all, he sprang at the colored man with a yell. But Eradicate did not stay to see what would happen. With a howl of terror, he raced out of the door, and, old and rheumatic as he was, he managed to gain the stable of his mule, Boomerang, over which he had his humble but comfortable quarters.

"Well, I guess he's safe for a while!" laughed Tom, as he saw the giant turn away, shaking his fist at the closed door, for Koku, big as he was, stood in mortal terror of the mule's heels.

Tom locked the door of the electrical shop and Went back to his interrupted problem. From Jackson he learned that Koku and Eradicate had merely happened to stroll into the forbidden place, which had been left open by accident. There, it appeared, Koku had handled some of the machinery, ending by switching on the current of the machine the handles of which he later unsuspectingly picked up. Then he received a shock he long remembered, and for many days he believed Eradicate had been responsible for it, and there was more than the usual hostile feeling between the two. But Eradicate was innocent of that trick, at all events.

"Though," said Tom, telling his father about it later, "Rad would have turned on the current if he had known he could make trouble for Koku by it. I never saw their like for having disagreements!"

"Yes, but they are both devoted to you, Tom," said the aged inventor. "But what is this you hinted at--a silent motor you called it, I believe? Are you really serious in trying to invent one?"

"Yes, Dad, I am. I think there's a big field for an aeroplane that could travel along over the enemy's lines--particularly at night--and not be heard from below. Think of the scout work that could be done.

"Well, yes, it could be done if you could get a silent motor, or propellers that made no noise, Tom. But I don't believe it can be done."

"Well, maybe not, Dad. But I'm going to try!" and Tom, after a further talk with his father, began work in earnest on the big problem. That it was a big one Tom was not disposed to deny, and that it would be a valuable invention even his somewhat skeptical father admitted.

"How are you going to start, Tom?" asked Mr. Swift, several days after the big idea had come to the young man.

"I'm going to experiment a bit, at first. I've got a lot of old motors, that weren't speedy enough for any of my flying machines, and I'm going to make them over. If I spoil them the loss won't amount to anything, and if I succeed --well, maybe I can help out Uncle Sam a bit more."

As Tom had said he would do, he began at the very foundation, and studied the fundamental principles of sound.

"Sound," the young inventor told Ned Newton, in speaking about the problem, "is a sensation which is peculiar to the ear, though the vibrations caused by sound waves may be felt in many parts of the body. But the ear is the great receiver of sound."

"You aren't going to invent a sort of muffler for the ears, are you, Tom?" asked Ned. "That would be an easy way of solving the problem, but I doubt if you could get the Germans to wear your ear-tabs so they wouldn't hear the sound of the Allied aeroplanes."

"No, I'm not figuring on doing the trick that way," said Tom with a laugh. "I've really got to cut down the sound of the motor and the propeller blades, so a person, listening with all his ears, won't hear any noise, unless he's within a few feet of the plane."

"Well, I can tell you, right off the reel, how to do it," said the bank employee.

"How?" asked Tom eagerly.

"Run your engine and propellers in a vacuum," was the prompt reply.

"Hum!" said Tom, musingly. "Yes, that would be a simple way out, and I'll do it, if you'll tell me how to breathe in a vacuum."

"Oh, I didn't agree to do that," laughed Ned.

But he had spoken the truth, as those who have studied physics well know. There must be an atmosphere for the transmission of sound, which is the reason all is cold and silent and still at the moon. There is no atmosphere there. Sound implies vibration. Something, such as liquid, gas, or solid, must be set in motion to produce sound, and for the purpose of science the air we breathe may be considered a gas, being composed of two.

Not only must the object, either solid, liquid, or gaseous, be in motion to produce sound, but the air surrounding the vibrating body must also be moving in unison with it. And lastly there must be some medium of receiving the sound waves--the ear or some part of the body. Totally deaf persons may be made aware of sound through the vibrations received through their hands or feet. They receive, of course, only the more intense, or largest, sound waves, and can not hear notes of music nor spoken words, though they may feel the vibration when a piano is played. And, as Ned has said, no sound is produced in a vacuum.

"But," said Tom, "since I can't run my aeroplane in a vacume, or even have the propellers revolve in one, it's up to me to solve the problem some other way. The propellers don't really make noise enough to worry about when they're high in the air. It's the exhaust from the motor, and to get rid of that will be my first attempt."

"Can it be done?" asked Ned.

"I don't know," was Tom's frank answer.

"They do it on an automobile to a great extent," went on Ned. "Some of 'em you cant hardly hear."

"Yes, but an aeroplane engine runs many, many times faster than the motor of an auto," said Tom, "and there are more explosions to muffle. I doubt if the muffler of an auto would cut down the sound of an aero engine to any appreciable extent. But, of course, I'll try along those lines."

"They have mufflers or silencers for guns and rifles," went on Ned. "Couldn't you make a big one of those contraptions and put it on an aeroplane?"

"I doubt it," said Tom, shaking his head. "Of course it's the same principle as that in an auto muffler, or on a motor boat--a series of baffle plates arranged within a hollow cylinder. But all such devices cut down power, and I don't want to do that. However, I'm going to solve the problem or--bust!"

And Tom came near "busting," Ned remarked later, when he and his friend talked over the progress of the invention.

Two weeks had passed since the start of his evolution of his new idea, and following the visiting of the representatives of the Universal Flying Machine Company. Since then neither Gale nor Ware had communicated with Tom.

"But I must be on the watch against them," thought the young inventor. "I'm pretty sure Gale heard me mention what I was going to try to invent, and he may get ahead of me, and put a silent motor on the market first. Not that I'm afraid of being done out of any profits, but I simply don't want to be beaten."

The details of Tom's invention cannot be gone into, but, roughly, it was based on the principle of not only a muffler but also of producing less noise when the charges of gasoline exploded in the cylinders. It is, of course, the explosion of gasoline mixed with air that causes an internal combustion engine to operate. And it is the expulsion of the burned gases that causes the exhaust and makes the noise that is heard.

Tom was working along the well-known line of the rate of travel of sound, which progresses at the rate of about 1090 feet a second when air is at the freezing point. And, roughly, with every degree increase in the atmosphere's temperature the velocity of sound increases by one foot. Thus at a temperature of 100 degrees Fahrenheit, or 68 degrees above freezing, there would be added to the 1090 feet the 68 feet, making sound travel at 100 degrees Fahrenheit about 1158 feet a second.

Tom had set up in his shop a powerful, but not very speedy, old aeroplane engine, and had attached to it the device he hoped would help him toward solving his problem of cutting down the noise. He had had some success with it, and, after days and nights of labor, he invited his father and Ned, as well as Mr. Damon, over to see what he hoped would be a final experiment.

His visitors had assembled in the shop, and Eradicate was setting out some refreshments which Tom had provided, the colored man being in his element now.

"What's all this figuring, Tom ?" asked Mr. Damon, as he saw a series of calculations on some sheets of paper lying on Tom's desk.

"That's where I worked out how much faster sound traveled in hydrogen gas than in the ordinary atmosphere," was the answer. "It goes about four times as fast, or nearly four thousand two hundred feet a second. You remember the rule, I suppose. 'The speed of sonorous vibrations through gases varies inversely as the squares of the weights of equal volumes of the gases,' or, in other words--"

"Give it to us chiefly in 'other words,' if you please, Tom!" pleaded Ned, with a laugh. "Let that go and do some tricks. Start the engine and let's see if we can hear it."

"Oh, you can hear it all right," said Tom, as he approached the motor, which was mounted on a testing block. "The thing isn't perfected yet, but I hope to have it soon. Rad! Where is that black rascal? Oh, there you are! Come here, Rad!"

"Yaas sah, Massa Tom! Is I gwine to help yo' all in dish yeah job?"

"Yes. Just take hold of this lever, and when I say so pull it as hard as you can."

"Dat's whut I will, Massa Tom. Golly! ef dat no 'count giant was heah now he'd see he ain't de only one whut's got muscle. I'll pull good an' hard, Massa Tom."

"Yes, that's what I want you to. Now I guess we're all ready. Can you see, Dad--and Ned and Mr. Damon?"

"Yes," they answered. They stood near the side wall of the shop, while Tom and Eradicate were at the testing block, on which the motor, with the noise-eliminating devices attached, had been temporarily mounted.

"All ready," called the young inventor, as he turned on the gas and threw over the electrical switch. "All ready! Pull the starting lever, Rad. and when it's been running a little I'll throw on the silencer and you can see the difference."

The motor began to hum, and there was a deafening roar, just as there always is when the engine of an aeroplane starts. It was as though half a dozen automobile engines were being run with the mufflers cut out.

"Now I'll show you the difference!" yelled Tom, though such was the noise that not a word could be heard. "This shows you what my silencer will do."

Tom pulled another lever. There was at once a cessation of the deafening racket, though it was not altogether ended. Then, after a moment or two, there suddenly came a roar as though a blast had been let off in the shop.

Tom and Eradicate were tossed backward, head over heels, as though by the giant hands of Koku himself, and Mr. Damon, Ned, and Tom's father saw the motor fly from the testing block and shoot through the roof of the building with a rending, crashing, and splintering sound that could be heard for a mile.