ng 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