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."