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 vacuum, 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