"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.