"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 aboutwhen 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.
