, it
was based on the principle of not only a muffler but also of producing
less noise when the charges of gasoline exploded in the cylinders. It
is, of course, the explosion of gasoline mixed with air that causes an
internal combustion engine to operate. And it is the expulsion of the
burned gases that causes the exhaust and makes the noise that is heard.

Tom was working along the well-known line of the rate of travel of
sound, which progresses at the rate of about 1090 feet a second when
air is at the freezing point. And, roughly, with every degree increase
in the atmosphere's temperature the velocity of sound increases by one
foot. Thus at a temperature of 100 degrees Fahrenheit, or 68 degrees
above freezing, there would be added to the 1090 feet the 68 feet,
making sound travel at 100 degrees Fahrenheit about 1158 feet a second.

Tom had set up in his shop a powerful, but not very speedy, old
aeroplane engine, and had attached to it the device he hoped would help
him toward solving his problem of cutting down the noise. He had had
some success with it, and, after days and nights of labor, he invited
his father and Ned, as well as Mr. Damon, over to see what he hoped
would be a final experiment.

His visitors had assembled in the shop, and Eradicate was setting out
some refreshments which Tom had provided, the colored man being in his
element now.

"What's all this figuring, Tom?" asked Mr. Damon, as he saw a series of
calculations on some sheets of paper lying on Tom's desk.

"That's where I worked out how much faster sound traveled in hydrogen
gas than in the ordinary atmosphere," was the answer. "It goes about
four times as fast, or nearly four thousand two hundred feet a second.
You remember the rule, I suppose. 'The speed of sonorous vibrations
through gases varies inversely as the squares of the weights of equal
volumes of the gases,' or, in other words--"

"Give it to us chiefly in 'other words,' if you please, Tom!" pleaded
Ned, with a laugh. "Let that go and do some tricks. Start the engine
and let's see if we can hear it."

"Oh, you can hear it all right," said Tom, as he approached the motor,
which was mounted on a testing block. "The thing isn't perfected yet,
but I hope to have it soon. Rad! Where is that black rascal? Oh, there
you are! Come here, Rad!"

"Yaas sah, Massa Tom! Is I gwine to help yo' all in dish yeah job?"

"Yes. Just take hold of this lever, and when I say so pull it as hard