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 as you can."