"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."
"Dat's whut I will, Massa Tom. Golly! ef dat no 'count giant was heah now he'd see he ain't de only one whut's got muscle. I'll pull good an' hard, Massa Tom."
"Yes, that's what I want you to. Now I guess we're all ready. Can you see, Dad—and Ned and Mr. Damon?"
"Yes," they answered. They stood near the side wall of the shop, while Tom and Eradicate were at the testing block, on which the motor, with the noise-eliminating devices attached, had been temporarily mounted.
"All ready," called the young inventor, as he turned on the gas and threw over the electrical switch. "All ready! Pull the starting lever, Rad, and when it's been running a little I'll throw on the silencer and you can see the difference."