were about seventy inches in
diameter, while the diameter of the leading and following truck-wheels
was but half that number of inches.

Mr. Swift had turned away from the locomotive when Tom put his head out
of the door again.

"Do you hear that, father?" he demanded in a puzzled tone.

"Hear what, Tom?" asked the old inventor, looking up.

"That ticking sound? I declare, I'd think it was one of those
death-watch beetles had got in here. Sounds like a big watch ticking. I
can't make it out."

"Where is it? What is it?" repeated Mr. Swift. "I hear nothing down
here on the floor of the shed."

"Well, it gets me," muttered Tom, and disappeared again. In a moment he
called out: "Say, you fellows! who left his bundle of overalls in here?
Better take 'em out to be manicured. Whose are these?"

Two or three of the mechanics working near looked up from their tasks.
Mr. Swift turned back to the door of the cab again.

"What is the matter now, Tom?" he asked, in added curiosity.

"That bundle, Dad."

Tom once more appeared and addressed the workmen: "Whose bundle of
dirty overalls is this in here? Come and take 'em away. They shouldn't
have been left here."

"Why, Mr. Tom," said the foreman who was near, "I didn't see any soiled
overalls in there when I left last evening. Any of you fellows," he
asked the group of hands, "know anything about any overalls?"

"The bundle is here all right. Pushed back against the third series
motors. Come up here, one of you fellows--"

Suddenly there was a noise at the end of the shed where the door to the
offices lay. Two figures burst through from the glass doors and charged
down the lanes between the lathes and cranes. Ned Newton led, Rad
Sampson, his face a mouse-gray with fear, followed.

"Massa Tom! Massa Tom!" shouted the colored man. "Look out fo' de bomb!
Look out fo' de bomb!"

The foreman sprang toward the high door of the locomotive where Tom
stood, staring out. The young inventor, quick as his mind usually
functioned, did not understand at all what Eradicate meant.

"There's something wrong in there, Mr. Tom!" shouted the foreman. "Come
down, sir, and let me get up there and see what it is."

But Mr. Barton Swift grasped the meaning of what was going on more
quickly than anybody else. Tom's father, Tom frequently said, had spent
so many years investigating chemical and mechanical mysteries that he
saw more clearly and more exactly into and through most problems