"Yes, sir," replied Jack.

"And now you had better report for your geometry lesson," went on
the professor. "I need the laboratory now for a class in
physics. Just tell the janitor to come here and sweep up the
broken glass. I am very glad neither of you boys was seriously
injured. You must be more careful next time."

"Oh, Mark was careful enough," said Jack. "It was all my fault.
I didn't think the gas was quite so powerful."

"All right," answered the professor with a smile as Jack and Mark
passed out on their way to another classroom.

The two lads, whom some of my readers have met before in the
previous books of this series, were friends who had become
acquainted under peculiar circumstances. They were orphans, and,
after having had many trying experiences, each of them had left
his cruel employers, and, unknown to each other previously, had
met in a certain village, where they were obliged to beg for
food. They decided to cast their lots together, and, boarding a
freight train, started West.

The train, as told in the first volume to this series, called
"Through the Air to the North Pole," was wrecked near a place
where a certain Professor Amos Henderson, and his colored helper,
Washington White, lived. Mr. Henderson was a learned scientist
who was constantly building new wonderful machines. He was
working on an airship, in which to set out and locate the North
Pole, when he discovered Jack and Mark, injured in the freight