"That, boys," said the scientist, with some pride, "is a modern
catapult--an up-to-the-minute catapult which, had it been known to the
ancients, would have enabled the hosts of Joshua, for instance, to
batter down the walls of Jericho without the trouble of marching so
many times around the city."

"And what has a compressed air catapult got to do with the
_Snowbird_?" queried Jack. "You propose launching your flying machine in
the usual way," said the professor. "I see you have wheel trucks all
ready to slip under her. We will not use those wheels, boys. I have a
better plan. We will launch the _Snowbird_ into the air from my
catapult."

"Great goodness, Professor!" cried Mark. "Is that practicable?"

"We'll know after we have tried it," retorted Professor Henderson,
drily.

"How did you happen to start working on this catapult idea?" asked
Jack.

"Well, I can't tell you everything," replied the inventor, "for it is
partly a secret."

"Huh," laughed Mark. "You're mysterious. You haven't joined forces
with some department of our government, or with another country?"

The professor smiled, thinking how keen this young man always proved
himself to be.