sure of my catapult, and we will try the two contrivances together."

In the morning all rose bright and early and prepared the _Snowbird_ for
her trial flight. Washington White had indeed disappeared--possibly in
search of his Shanghai rooster--and Andy Sudds was off on a hunt.
Therefore the professor and his two young comrades essayed the trip
alone.

Jack and Mark tossed a coin to see who should first guide the great
air machine, and Mark won the preference. He, as well as his chum and
the professor, had already donned their aeronautic uniforms, and he
now strapped himself into the pilot's seat. The steering apparatus,
the levers that controlled the planes, and the motor switch were all
under his hand. While in flight the _Snowbird_ need be under the
control of but one person at a time.

The professor had rigged his catapult so that he could release the
trigger from the flying machine. Mark said he was ready; the professor
reached for the cord which would release the trigger.

"Start your motor, Mark, a fraction of a second before I release the
compressed air," commanded Mr. Henderson. "Now!"

The motor of the flying machine buzzed faintly. Jack's eyes were on
the speed indicator. He suddenly felt the great, quivering flying
machine, which had been run out of the hangar on to the steel plank
of the catapult, lurch forward. The feeling affected him just as the
sudden dropping of an elevator from a great height affects its
passengers.