"You must not say such things!" cried the aviator. "You have no
proof!"

"I have all the proof I want as far as he is concerned," declared
Jack. "Maybe he didn't intend to kill us, or hurt us, but he sure
did want to wreck the machine when he tampered with the gyroscope."

"What is the gyroscope?" asked Dick.

"It is an invention of mine, and one over which Lieutenant Larson
and I had some argument," said Mr. Vardon.

"You probably know," the aviator went on, while Dick, Paul, and
Innis, with several other cadets, listened interestedly, "you
probably know that one of the great problems of aviation is how to
keep a machine from turning turtle, or turning over, when it strikes
an unexpected current, or 'air pocket' in the upper regions. Of
course a birdman may, by warping his wings, or changing the
elevation of his rudder, come out safely, but there is always a
chance of danger or death.

"If there was some automatic arrangement by which the airship would
right itself, and take care of the unexpected tilting, there would
be practically no danger.

"I realized that as soon as I began making airships, and so I
devised what I call a gyroscope equilibrizer or stabilizer. A
gyroscope, you know, is a heavy wheel, spinning at enormous speed,
on an anti-friction axle. Its great speed tends to keep it in
stable equilibrium, and, if displaced by outside forces, it will