accepted the
challenge of the _Clarion_, in fact. Is the Sky-Bird supplied with
gasoline?"

"Yes, sir," said John; "we just got through with that job. We have
easily enough fuel aboard now for a couple of hours' flight, and that
will be long enough for a first one. New engines are always 'stiff'
and should not be run too long at a stretch."

"Have you run this pair yet?"

"Oh, yes," said Bob. "We have tried them out several times, dad, and
in connection with the propeller, too. They work tip-top, either
connected or disconnected. I tell you, when they're in connection they
certainly do make this big propeller hum!"

"I can't understand how you can operate the propeller in here," said
Mr. Giddings, much puzzled. "All the airplanes I have seen have always
dashed forward as soon as their propellers began to revolve under
impulse of the motor or motors; there was no restraining them. I
should think this machine would run through the front end of the hangar
here as soon as you--"

"Pardon me, sir," interrupted John, "but we have gone those fellows one
better. You forget that in the drawings we showed you there was a set
of brakes designed to be worked by a control within reach of the pilot,
brakes which will engage these ground wheels a good deal the same as
brakes work on automobiles--by a flexible band of steel and grit-filled
cotton which is made to compress over a large sort of hub on the inner
side of each wheel."

"Very good," said Mr. Giddings; "but I understand that has been tried
before, with the result that the airplane at once tipped forward and
stuck its nose into the ground, or rather tried to, smashing its
propeller to smithereens."

"They will do that every time unless something has been devised to
counteract this tendency to pitch over," explained John. "We have
devised the thing to prevent it, Mr. Giddings."

"See here, dad," put in Bob at this point. "Stoop down a bit and look
under the forward end of the body here."

His father did as requested, and Bob pointed out a circular opening
about the size of a saucer, from which protruded the end of an
aluminum-encased shaft bearing a small rubber-tired wheel of very
sturdy proportions.

"That is our preventer, dad," smiled his son.

"In a few minutes we'll show you how it works," added John Ross. "I
see you are wearing a cap, sir, as I suggested. That is all the
special dress you will need, as our enclosed cabin makes helmets and
clo