le.

"Ready!" came the answer in unison.

Paul touched the little valve in the tank chamber of the fuselage. The
next moment there was a quiver, and then the propeller began fairly to
hum. A strong, steady gust of air began to blow in the faces of the
Giddingses, while they had to hang on grimly in order to keep their
little charge from jumping out of their arms and dashing away into the
air. For fully three minutes the propeller continued to whirl with
undiminished speed, then slowly it began to slow up, and finally
stopped.

Both Mr. Giddings and his son wiped their hot brows as they handed the
plane over to its makers.

"Whew!" said Bob, "that little mule has got a lot of pull to her."

"That she has," supplemented his father. "What sort of material is her
frame made of?"

"Balsa-wood," said John.

"I never heard of that. Is it something new?"

"Yes,--to the arts of civilization, but I presume it has been used by
the Indians of Ecuador, where it grows, for scores of years in the
making of rafts, for which it is particularly well adapted. The tree
looks much like our southern cottonwood, and the wood apparently has no
grain. It has a surprising toughness and strength, and is a trifle
over half the weight of cork, weighing only 7.8 pounds per cubic foot,
while the same sized piece of cork weighs 13.7 pounds."

"Has this wood ever been used in constructing full-sized airplanes?"
asked Mr. Giddings.

"I think not; but Paul and I believe it will be the coming wood for
them," said John with enthusiasm. "We have used it plain on this
machine. On a large airplane it ought to be reinforced with transverse
sections of very thin spruce laid latticewise. That would add
considerably to its natural strength, and increase the total weight
very little."

"H'm, h'm!" said the great newspaper publisher, "this is very
interesting, I am sure. Now let us see how this little affair behaves
itself in the air."

Paul and his brother led the way out into one corner of the big field,
so as to bring what slight breeze might spring up into the head of the
airplane, explaining that machines without a pilot would keep a better
keel under such conditions. John then carefully attached the
bicycle-pump and recharged the air-tank, following which he took out
his watch to time the flight. Mr. Giddings and Bob also took out their
watches.

Paul set the little Sky-Bird down on the hard earth, in a spot where
there was no