at all, sir," assured Paul.

Mr. Giddings began twirling the little twelve-inch two-bladed propeller
at the nose of the model airplane. "What do you use for power to turn
this propeller?" he asked, after admiring its perfect proportions for a
moment. "I don't see any rubber-bands, such as Robert here has always
used on his little machines."

John deftly lifted off the thin veneer hood of the airplane, and
disclosed a very small four-cylindered rotary pneumatic engine of
bewitching simplicity and lightness, which a baby could have held out
in its pudgy palm.

"Paul has worked this little motor out of aluminum and brass and steel,
from odds and ends," said John.

"With more or less help on the part of my elder brother," interjected
Paul.

"Well, perhaps with a little," admitted John, "more suggestive than
otherwise."

"What sets it going?" questioned Bob, curiously.

"The fuselage is divided into three sections," said Paul. "The forward
section contains the engine here; the rear section is an airtight
chamber containing helium; and the central section is also an airtight
chamber, but contains ordinary air which has been pumped into it
through a valve, using the bicycle pump John is carrying, until it is
under strong pressure. When I turn this little valve an outlet is
opened for the air to escape by a tube into branches communicating with
each of these four cylinders. This works the tiny pistons, much the
same as gas in a gasoline-motor, and they turn the little crank-shaft
to which they are connected, and the crank-shaft in turn revolves the
propeller on its end."

"Wonderfully simple!" Mr. Giddings exclaimed. "Wonderfully ingenious,
too! Is this your invention, young men?"

"Partly, sir," replied Paul. "I understand, a company in New York is
making a somewhat similar pneumatic motor for model airplanes, but John
and I have made some radical improvements, to our notion. To-day's
test will tell the story."

"Let's see the propeller spin 'er up once for the fun of it," suggested
Bob. "It won't do any harm, will it? Dad and I will hold on to the
airplane."

"Get a good grip then," warned John Ross, "for you will find there's a
terrific pull to the little rascal. Paul and I tried her in that
fashion early this morning down in the basement."

Bob and his father secured firm holds of the little Sky-Bird, one on
each side, where the propeller could not strike them.

"Ready?" asked Paul, with a smi