But that attachment was not in use at Kentfield then.

The powerful motor hummed and throbbed, for a muffler was temporarily
dispensed with on account of its weight. Every unnecessary ounce
counts on an airship, as it is needful to carry as much oil and
gasolene as possible, and the weight given over to a muffler could be
more advantageously applied to gasolene, on the smaller craft.

Faster and faster whirled the big blades, cutting through the air. The
captain kept his eyes on a balance scale, by which was registered the
pull of the propellers.

"That's enough!" he cried. "Let her go!"

Dick felt the machine move slowly forward on the rubber tired bicycle
wheels over the grassy starting ground, gradually acquiring speed
before it would mount upward into the air.

Perhaps a word of explanation about airships may not be out of place.
Those of you who know the principle on which they work, or who have
seen them, may skip this part if you wish.

The main difference between a balloon and an aeroplane, is that the
balloon is lighter than air, being filled with a very light gas, which
causes it to rise.

An aeroplane is heavier than air, and, in order to keep suspended, must
be constantly in motion. The moment it stops moving forward it begins
to fall downward.

There are several kinds of airships, but the principle ones are
monoplanes and biplanes. Mono means one, and monoplane has but one set
of "wings," being built much after the fashion of a bird.

A biplane, as the name indicates, consists of two sets of planes, one
above the other. There are some triplanes, but they have not been very
successful, and there are some freak aeroplanes built with as many as
eight sets.

If you will scale a sheet of tin, or a thin, flat stone, or even a
slate from a roof, into the air, you will have the simplest form of an
aeroplane. The stone, or tin, is heavier than the amount of air it
displaces, but it stays up for a comparatively long time because it is
in motion. The moment the impulse you have given it by throwing fails,
then it begins to fall.

The engine, or motor, aboard an aeroplane keeps it constantly in
motion, and it glides along through the air, resting on the atmosphere,
by means of the planes or wings.

If you will take a clam shell, and, holding it with the concave side
toward the ground, scale it into the air, you will see it gradually
mount upward. If you hold the convex side toward the ground and