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.
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