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