flat surface with
a clean, sharp edge. Holding the cardboard at arm's length, withdraw
your hand, leaving the cardboard without support. What is the result?
The cardboard, being heavier than air, and having nothing to sustain
it, will fall to the ground. Pick it up and throw it, with considerable
force, against the wind edgewise. What happens? Instead of falling to
the ground, the cardboard sails along on the wind, remaining afloat so
long as it is in motion. It seeks the ground, by gravity, only as the
motion ceases, and then by easy stages, instead of dropping abruptly as
in the first instance.

Here we have a homely, but accurate illustration of the action of the
flying machine. The motor does for the latter what the force of your arm
does for the cardboard--imparts a motion which keeps it afloat. The only
real difference is that the motion given by the motor is continuous and
much more powerful than that given by your arm. The action of the latter
is limited and the end of its propulsive force is reached within a
second or two after it is exerted, while the action of the motor is
prolonged.

Another Simple Illustration.

Another simple means of illustrating the principle of flying machine
operation, so far as sustentation and the elevation and depression of
the planes is concerned, is explained in the accompanying diagram.

A is a piece of cardboard about 2 by 3 inches in size. B is a piece of
paper of the same size pasted to one edge of A. If you bend the paper
to a curve, with convex side up and blow across it as shown in Figure
C, the paper will rise instead of being depressed. The dotted lines show
that the air is passing over the top of the curved paper and yet, no
matter how hard you may blow, the effect will be to elevate the paper,
despite the fact that the air is passing over, instead of under the
curved surface.

In Figure D we have an opposite effect. Here the paper is in a curve
exactly the reverse of that shown in Figure C, bringing the concave side
up. Now if you will again blow across the surface of the card the action
of the paper will be downward--it will be impossible to make it rise.
The harder you blow the greater will be the downward movement.

Principle In General Use.

This principle is taken advantage of in the construction of all
successful flying machines. Makers of monoplanes and biplanes alike
adhere to curved bodies, with the concave surface facing downward.
Straight planes were