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
throw it, you will see the clam shell curve downward.

That is the principle on which airships mount upward and descend
while in motion. In a biplane there is either a forward or rear
deflecting rudder, as well as one for steering from side to side.
The latter works an the same principle as does the rudder of a boat
in the water. If this rudder is bent to the right, the craft goes
to the right, because of the pressure of air or water on the rudder
twisted in that direction. And if the rudder is deflected to the
left, the head of the craft takes that direction.

Just as the curve of a clam shell helps it to mount upward, so the
curve of the elevating or depressing rudder on an airship helps it
to go up or down. If the rudder is inclined upward the aeroplane
shoots toward the clouds. When the rudder is parallel to the plane
of the earth's surface, the airship flies in a straight line. When
the rudder is tilted downward, down goes the craft.

I hope I have not wearied you with this description, but it was,
perhaps, needful, to enable those who have never seen an aeroplane
to understand the working principle. One point more. A gasolene
motor, very powerful, is used to whirl the wooden propeller blades
that shove the airship through the air, as the propeller of a motor-
boat shoves that craft through the water.

Faster and faster across the grassy ground went the biplane
containing Dick Hamilton and the army officers. It was necessary
to get this "running start" to acquire enough momentum so that the