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 tried for a time, but found greatly lacking in the
power of sustentation. By curving the planes, and placing
the concave surface downward, a sort of inverted bowl
is formed in which the air gathers and exerts a buoyant
effect. Just what the ratio of the curve should be is a
matter of contention. In some instances one inch to the
foot is found to be satisfactory; in others this is doubled,
and there are a few cases in which a curve of as much as
3 inches to the foot has been used.

Right here it might be well to explain that the word
"plane" applied to flying machines of modern construction
is in reality a misnomer. Plane indicates a flat,
level surface. As most successful flying machines have
curved supporting surfaces it is clearly wrong to speak
of "planes," or "aeroplanes." Usage, however, has made
the terms convenient and, as they are generally accepted
and understood by the public, they are used in like manner
in this volume.

Getting Under Headway.

A bird, on first rising from the ground, or beginning
its flight from a tree, will flap its wings to get under
headway. Here again we have another illustration of