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 the manner in which a flying machine gets under
headway--the motor imparts the force necessary to put the machine into
the air, but right here the similarity ceases. If the machine is to be
kept afloat the motor must be kept moving. A flying machine will not
sustain itself; it will not remain suspended in the air unless it is
under headway. This is because it is heavier than air, and gravity draws
it to the ground.

Puzzle in Bird Soaring.

But a bird, which is also heavier than air, will remain suspended, in a
calm, will even soar and move in a circle, without apparent movement
of its wings. This is explained on the theory that there are generally
vertical columns of air in circulation strong enough to sustain a bird,
but much too weak to exert any lifting power on a flying machine, It is
easy to understand how a bird can remain suspended when the wind is
in action, but its suspension in a seeming dead calm was a puzzle
to scientists until Mr. Chanute advanced the proposition of vertical
columns of air.

Modeled Closely After Birds.

So far as possible, builders of flying machines have taken what may
be called "the architecture" of birds as a model. This is readily
noticeable in the form of construction. When a bird is in motion its
wings (except when flapping) are extended in a straight line at ri