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 right