ght
angles to its body. This brings a sharp, thin edge against the air,
offering the least possible surface for resistance, while at the same
time a broad surface for support is afforded by the flat, under side of
the wings. Identically the same thing is done in the construction of the
flying machine.

Note, for instance, the marked similarity in form as shown in the
illustration in Chapter II. Here A is the bird, and B the general
outline of the machine. The thin edge of the plane in the latter is
almost a duplicate of that formed by the outstretched wings of the bird,
while the rudder plane in the rear serves the same purpose as the bird's
tail.

CHAPTER IV. VARIOUS FORMS OF FLYING MACHINES.

There are three distinct and radically different forms of flying
machines. These are:

Aeroplanes, helicopters and ornithopers.

Of these the aeroplane takes precedence and is used almost exclusively
by successful aviators, the helicopters and ornithopers having been
tried and found lacking in some vital features, while at the same
time in some respects the helicopter has advantages not found in the
aeroplane.

What the Helicopter Is.

The helicopter gets its name from being fitted with vertical propellers
or helices (see illustration) by the action of which the machine is
raised directly from the ground into the air. This does away with the
necessity for getting the machine under a gliding headway before
it floats, as is the case with the aeroplane, and consequently the
helicopter can be handled in a much smaller space than is required for
an aeroplane. This, in many instances, is an important advantage, but it
is the only one the helicopter possesses, and is more than overcome
by its drawbacks. The most serious of these is that the helicopter is
deficient in sustaining capacity, and requires too much motive power.

Form of the Ornithopter.

The ornithopter has hinged planes which work like the wings of a bird.
At first thought this would seem to be the correct principle, and most
of the early experimenters conducted their operations on this line. It
is now generally understood, however, that the bird in soaring is in
reality an aeroplane, its extended wings serving to sustain, as well as
propel, the body. At any rate the ornithoper has not been successful in
aviation, and has been interesting mainly as an ingenious toy. Attempts
to construct it on a scale that would permit of its use by man in actual
aerial fli