or shape, or area of the wing, is also beyond question.
The records show that every conceivable
type of outlined structure is used by nature; the
material and texture of the wings themselves differ
to such a degree that there is absolutely no
similarity; some have concaved under surfaces,
and others have not; some fly with rapidly beating
wings, and others with slow and measured
movements; many of them fly with equal facility
without flapping movements; and the proportions
of weight to wing surface vary to such an extent
that it is utterly impossible to use such data as a
guide in calculating what the proper surface
should be for a correct flying machine.

THE ANGLE OF MOVEMENT.--How, then, it may
be asked, do they get their support? There must
be something, in all this variety and diversity of
form, of motion, and of characteristics, which
supplies the true answer. The answer lies in the
angle of movement of every wing motion, which
is at the control of the bird, and if this is examined
it will be found that it supplies the correct
answer to every type of wing which nature has
made.

AN INITIAL IMPULSE OR MOVEMENT NECESSARY.--
Let A, Fig. 9, represent the section of a bird's
wing. All birds, whether of the soaring or the
flapping kind, must have an initial forward movement