Some have these movements in a greater or
less degree, or capable of a greater range; but
the joint is the same, with scarcely an exception.
When the stroke of the wing is downwardly the
rear margin is higher than the front edge, so
that the downward beat not only raises the body
upwardly, but also propels it forwardly.

THE WING MOVEMENT.--The moment the wing
starts to swing upwardly the rear end is
depressed, and now, as the bird is moving forwardly,
the wing surface has a positive angle of
incidence, and as the wing rises while the forward
motion is taking place, there is no resistance
which is effective enough to counteract the
momentum which has been set up.

The great problem is to put this motion into a
mechanical form. The trouble is not ascribable
to the inability of the mechanic to describe this
movement. It is an exceedingly simple one.
The first difficulty is in the material that must
be used. Lightness and strength for the wing
itself are the first requirements. Then rigidity
in the joint and in the main rib of the wing, are
the next considerations.

In these respects the ability of man is limited.
The wing ligatures of flying creatures is exceedingly
strong, and flexible; the hollow bone formation