ont edge
of the wing is stiff and strong, it retains its hollow shape, and
prevents the air from sliding out in this direction, but the pressure of
the air is enough to bend up the thin, flexible ends of the feathers at
the hinder border of the wing, so the air makes its escape there, and
slides out backward and upward. The weight of the bird is all the time
pulling it down toward the earth; so, at the same time that the air
slides out upward and backward past the bent edge of the wing, the wing
itself, and with it the bird, slides forward and downward off from the
confined air. You will have a much better idea of this if you will cut
out a little paper model of a bird's wing and watch the way in which it
falls through the air.

[Illustration]

Take a sheet of stiff paper and cut it in the shape shown in the diagram
above, but considerably larger. Be very careful to have the two sides
alike, so that they shall balance each other. Now fold up the front
margin of each wing, along the dotted lines _a_, _a_, _a_, _a_, to form
a stiff rim to represent the rim of bone along the front edge of a
bird's wing, and cut out a small strip of wood, about as thick as a
match and twice as long, and run this through the two slits, _b_, _b_,
to represent the body of the bird. If you hold this model about three
feet from the ground, and allow it to fall gently, you will see that,
instead of falling straight to the ground, it will slide forward, and
strike the ground two or three feet ahead of you. It is really its
weight which causes it to do this, so that the statement that a bird
flies by its own weight is strictly true.

[Illustration: A SKILLFUL FLYER.]

This is true, also, of insects and bats. They all have wings with stiff
front edges, and flexible hind edges which bend and allow the air to
pass out, so that flying is nothing but sliding down a hill made of air.
A bird rises, then, by flapping its wings, and it flies by falling back
toward the earth and sliding forward at the same time. At the end of
each stroke of its wings it has raised itself enough to make up for the
distance it has fallen since the last stroke, and accordingly it stays
at the same height and moves forward in a seemingly straight line. But
if you watch the flight of those birds which flap their wings slowly,
such as the woodpecker, you can see them rise and fall, and will have no
trouble in seeing that their path is not really a straight line, but is
made u