the central shaft or quill, there is a broad, thin portion,
which is called the _vane_. The vane on one side of the shaft is quite
broad and flexible, while that on the other side is narrow and stiff;
and by looking at a wing with the feathers in their places, you will
find that they are placed so that they overlap a little, like the slats
on a window-blind. Each broad vane runs under the narrow vane of the
feather beside it, so that, when the wing is moved downward, each
feather is pressed up against the stiff narrow vane of the one beside
it, and the whole wing forms a solid sheet like a blind with the slats
closed. After the down-stroke is finished and the up-stroke begins, the
pressure is taken off from the lower surface of the wing, and begins to
act on the upper surface and to press the feathers downward instead of
upward. The broad vanes now have nothing to support them, and they bend
down and allow the air to pass through the wing, which is now like a
blind with the slats open. By these two contrivances,--the shape of the
wing, and the shape and arrangement of the feathers,--the wing resists
the air on its down-stroke and raises the bird a little at each flap,
but at each up-stroke allows the air to slide off at the sides, and to
pass through between the feathers, so that nothing is lost.

[Illustration: QUAIL (SCRATCHERS).]

So much for the way in which the bird is raised into the air. Rising in
the air is not flying, for a balloon and a kite rise but do not fly.
Now, how is a bird able to move forward? This is not quite as easy to
understand as the other, but I hope to be able to make it clear to you.
I must first say, however, that it is not done by rowing with the wings,
for they move up and down, not backward and forward, and no amount of
rowing up and down would drive a bird forward, any more than rowing
backward and forward would lift a boat up into the air.

You will find, if you carefully examine a bird's wing, that all the
bones and muscles are placed along the front edge, which is thus made
very stiff and strong. The quill feathers are fastened in such a way
that they point backward, so that the hind edge of the wing is not stiff
like the front edge, but is flexible and bends at the least touch. As
the air is not a solid, but a gas, it has a tendency to slide out from
under the wing when this is driven downward, and of course it will do
this at the point where it can escape most easily. Since the fr