to examine only one of these,--the way in which
the wing is adapted to its use.

Did you ever look at a bird's wing carefully, and try to find out from
it the way in which it is used? People usually suppose, either that a
bird flies because it is lighter than the air, like a balloon, or that
it rows itself along as a boat is rowed through the water. Neither of
these suppositions is true. A bird is not lighter than the air, and does
not float; for when a bird is shot on the wing it falls to the ground
just as quickly as a squirrel. On the contrary, a bird flies by its own
weight, and could not fly at all if it were not heavier than the air.

You know that when you move a large, flat surface rapidly through the
air, it meets with considerable resistance. A bird's wing is so large,
and is moved so rapidly, that the resistance of the air is enough to
raise the bird a short distance each time the wings are flapped
downward; but after each down-flap there must be an up-flap, and the air
resists this just as it does the down-flap; so, unless there were some
arrangement to prevent it, the bird would drive itself down each time it
raised its wings, just as far as it had raised itself by the down-stroke
before, so that it would never get into the air at all. To meet this
difficulty, the wing is so shaped that it is concave or hollow upon its
lower surface, so that it gathers the air together and prevents it from
escaping; while the upper surface is convex or bulging, so that the air
slides off from it when the wing is moved upward. If you have ever been
caught in a sudden squall of wind with an open umbrella, you will easily
understand how great a difference in resisting power this difference in
the shape of the two sides of the wing will make. As long as you can
keep the bulging side of the umbrella pointed toward the wind, you find
no difficulty in holding it; but if the wind strikes the hollow
under-side of the umbrella, it pulls so violently that, unless you are
able to turn around and face the wind, the chances are that the umbrella
will either be pulled away from you or turned inside out. But in the
latter case, the wind slides out over the edges again, so that there is
no trouble in holding on to the umbrella.

The peculiar shape of the wing is only one of the ways by which the
down-stroke is made to strike the air with more force than the
up-stroke. If you will look at a quill-feather, you will see that, on
each side of