FLYING AND SWIMMING.
You can jump off from the ground just a little way into the air, but
you can not fly into it, as the birds do. It is because you have no
wings. But how is it that the birds fly with their wings? They push
themselves up with them into the air. But perhaps you will say that
they do not have any thing to push against, for there is nothing but
air about them. Now it is the air itself that they push against. They
press down upon the air with their wings, just as you press with your
feet on the ground when you jump up; and as the bird, when it gets
once started, keeps working its wings, it goes up and up, pushing down
against the air each time that its wings are moved.
[Sidenote: How wings raise the birds in the air.]
[Sidenote: Why they are so large.]
It is necessary that birds should have very large wings to raise
themselves up thus in the air. If their wings were small, they would do
no good, because they would not press upon enough of the air. You can
move your hands in the same way that the bird does its wings, but you
can not raise yourself off from the ground. Why? Because your hands are
so small that they press only upon a little of the air. If your hands
were as broad for you as the wings of birds are for them, and you had
the proper muscles to work them, you could fly.
[Sidenote: Flying in water.]
[Sidenote: The kite.]
You can learn to fly, but it is in the water, and not in the air, that
you can do it. Swimming is really flying in water. The hands and feet
do for the swimmer what the wings do for the bird. He presses against
the water with his hands and feet in the same way that the bird does
against the air with its wings. Sometimes you see a bird dive down from
a great way up in the air, in the same way that the swimmer does in the
water. When it does this its wings are very still, and are folded close
to its side, as you see here in the kite; but when it goes up again it
works its wings up and down, just as the swimmer works his feet and
hands when he is rising in the water.
[Illustration]
[Sidenote: The tail of a fish like a sculling oar.]
Fishes swim chiefly with their tails. The tail is to a fish in the
water what wings are to a bird in the air. It acts like a sculling
oar in a boat, as I told you in Part Second, Chapter XXIII. The fins
are the balancers, while the tail works the fish forward by its quick
movements to one side and the other. You can see this very plainly if
you watch gold-fishes as you see them in a glass vessel.
[Sidenote: Why we can not fly in the air with our hands.]
Observe why it is that you can not fly with your hands in the air in
the same way that you can swim with them in the water. The water gives
way under your hands just as the air does, but the air gives way much
more easily than the water, because it is so much lighter. As the air
gets out of the way so easily, you can not fly in it unless you have
something very broad, so as to press down on a great deal of it at the
same time. To fly, you must have large wings instead of small hands.
You can see what a difference there is between hands and wings by
trying a little experiment. Move about your hand in the air. You do it
with perfect ease, and the air does not seem to resist the hand at all.
Now take a large palm-leaf fan and move that about. You can not do this
so easily as you moved your hand, unless you move it edgewise. Why is
this? Because it presses upon so much more air than your hand does,
and the resistance of so much air to the fan you can feel as you push
it out of the way. The fan takes hold, as we may say, of more air than
your hand does, and so does also the wing of a bird.
Did you ever think how large wings you would need to fly with? You
would have to press upon a great deal of air to carry your body up as
the birds do theirs. See how large the wings of a bird are, as they are
stretched out. They are both very long and very broad; and, besides,
the bird is not so large as he seems to be. You will see this if all
the feathers are stripped from its body. If this be done while the
wings are left whole, it will seem to you that it takes very large
wings to raise a very little body. You can see, then, that it would
require very large wings indeed to carry your body up in the air; and
still larger ones to carry up a man.
[Illustration]
[Sidenote: Wings of the swift.]
Here is a bird that flies so fast that it is called the swift. Its
wings, you see, are very long. You do not see how broad they are,
because they are not fully spread out in the figure.
[Illustration]
[Sidenote: Wings of the bat.]
But there is no animal that has a greater extent of wing than the bat,
unless it be some of the insects. This is the reason why it flies so
swiftly. You can see in this figure of the long-eared bat what a large
amount of air its wings press upon as it works them. The wings of
insects that fly very swiftly are very large in proportion to their
bodies. This you can see in the butterfly that flies so nimbly from
flower to flower. Those that fly rather slowly, as the bumble-bee, have
not very large wings.
[Illustration]
[Sidenote: The flying fish.]
I believe that there is only one kind of fish that can fly in the air.
It is represented here. You can see that the fins with which it flies
are not nearly so large as the wings of a bird of the same size would
be. It therefore can not fly very high or far. The highest that it was
ever known to fly is twenty feet, and usually it skims along only two
or three feet above the water. It does not go up into the air in the
same way that a bird does. It gets its upward start from the water, and
all that it does with its wing-like fins is to keep itself up, which
it sometimes does for perhaps five or six hundred feet. It takes this
flight in the air in fleeing from some large fish, and in this way
often escapes being devoured.
[Illustration]
[Sidenote: The flying squirrel.]
That beautiful animal, the flying squirrel, which you see here, has a
fold of skin extending from the fore leg to the hind leg on each side.
These folds answer somewhat as wings when they are stretched out. Very
graceful is the movement when the animal takes a long, flying sweep
from one tree to another. But he can not go up in the air as a bird
does, for the folds are not nearly so large as real wings, and so do
not press upon enough air to carry him up. He can only take the sweep
that I have mentioned.
[Sidenote: Shape of the wings of birds.]
[Sidenote: How they are used in flying.]
Observe the shape of the wings of birds. They are rather rounded on the
upper surface, and hollowed out underneath. They are shaped in this way
to make the flying easy. This I will explain to you. When raising the
wing, the air goes easily off from the rounded surface; but when it
is moved downward, the air can not get away easily from the hollowed
surface. The wing gets hold, as we may say, of some of the air, and,
pressing upon it, raises up the bird.
You can see how this is by moving an open umbrella in the air. You can
move it very easily if you push the outer rounded surface straight
forward against the air. This is because the air moves off from the
round surface of the umbrella as easily as it does from the upper
surface of the bird’s wing. But if you move the umbrella with the inner
hollowed surface against the air, you find it rather hard work. Why? It
is because the air is caught in the hollow of the umbrella as it is in
the hollow of the bird’s wing.
But this is not all. The bird, in raising its wing, does not move it
straight upward. It moves it in such a way that it rather cuts the air
with its forward edge. It does this to get it up with little resistance
from the air. But when it moves it downward, it wants to get as much
resistance from the air as it can, so it moves it straight down, and
not edgewise. You can see how this works by moving a palm-leaf fan
about in the air. Move it edgewise, and it goes very easily. This
is like the upward motion of the bird’s wing. But move it broadside
against the air, and you feel considerable resistance. That is, the air
resists the pressure of the fan, just as it resists the pressure of the
wing in the downward stroke.
[Sidenote: How the hands are used in swimming.]
The swimmer manages his hands in the water in the same way that the
bird does its wings in the air. When he raises his hands forward, he
does it edgewise; but when he presses them down, he moves them flat
against the water, so as to press upon as much water as he can.
_Questions._--How is it that birds fly? Why do they have large wings?
Why can you not fly? How is swimming like flying? What do fishes swim
with? Why can not you fly in the air as well as swim in the water?
Tell about the experiment with the fan. What is said about the size
of birds’ wings? Tell about the bird called the swift. Tell about
the bat. What is said about the flying fish? What about the flying
squirrel? What is said of the shape of wings of birds? Give the
comparison of the umbrella. Tell how the bird moves its wings upward
and downward. Give the comparison of the fan. Give the comparison
about swimming.