How wings raise the birds in the air.
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.
Flying in water.
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.
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.
Why we can not fly in the air with our hands.