St. Nicholas Magazine for Boys and Girls, Vol. 5, September 1878, No. 11

by Various

PENGUINS (SWIMMERS AND DIVERS).

I could easily fill the whole of this chapter with an account of the different ways in which the body of a bird is fitted for life in the air, but we have room 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 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.

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 front 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.