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.