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.

[Illustration:  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.