escape, and equalize the pressure again.  Now, many birds, the condor, for example, fly over the tops of the highest mountains, and nearly all birds, either occasionally or habitually, ascend to very great altitudes, and, unless there were some plan for regulating the pressure of the air inside their bodies, they would suffer great inconvenience and even pain and danger.  But they are provided with an arrangement by which the air within them can escape easily as it expands and thus keep the pressure within just equal to that outside, so that they can ascend and descend as rapidly as they wish, without feeling the least inconvenience.  In the body of the bird there are several large bags, like the lungs, called air-chambers; many of their bones are hollow, and others are pierced with long winding tubes called air-tubes.  All these air-chambers and air-tubes are connected with the lungs so that air can pass into and out of them at each breath.  The connection between these chambers and the lungs is so complete that a wounded hawk can breathe through a broken wing almost as well as through its mouth.  When a bird mounts upward, the air inside its body gradually expands, but the bird does not feel any inconvenience; for, at each breath, part of the air passes from the air-chambers into the lungs, so that the pressure on the inside does not become greater than that on the outside.

[Illustrations:  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