The chief peculiarity of birds is their power of flight, and, although there are a few birds which do not fly, most of them do, and the various organs of their bodies are all constructed in such a way as to fit them for a life in the air. Their bodies are very solid and compact, in order that most of their weight shall be near the place where the wings are attached. The feet, legs, head, and neck are light, and so arranged that they may be drawn up close to the body while the bird is flying. As the neck is long and very flexible, the body does not need to be pliant, as with most creatures having backbones; but it is important that the wings should have a firm support, so the bones of the back are united. The body of a bird must also be well protected from the cold; for, as it ascends and descends through the air, it passes through regions of very different temperatures, and it must be provided with a thick and warm covering in order to be able to endure these sudden changes, and one also which shall be very light and able to shed the water; for, otherwise, a bird would be unable to fly. The feathers of a bird answer to all these needs, and are so placed upon the body that they form a smooth surface which does not catch against the air when the bird is passing through it. In its rapid ascents and descents, the bird is exposed to another danger even greater than the sudden changes of temperature. You all know that air presses in every direction with great force, and that we do not feel it because there is air in all parts of our bodies as well as outside them, and the pressure of the air inside exactly balances that of the outside air. If we should suddenly take away the outside air in any way, such as covering a person up with an air-pump receiver, and quickly and completely exhausting the air, the consequences of the inside pressure would be very terrible, and if the experiment could be tried quickly enough the body would burst like an exploding gun, with a loud noise. [Illustration: THE EAGLE (BIRD OF PREY).]