When we look at the structure of the flying birds, we see at once that they were formed for swift locomotion through the air, just as plainly as the lithe skiff was made to glide over the water or the carriage to spin over the land. In the first place, the body of the bird is comparatively light--that is, in proportion to the width, strength, and extent of its wings. By its thick, light, airy covering of feathers its body is made still more buoyant, besides presenting a larger surface to the supporting air with very little additional weight. The tail, too, with its long, closely woven quills spread out like a fan, not only serves the purpose of a rudder for guiding the aerial craft, but is still more useful in helping to sustain the bird's weight in the up-buoying element. It is interesting to note that the feathers on the bodies of the flying birds are arranged in tracts, with intervals here and there of quite, or almost, bare skin, called "apteria." Now, when a bird is carefully skinned, it will be seen that the feathered spaces have their own special slips of muscles inserted into the roots of the feathers, and when these muscles are contracted, they serve to raise the feathers, and must, therefore, be of some subsidiary value in flying, by making the bird's body more buoyant. Suggestive, indeed, is the fact that the plumes of the non-flyers are not arranged in tracts, but are evenly distributed over the body. Nor is that all that Nature has done to carry out her evident purpose of making the bird a natural "flying machine." The body of the bird contains numerous air sacs, all connected with the lungs, and these, when inflated, are a great help in flying by making the bird light. More than that, many of the bones, though strong, have thin walls and are hollow, the cavities being connected with the lungs and air sacs, from which they are also filled with air, contributing another element of lightness to the aerial navigator. That the bird's bones are capable of being permeated with air can be demonstrated by actual experiment, and is, therefore, a scientifically established fact. It is easy enough to prove it in this way: Take a dead bird that has been beheaded, pass a syringe into its windpipe, tie it carefully so that the air cannot escape at the sides, then blow the air down through the tube, and you will be able to follow the passage of the air into the skin and other parts of the body. Now, if you will cut off one of the bones, you can detect the air passing from the cut surface; and, more than that, as a scientific English writer says, "if the experiment be made by using colored fluid instead of air--which is pumped in by a syringe--the fluid can be seen to ooze from the ends of any bone or muscle that has been cut across." Thus it is seen that the whole body of the fowl is so constructed that it can be pervaded with air.