"Take, for instance, a plain feather--say the tail feather of an eagle. The long quill is made of feather-bone, that wonderfully light, yet strong material that forms the rigid part of all feathers, so tough that it is almost impossible to break it, yet so flexible it will bend into a circle and then spring back like a bit of whalebone! Nothing that man has ever been able to make can equal it. "There is no blood, no nerves, no circulation and apparently no life in a full grown feather, yet it does not decompose; indeed, it is one of the hardest things in the world to destroy by any process of decomposition. It retains its resiliency and all its flexibility for years--all that is necessary is to keep it dry. It is finished all along the rib (or quill) with a hard, glossy enamel on the outside and this enamel keeps its polish as long as the feather lasts. "From [tr. note: sic on punctuation] an engineering standpoint, or the standpoint of the mechanic or artisan, there is absolutely no suggestion of betterment to be made, for the feather is an exact, perfectly finished product. Its long central quill tapers from base to point with geometric precision, thereby giving perfect resistance to bending force, and this is one of the combination of secrets that enables the bird to fly as easily as man can walk. Also this long quill is hollow, thereby all extra weight is done away with and added strength gained because of the tube contraction; and to make it perfect from a mechanical standpoint, the under side of the quill is reinforced by a doublerolled thickening of the shell of the quill itself so that strains are equalized. "This long quill is also curved slightly, to meet air resistance again and overcome it when the whole tail is spread, fan-like, to suddenly alter a direction or check speed in flight. "The long, soft side masses are formed of a multitude of tiny feathers, each one perfectly equipped, perfectly made, mechanically and geometrically without fault. Each of these tiny side feathers has its own midrib that tapers from base to tip, and each of these midribs carries its own equipment of side 'hairs' so beautifully constructed that it locks automatically into the one on each side of it in such a way that it makes a solid yet flexible mass of the whole surface, against which the air flows as the bird flies. "If these side feathers be split apart they will come back into place so exactly that the split cannot be detected. Nothing else in nature repairs itself with such precision. Many things, for instance the claw leg of the crawfish, will replace itself exactly when destroyed, but the feather alone _repairs_ its own breaks precisely and automatically.