Page:Popular Science Monthly Volume 4.djvu/558

540, but they are kites which continually change their surfaces and position in respect to the air, which artificial kites do not. An important difference between them is the rigidity of the one and the wonderful flexibility of the other. The kite rises as its oblique surface is pressed against the elastic air; the same is true of the wing. But the wing rotates, so that the proper obliquity of its parts is continually maintained; it rolls on and off the wind; it rotates not only throughout its length, but in each of its parts. The quills, which are convex, rotate, and present closed or oblique surfaces, which hold or discharge the air.

We have space for but one more of the numerous diagrams and figures which Dr. Pettigrew has prepared, illustrating the phenomena of flight. Fig. 16 is of the extended wing of a partridge, seen from beneath and from behind.



The wings, when flexed and extended in flight, assume curved surfaces, which change at each instant and carve the air, as does the tail of the fish the water, into complex wave-lines; and such is the structure of the wing that these results are inevitable when it is put in motion. "The muscles, bones, ligaments, and feathers, are so adjusted with reference to each other that, if the wing is moved at all, it must move in the proper direction." The bird no more thinks of its motions in flying than we do of ours in walking; the actions are mechanical and instinctive. An opinion long prevailed that heated air in the hollow bones of the bird gave it buoyancy and power of flight. This is shown to be a fallacy.

Three principal forces are expressed in flight: muscular and elastic force of the wing, weight of the body, and recoil of the air. By the mechanical structure of the wing, these forces act, react, and combine. Thus birds traverse the aerial ocean; the wild-goose drives his train along invisible tracks; the albatross and petrel are at home in the gale, undisturbed by its clamor; and the condor, with easy motion, treads with his pinions the elastic floors of the upper air.

The more rapid the strokes of the wing the greater the achievement. Not so with one of the most ingenious of human contrivances for progression. The screw, if urged beyond a certain velocity, holds and carries with it the water, and its propelling power is lost. It wants the flexibility of the wing and the fin—the adaptation is not complete.