concave or convex side of an umbrella against the wind; one side holds the air, the other discharges it. The wing of the albatross shows how completely the feathers are adjusted, on the upper side, to avoid any* hold upon the air.
This arrangement, with the flexibility and screw-like motion of the wings of the gull, shown in Fig. 14, explains the exceedingly small resistance experienced in the upward movement, and also the forward impetus which it communicates.
It is in the down-stroke, or, as Dr. Pettigrew insists, in the beginning of the down-stroke, that force is chiefly expended. This movement is essentially a muscular act, and by this force alone no bird could sustain long-continued flight. The lark, whose flight is upward, soon descends to the earth. It lifts itself against gravity, simply by expenditure of vital force. But, the moment forward motion is attained, other forces relieve the strain upon the pinions, and their inclined surfaces convert gravity into a propelling power. It is obvious, however, that flight is attended with considerable muscular exertion. Migrating birds alight in unsuitable positions for rest, but the swallow will fly 1,000 miles in a single journey, and the condor attains an altitude of six miles.
The heron will strike the air 60 times in a minute, which, with 60 up-strokes, gives 120 movements, and this is continued through long
flights; and the same is true of many ducks and land-birds which strike the air with extreme and apparently exhausting rapidity. So swift are the motions of the wings of the humming-bird that they produce only a blurred spot before the eye.
That wings act as true kites, when in motion, is a familiar obser-