impermeable yet flexible plane—all this has been often insisted on by writers on design in Nature. But there are two points not so often noticed which especially concern us here.
1. Of the two vanes of each feather, the hinder one is much the broader. This, together with the manner of overlap, causes
Fig. 1.—Longitudinal Section of the Wing Plane and Cross-section of Three Feathers. a, shaft; v, v', vane.
the feathers to rotate and close up into an impervious plane in the downstroke, and to open and allow the air to pass freely through in the upstroke, as shown in the figure (Fig. 1). This structure and arrangement produce the greatest possible effectiveness of the downstroke and the least possible loss in recovery for another stroke.
2. The plane of the wing is supported not along the middle, but along the extreme anterior border, as shown in Fig. 2, which
Fig. 2.—Diagrammatic Cross-section of Bird's Wing. a, wing bones; b, plane.
is a diagrammatic cross-section of the wing. The effect of the down stroke is to tip up the wing behind, as shown in Fig. 3. The whole force of the stroke, a b, is resolved into two components— one, a c, sustaining, and the other, b c, propelling onward. In easy flight, therefore, only downward flapping is necessary, although
Fig. 3.—Diagrammatic Cross-section of Bird's Wing during Downstroke. a, b, whole force; a, c, part sustaining; b, c, part propelling.
in rapid flight doubtless the stroke is also a little backward.
The same admirable adaptation is carried out in every part of the bird. The whole bird is an exquisitely constructed flying machine. The smallness of the head, the feet, and the viscera,