1872.] Natural and Artificial Flight. 197 
inevitably and invariably darts forward (Fig. 11), it follows as 
a consequence that the wing, as already partly explained, flies 
forwards as a true kite, both during the down and up strokes, 
as shown atcdefghijkim of Fig.12, and that its under 
concave or biting surface, in virtue of the forward travel 
communicated to it by the body in motion, is closely applied 
to the air, both during its ascent and descent, a fact hitherto 
overlooked, but one of considerable importance, as showing 
how the wing furnishes a persistent buoyancy, alike when 
it rises and falls.” 
“‘Where the Kite Formed by the Wing Differs from the Boy’s 
Kite.—The natural kite formed by the wing differs from the 
artificial kite only in this, that the former is movable in all 
its parts, and more or less flexible and elastic, the latter 
being comparatively rigid. The flexibility and elasticity of 
the kite formed by the natural wing is rendered necessary by 
the fact that the wing is articulated or hinged at its root ; 
its different parts travelling at various degrees of speed in 
proportion as they are removed from the axis of rotation. 
Thus the tip of the wing travels through a much greater 
space in a given time than a portion nearer the root. If the 
wing was not flexible and elastic, it would be impossible to 
reverse it at the end of the up and down strokes, so as to pro- 
duce a continuous vibration. The wing is also practically 
hinged along its anterior margin, so that the posterior 
margin of the wing travels through a much greater space in 
a given time thana portion nearer the anterior margin. The 
compound rotation of the wing is greatly facilitated by the 
flexible and elastic properties of the pinion.” 
“Compound Rotation of the Wing.—The wing during its 
vibration rotates upon two separate centres, the tip rotating 
upon the root of the wing as an axis (short axis of wing), the 
posterior margin rotating upon the anterior margin (long 
axis of wing). Vide abandcd of Fig. 13. This compound 
rotation goes on throughout the entire down and up strokes, 
and is intimately associated with the power which the wing 
possesses of alternately seizing and evading the air. It is 
this arrangement which enables the wing to present an in- 
finity of inclined surfaces to the air—the angle of inclina- 
tion being increased or diminished at any stage of the down 
and up stroke.” 
“* The Wing of the Bird Cranked slightly Forwards—the Com- 
pound Rotation of the Rowing Feathers.—It will be observed 
from Fig. 13 that the wing is cranked somewhat forwards 
(compare position of axis a 6 with that of axis cd), a very 
slight movement of rotation at cd being accompanied by a 
