166 



A.NIMAL LOCOMOTION. 



se(|uence that the wing, as already partly explained, flies 

 forward as a true kite, both during the down and up strokes, 

 as shown Sitcdefghijklm of fig. 88; am? 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. 



Fig. 88. 



In fig. 88 the greater impulse communicated during the 

 down stroke is indicated by the double dotted lines. The 

 angle made by the wing with the horizon (a b) is constantly 

 varying, as a comparison of c with d, d with e, e with /, / 

 with g, g with A, and h with i will show ; these letters having 

 reference to supposed transverse sections of the wing. This 

 figure also shows that the convex or non-biting surface of the 

 wing is always directed upwards, so as to avoid unnecessary 

 resistance on the part of the air to the wing during its ascent; 

 whereas the concave or biting surface is always directed down- 

 wards, so as to enable the wing to contend successfully with 

 gravity. 



TVhere the Kite formed hj the Wing differs from the Boys Kite, 

 — The natural kite formed by the wing diff'ers from the arti- 

 ficial kite only in this, that the former is capable of being 

 moved in all its parts, and is 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 diflerent 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 



