346 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



wing. Figure 15 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 downwards, so as to enable the wing to contend successfully 

 with gravity. 



On comparing c d efg of figure 15, p. 345, with a be dot figures 3 and 5, p. 338, 

 it will be seen that the principle involved in the flight of the wing of the insect, 

 bat, and bird is essentially the same. The wing is, in short, in every instance, a 

 true kite, and flies forward in accordance with natural laws. 



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 capable of being moved in all its parts, and is more or less flexible and elastic, 



thelatter 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 produce 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 than a portion nearer the anterior margin. The compound rotation 



of the wing is greatly facilitated by the flexible and elastic properties of the 



pinion. It causes the pinion to twist upon its long axis during its vibration, as 



already fully explained (see g, r and a, s of fig. 2, p. 336). The twisting 



referred to is partly a vital and partly a mechanical act ; that is, it is occasioned 



in part by the action of the muscles, and in part by the greater momentum 



acquired by the tip and posterior margin of the wing, as compared with the 



root and anterior margin ; the speed acquired by the tip and posterior margin 



causing them to reverse always subsequently to the root and anterior margin, 



which has the effect of throwing the anterior and posterior margins of the 



wing into figure of 8 curves. It is in this way that the posterior margin of the 



outer portion of the wing is made to incline forwards at the end of the down 



stroke (fig. 2 g, p. 336), when the anterior margin is inclined backwards, and 



that the posterior margin of the outer portion of the wing is made to incline 



backwards at the end of the up stroke (fig. 2 a, p. 336), when a corresponding 



portion of the anterior margin is inclined forwards. 



The Angles formed by the Wing in Action. — Not the least interesting feature 

 of the compound rotation of the wing, of the varying degrees of speed attained 

 by its different parts, and of the twisting or plaiting of the posterior margin 

 around the anterior, is the great variety of kite-like surfaces developed upon 



