PROGRESSION IN OR THROUGH THE AIR. 



167 



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 greater space in a given time 

 than a portion nearer the anterior margin (fig. 80, p. 149). 

 The compound rotation of the wing is greatly facilitated by the 

 wing being flexible and elastic. This causes the pinion to twist 

 upon its long axis during its vibration, as already stated. The 

 twisting is partly a vital, and partly a mechanical act ; that 

 is, it is occasioned in part by the action of the muscles, in part 

 by the reaction of the air, and in part by the greater momen- 

 tum 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 eff'ect 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, when 

 the anterior margin is inclined backwards ; the posterior 

 margin of the outer portion of the wing being made to 

 incline backwards at the end of the up stroke, when a cor- 

 responding portion of the anterior margin is inclined forwards 

 (figs. 69 and 70, g, a, p. 141 : fig. 86, /,/, p. 161). 



The Angles formed by the Wing during its Vibrations. — 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 its dorsal and ventral aspects. Thus the tip 

 of the wing forms a kite which is inclined upwards, forwards, 

 and outwards, while the root forms a kite which is inclined 

 upwards, forwards, and inwards. The angles made by the 

 tip and outer portions of the wing with the horizon are less 

 than those made by the body or central part of the wing, and 

 those made by the body or central part less than those made 

 by the root and inner portions. The angle of inclination 

 peculiar to any portion of the wing increases as the speed 

 peculiar to said portion decreases, and vice versa. The wing 

 is consequently mechanically perfect ; the angles made by its 



