164 



ANIMAL LOCOMOTION. 



wing descends in the direction c d\ but the moment it begins 

 to descend the body moves upwards and forwards (see arrows) 

 in a curved line to e. As the wing is attached to the body 

 the wing is made gradually to assume the position /. The 

 body (e), it will be observed, is now on a higher level than 

 the wing (/) ; the under surface of the latter being so adjusted 

 that it strikes upwards and forwards as a kite. It is thus 

 that the wing sustains and propels during the up stroke. The 

 body (e) now falls downwards and forwards in a curved line 

 to g, and in doing this it elevates or assists in elevating the 

 wing to j. The pinion is a second time depressed in the 

 direction k I, which has the effect of forcing the body along a 

 waved track and in an upward direction until it reaches the 

 point m. The ascent of the body and the descent of the 

 wing take place simultaneously (jn n). The body and wing, 

 are alternately above and beneath a given line x x\ 



A careful study of figs. 84, 85, 86, and 87, pp. 160, 161, 

 and 163, shows the great importance of the twisted configura- 

 tion and curves peculiar to the natural wing. If the wing 

 was not curved in every direction it could not be rolled on 

 and off the wind during the down and up strokes, as seen 

 more particularly at fig. 87, p. 163. This, however, is a vital 

 point in progressive flight. The wing (b) is rolled on to the 

 wind in the direction b a, its under concave or biting surface 

 being crushed hard down with the effect of elevating the body 

 to e. The body falls to g, and the wing (/) is rolled off* the 

 wind in the direction fj, and elevated until it assumes the 

 position j. The elevation of the wing is effected partly by 

 the fall of the body, partly by the action of the elevator 

 muscles and elastic ligaments, and partly by the reaction of 

 the air, operating on its under or concave biting surface. 

 The wing is therefore to a certain extent resting during the 

 up stroke. 



The concavo-convex form of the wing is admirably adapted 

 for the purposes of flight. In fact, the power which the wing 

 possesses of always keeping its concave or under surface 

 directed doivmvards and fonvards enables it to seize the air at 

 every stage of both the up and down strokes so as to supply 

 a persistent buoyancy. The action of the natural wing is 

 accompanied l)y remarkably little slip — the elasticity of the 



