168 



ANIMAL LOCOMOTIOK. 



several parts with the horizon being accurately adjusted to 

 the speed attained by its different portions during its travel 

 to and fro. From this it follows that the air set in motion 

 by one part of the wing is seized upon and utilized by 

 another ; the inner and anterior portions of the wing supply- 

 ing, as it were, currents for the outer and posterior portions. 

 This results from the wing always forcing the air outwards 

 and backwards. These statements admit of direct proof, and 

 I have frequently satisfied myself of their exactitude by ex- 

 periments made with natural and artificial wings. * 



In the bat and bird, the twisting of the wing upon its long 

 axis is more of a vital and less of a mechanical act than in 

 the insect ; the muscles which regulate the vibration of the 

 pinion in the former (bat and bird), extending quite to the 

 tip of the wing (fig. 95, p. 175 ; figs. 82 and 83, p. 158). 



The Body and Wings move in opposite Curves. — I have stated 

 that the wing advances in a waved line, as shown at acegi 

 of fig. 81, p. 157; and similar remarks are to be made of 

 the body as indicated at 1,2, 3, 4, 5 of that figure. Thus, 

 when the wing descends in the curved line a c, it elevates 

 the body in a corresponding but minor curved line, as at 

 1, 2 ; when, on the other hand, the wing ascends in the 

 curved line c e, the body descends in a corresponding but 

 smaller curved line (2, 3), and so on ad infiniturrii The un- 

 dulations made by the body are so trifling when compared 

 with those made by the wing, that they are apt to be 

 overlooked. They are, however, deserving of attention, as 

 they exercise an important influence on the undulations made 

 by the wing; the body and wing swinging forward alternately, 

 the one rising when the other is falling, and vice versa. 

 Flight may be regarded as the resultant of three forces : — the 

 muscidar and elastic force, residing in the wing, which causes 

 the pinion to act as a true kite, both during the down and up 

 strokes ; the weight of the body, which becomes a force the 

 instant the trunk is lifted from the ground, from its tendency 

 to fall downwards and forwards ; and the recoil obtained from 

 the air by the r^apid action of the wing. These three forces 

 may be said to be active and passive by turns. 



When a bird rises from the ground it runs for a short 



