158 



ANIMAL LOCOMOTION. 



this is practically impossible, as the wing is driven with such 

 velocity that there is positively no time to waste in waiting 

 for the purely mechanical ascent of the wing. That the 



Fig. 82. 



Fig. 83. 



Figs. 82 and 83 show that when the wings are elevated (e, /, g of fig. 82) the 

 body falls (s of fig. 82) ; and that when the wings are depressed (h, i, j of 

 fig. 83) the body is elevated (r of fig. 83). Fig. 82 shows that the wings are 

 elevated as short levers (e) until towards the termination of the up stroke, 

 when they are gradually expanded (/, g) to i)repare them for making the 

 down stroke. Fig 83 shows that the wings descend as long levers (h) until 

 towards the termination of the down stroke, when they are gradually folded 

 or flexed (i, j), to rob them of their momentum and prepare them for making 

 the up stroke. Compare with figs. V4 and 75, p 145. By this means the air 

 beneath the wings is vigorously seized during the down stroke, while that 

 above it is avoided during the up stroke. The concavo-convex form of the 

 wings and the forward travel of the body contribute to this result. The 

 wings, it will be observed, act as a parachute both during the up and down 

 strokes. Compare with fig. 55, p. 112. Fig. 83 shows, in addition, the com- 

 j)Ound rotation of the wing, how it rotates upon a as a centre, with a radius 

 m h 11, and upon a c 5 as a centre, with a radius k I. Compare with fig. 80, 

 p. U9. — Original. 



ascent of the pinion is not, and ought not to be entirely due 

 to the reaction of the air, is proved by the fact that in flying 

 creatures (certainly in the bat and bird) there are distinct 



