PROGRESSION IN OR THROUGH THE AIR. 



163 



ligaments and other structures combined with the resistance 

 experienced from the air rotating the posterior or thin 

 margin of the pinion in an upward direction, as shown at 

 defg and dfg of figs. 84 and 85, p. 160. The wing by 

 this partly vital and partly mechanical arrangement is rotated 

 off the wind in such a manner as to keep its dorsal or non- 

 biting surface directed upwards, while its concave or biting 

 surface is directed downwards. The wing, in short, has its 

 planes so arranged, and its angles so adjusted to the speed 

 at which it is travelling, that it darts up a gradient like a 

 true kite, as shown at c d e f g of figs. 84 and 85, p. 160, 

 or ghi of fig. 88, p. 166. The wing consequently ele- 

 vates and propels during its ascent as well as during its 

 descent It is, in fact, a kite during both the down and up 

 strokes. The ascent of the wing is greatly assisted by the 

 forward travel^ and downward and forward fcdl of the body. 

 This view will be readily understood by supposing, what is 

 really the case, that the wing is more or less fixed by the air 

 in space at the point indicated by 2 of figs. 84 and 85, p. 

 160; the body, the instant the wing is fixed, falling down- 

 wards and forwards in a curve, which, of course, is equivalent 

 to placing the wing above, and, so to speak, behind the volant 

 animal — in other words, to elevating the wing preparatory to 

 a second down stroke, as seen at g of the figures referred to 

 (figs. 84 and 85). The ascent and descent of the wing is 

 always very much greater than that of the body, from the fact 

 of the pinion acting as a long lever. The peculiarity of the 

 wing consists in its being a flexible lever which acts upon 

 yielding fulcra (the air), the body participating in, and to a 

 certain extent perpetuating, the movements originally produced 



Fig. 87. 



by the pinion. The part which the body performs in flight ii 

 indicated at fig. 87. At a the body is depressed, the wing 

 being elevated and ready to make the down stroke at h. The 



