DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 345 



aided by the recoil obtained from the air. In other words, it is not necessary 

 to elevate the wing forcibly in the direction c d to obtain the upward and for- 

 ward movement c e. One single impulse communicated at a, causes the wing 

 to travel to e, and a second impulse communicated at e, causes it to travel to i. 

 It follows from this that a series of vigorous down impulses would, if a certain 

 interval was allowed to elapse betiveen them, beget a corresponding series of up 

 impulses, in accordance with the law of action and reaction, the wing and the 

 air under these circumstances being alternately active and passive. I say if a 

 certain interval was allowed to elapse between every two down strokes, but 

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

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

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

 elastic ligaments, delegated to the performance of this function. The reaction 

 of the air is therefore only one of the forces employed in elevating the wing ; 

 the others, as I shall show presently, are vital and vito mechanical in their nature. 

 The falling downwards and forwards of the body when the wings are ascending 

 also contribute to this result. 



The Wing acts as a true Kite both during the Down and Up Strokes. — If, as 

 I have endeavoured to explain, the wing, even when elevated and depressed in 

 a strictly vertical direction, inevitably and invariably darts forward (figure 14, p. 

 344), it follows as a consequence that the wing, as already partly explained, flies 

 forwards as a true kite, both during the down and up strokes, as shown at 

 cdefghijklmoi fig. 15, and that its under concave or biting surface, in 

 virtue of the forward travel communicated to it by the body in motion, is closely 

 applied to the air, both during its ascent and descent, a fact hitherto overlooked, 

 but one of considerable importance, as showing how the wing furnishes a per- 

 sistent buoyancy, alike when it rises and falls. 



Fig. 15. 



In figure 15 the greater impulse communicated during the down stroke is 

 indicated by the double dotted lines. The angle made by the wing of the bat and 

 bird with the horizon [a b of figure 15) is constantly varying, as in the insect wing, 

 as a comparison of c with d, d with e, e with /, and /with g of figure 15 will 

 show, these letters having reference to supposed transverse sections of the 



