PETTIGREW ON THE MECHANISM OF FLIGHT. 213 



as shown at be. It is at this stage of extension that the axillary and distal curves 

 reverse. In fig. 75 the wing is fully extended, the convexity of the axillary curve (c) 

 being directed downioards , that of the distal one (a b) upwards. This, I need scarcely 

 remark, is the reverse of what occurred in flexion, as a comparison of b a c of PL XV. fi 

 73 with the corresponding letters of Pig. 75 will satisfactorily show. The axillary and 

 distal curves formed by the posterior or thin margin of the wing when fully extended 

 are also well shown at PL XV. fig. 62 b a c. 



The cross Pulsations produced by the Wing in motion. 'Resemblance between the Flight 



t> 



)f Birds, the Swimming of Fishes, 8fc. — Prom this description it follows that, when 



the different portions of the anterior margin are elevated, corresponding portions of 

 the posterior margin are depressed, the different parts of the wing moving in opposite 

 directions, and playing, as it were, at cross purposes for a common good — the object 

 being to rotate or screw the wing down upon the wind at a gradually increasing 

 angle during extension, and to rotate it in an opposite direction and withdraw it at 

 a gradually decreasing angle during flexion. It also happens that the axillary and 

 distal curves coordinate each other and bite alternately, the distal curve posteriorly 

 seizing the air in extreme extension with its concave surface (while the axillary 

 curve relieves itself by presenting its convex surface), the axillary curve, on the 

 other hand biting durine flexion with its concave surface (while the distal one relieves 



O ~""~o 



itself by presenting its convex one). The wing may therefore be regarded as exer- 

 cising a fourfold function, the pinion being made to move from within outwards and 

 from above downioards during extension in the effective or down stroke, m&Jrom without 

 inwards and from below upwards during flexion in the up or return stroke. The wing 

 consequently acts as an elevator and propeller, during both the down and up strokes, 

 the anterior and posterior margins of the pinion presenting concave and convex surfaces 

 alternately to suit the varying position of the wing during its descent and ascent, it 

 heins so arranged that there is always a concave or biting surface to lay hold of the - 



and a convex one to facilitate its escape when its presence might retard the upward 

 and onward movement of the bird. This oscillating of the wing on two separate ax* 

 the one running parallel with the body of the bird, the other at right angles to it, is well 

 worthy of attention as showing that the wing attacks the air on which it operates in 

 every direction and at almost the same moment, viz. from within outwards and from 

 above downwards, during the down or effective stroke, and from witliou inwards and 

 from below upwards during the up or return stroke. As a corollary to the foregoing, 

 the wing may be said to agitate the air in two principal directions, viz /m* wUJnn out- 



, * i t • 7 s M ,.„„rj m Hip reverse, the asitation m question 

 wards or the reverse, and from behind forwards or the t everse, »g i 



, -. i. i u^rKnoi ind n lateral. As, however, the curves 



producing two powerful pulsations, a longitudinal and a lateiai. jx , , 



* « • i • i * l Mh»v wben the win- is in motion, so the one pulsation merges 



to PP . 204 , 2 05, id 206 Will show, also produces ^g^JTZSZ 



one being from before backwards or the reverse, the other trom siae i 



be said to drill the water in two directions, viz. from behind ^»^« 



screwing of the body on its long axis, and from side to sxde by causmg Us antenor and 



