DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 375 



be made to play equally above {a b) and below (c d) the body, the tendency is to 

 drive the body in an undulating line, away from x, in the direction s x. As, 

 however, the opposite wing tends to push the body in a precisely contrary 

 direction, the forces exercised by the two wings neutralise each other in the mesial 

 line of the bird, the force which ultimately prevails being that of gravity. To 

 destroy the power of gravity, and to elevate and propel the bird, it is necessary 

 that the wings descend further than they ascend, and that the posterior mar- 

 gins of the wings be constantly kept on a lower level than the anterior ones. 

 It is also necessary that the wings be convex on their upper surfaces, and con- 

 cave on their under ones, and that the concave or biting surfaces be brought 

 more violently in contact with the air during the down stroke, than the con- 

 vex ones during the up stroke. The greater range of the wing below than 

 above the body, and of the posterior margin below than above a given line, may 

 be readily made out by watching the flight of the larger birds. It is also well 

 seen in the upward flight of the lark. The range of the wing of the gull in 

 ordinary flight is shown at Plate XIV. fig. 19. When the wing is elevated, high 

 above the body, as represented at 3 of figures 5 and 18, Plates XI. and XIV., 

 it is generally in the effort of rising, or in picking up garbage from the surface 

 of the sea, or in suspending or letting the body down gradually prior to alight- 

 ing. In such cases the wings expend their greatest force when a little above or 

 on a level with the body, as is well exemplified in the hovering of the kestrel. 



Compound Rotation of the Wing of the Bird. — To work the tip and posterior 

 margin of the wing independently and yet simultaneously, two axes are neces- 

 sary, one axis (the short axis) corresponding to the root of the wing ; the second 

 (the long axis) to the anterior margin. This renders the wing eccentric in its 

 nature. The primary or rowing feathers are also eccentric, the shaft of each 

 feather being placed nearer the anterior than the posterior margin, an arrange- 

 ment which enables the feathers to open up and separate during the up stroke, 

 and approximate and close during the down one. The axes of rotation in the 

 wing of the bird are given at figure 19, Plate XIV., a representing the short 

 axis around which the tip of the wing rotates with a radius e bf; c, the long axis, 

 around which the posterior margin of the wing revolves with a radius g d h. 



These points are more fully illustrated at figure 45, p. 376, where a b repre- 

 sents the short axis (root of wing), with a radius ef; e d, the long axis (anterior 

 margin of wing), with a radius g p. 



The Wing of the Bird cranked slightly Forwards — the Compound Rotation 

 of the Rowing Feathers. — It will be observed from figure 45 (p. 376), that the wing 

 is cranked somewhat forwards (compare position of axis a b with that of axis 

 c d), a very slight movement of rotation along the anterior margin (c d) being 

 accompanied by a considerable rotation of the posterior margin (h ij k I). This 

 figure also shows that the individual primary, secondary, and tertiary feathers 



VOL. XXVI. PART II. 5 E 



