DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 347 



its dorsal and ventral aspects. Thus the tip of the wing forms a kite 

 which is inclined upwards, forwards, and outwards, while the root forms a 

 kite which is inclined upwards, forwards, and inwards. The angles made by 

 the tip and outer portions of the wing with the horizon are less than those 

 made by the body, and those made by the body less than those made by 

 the root and inner portions. The inclined surfaces peculiar to any portion of 

 the wing become more inclined as the speed peculiar to said portion decreases, 

 and vice versa. The wing is consequently mechanically perfect, the angles made 

 by its several parts with the horizon being accurately adjusted to the speed 

 attained by its different portions during its travel to and fro. From this 

 it follows that the air set in motion by one part of the wing is seized upon 

 and utilised by another, the inner and anterior portions of the wing supplying, 

 as it were, currents for the outer and posterior portions. This results from the 

 wing always forcing the air outwards and backwards. These statements admit 

 of direct proof, and I have frequently satisfied myself of their exactitude by 

 experiments made with natural and artificial wings. 



In the bat and bird the twisting of the wing upon its long axis is more of a 

 vital and less of a mechanical act than in the insect, the muscles which regulate 

 the vibration of the pinion in the former (bat and bird), extending quite to the 

 tip of the wing. 



The Body and Wings move in Opposite Curves. — I have stated that the wing 

 advances in a waved line, as shown at a c eg i of figure 14, p. 344 ; and the same 

 remark holds true, within certain limits, of the body as indicated at 1, 2, 3, 4, 

 and 5 of the same figure. Thus, when the wing descends in the curved line 

 a c, it elevates the body in a corresponding but minor curved line, as shown at 

 1, 2 ; when, on the other hand, the wing ascends in the curved line a e, the body 

 descends in a corresponding but smaller curved line (2, 3), and so on ad infinitum. 

 The undulations made by the body are so trifling when compared with those 

 made by the wing that they are apt to be overlooked. They are, however, 

 deserving of attention, as they exercise an important influence on the undula- 

 tions made by the wing, the body and wing swinging forward alternately, the 

 one rising when the other is falling, and vice versa. Flight may be regarded 

 as the resultant of three forces : — the muscular and elastic force, residing in 

 the wing, which causes the pinion to act as a true kite, both during the down 

 and up strokes ; the iveight of the body, which becomes a force the instant 

 the trunk is lifted from the ground, from its tendency to fall downwards and 

 forwards; and the recoil obtained from the air by the rapid action of the wing. 

 These three forces may be said to be active and passive by turns. 



Analysis of the Down and Up Strokes in the Insect — the Terms Extension and 

 Flexion defined. — As considerable confusion exists in the minds of most inves- 

 tigators as to the precise changes induced in the wing during the down and up 



VOL. XXVI. PART II. 4 X 



