DR FETTIGREW ON THE PHYSIOLOGY OF WINGS. 343 



When the Wing Ascends the Body Descends, and vice versa. — As the body 

 of the insect, bat, and bird falls forwards in a curve when the wing ascends, and 

 is elevated in a curve when the wing descends, it follows that the trunk of the 

 animal is urged along a waved line, as represented at 1, 2, 3,4, 5 of figure 14, p. 344, 

 the waved line ac eg i of the same figure giving the track made by the wing. 

 I have distinctly seen the alternate rise and fall of the body and wing when 

 watching the flight of the gull from the stern of a steam-boat. 



The direction of the stroke in the insect (figs. 3, 5, and 8, pp. 338, 340), as 

 I have already explained, is much more horizontal than in the bat or bird (figs. 

 12 and 13, p. 342). In either case, however, the down stroke must be delivered 

 in a more or less forward direction. This is necessary for support and pro- 

 pulsion. A horizontal to and fro movement will elevate, and an up and down 

 vertical movement propel, but an oblique forward motion is requisite for pro- 

 gressive upward flight." 



The Wing during its Vibrations moves on the Surface of an Imaginary 

 Sphere. — All wings are convex above and concave below. This shape is neces- 

 sary to enable the wing to evade the air during the up stroke, and to seize it 

 during the down one. The concave surface is presented during the up stroke, 

 and the concave one during the down stroke — the resistance experienced by a 

 concave surface when compared with a convex one being something like two to 

 one. The resistance is further increased by the wing being made to descend 

 with greater rapidity than it ascends. In whatever direction the wing turns 

 during the up stroke its movements are calculated to evade the air, and in 

 whatever direction it turns during the down stroke they are calculated to 

 seize it. This arises alike from the shape of the wing and the manner in which 

 it is applied to the air. Thus, in the insect in progressive flight the wing during 

 the up stroke describes a curve which is directed upwards and forwards. In the 

 bat and bird, where the wing is drawn towards the body during the up stroke, 

 the wing describes a second curve, this curve being directed upwards and inwards 

 with reference to the body. The under or concave surface of the wing may, 

 therefore, be said to be moving on the surface of an imaginary sphere during 

 the up stroke— an arrangement which enables it to avoid the superincumbent 

 air by its upper or convex surface, while it affords a certain amount of support 

 and ascensional power by its under or concave surface, this latter acting partly 

 as a kite and partly as a parachute. The wing may, in fact, be said to climb 

 during the up stroke ; and this climbing is so adroitly performed that two objects 

 are served by it — the superimposed air being avoided, and the body bearing 

 the wing being supported. In the climbing movement the anterior margin of 

 the wing cleaves a passage from behind upwards and forwards for the body 



* On the Mechanism of Flight, by the Author, Trans. Linn. Society, vol. xxvi. pages 214, 255, 

 and 256. 



VOL. XXVI. PART II. 4 U 



