DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 367 



application of artificial wings, and I find that by employing a ball and socket 

 joint, and a cross system of elastic bands at the root of the wing, I can imitate 

 the movements of the natural wing with remarkable precision. By adopting 

 the springs referred to — by making the wing elastic in all its parts, even along 

 its anterior or thick margin (natural wings are elastic in this situation), and by 

 applying a power which varies in intensity, I can communicate to an artificial 

 wing a vibratory motion, completely devoid of pauses or dead points. The 

 working of the wing in question is accompanied with very little slip. Indeed, 

 the slip is so little that the wing may be said to supply a persistent buoying 

 and propelling power. When the wing is made to vibrate briskly in a more or 

 less vertical direction, it leaps forward in a series of curves, the down stroke run- 

 ning into the up one and vice versa, to form a continuous upward wave track. 

 The power applied is greatest at the beginning of the down stroke. It is 

 decreased at the end of the down stroke, slightly increased at the beginning of 

 the up stroke, and again decreased towards the termination of that act. Those 

 changes in the intensity of the driving power are necessary to allow the air 

 time to react on the under surface of the wing, and to bring the elastic pro- 

 perties of the springs and of the wing into play. The springs should be arranged 

 at right angles and obliquely, that is, there should be a superior, inferior, ante- 

 rior, and posterior set running at right angles to each other, and between these 

 as many oblique springs as are deemed necessary. The springs ought to vary 

 as regards their length and their strength. Thus, the superior springs, which 

 assist in elevating the wing, ought to be longer and stronger than the inferior 

 ones ; and the posterior springs, which restrain the wing from leaping forwards 

 during its vibrations, should be longer and stronger than the anterior ones, the 

 wing having no tendency to travel backwards. A detailed account of the structure 

 and movements of artificial wings will be found at the end of the present memoir. 

 In the bat and bird the wing is extended or pushed away from the body prior 

 to the down stroke, and folded or drawn towards the body prior to the up stroke. 

 The unfolding or extending of the wing prior to the down stroke, as seen in 

 the gull, is shown at Plate XI. figures 3, 2, 1, 5 ; Plate XIV. figure 18. 



When the wing is being extended or opened out it is also being elevated, 

 as shown at 1, 2, 3 of Plate XL figure 5, and Plate XIV. figure 18. When 

 the wing is flexed, as at t p of figure 3, Plate XL, the under surface of the wing 

 (s q) is nearly on a level with the horizon (b d). When, however, the wing- 

 is partially extended, as at Plate XL figure 2, the angle which its under surface 

 makes with the horizon is considerable, c b d representing the angle, and b d 

 the horizon. When the wing is fully extended, and ready to give the down 

 stroke, the angle which the under surface of the wing makes with the horizon 

 is still more increased, as shown at Plate XL figure 1, c b d indicating the 

 angle, and b d the horizon. The angle made by the under surface of the root 



VOL. XXVI. PART II. 5 C 



