338 



DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



widely from the screw propellors at present in use — the blades of these propel- 

 lors always striking at a given angle and in the same direction. The advantage 

 in favour of the wing as compared with the screw as employed in navigation 

 is very great, and not at present understood.'" The area mapped out by the 

 wing during the effective stroke and while reversing; the various angles made 

 by the surfaces of the wing with the horizon in its passage to and fro ; the 

 rotating and twisting of the posterior or thin margin of the wing round the 

 anterior or thick margin ; and the figure of 8 track made by the tip of the wing 

 during its action, as seen in the wasp, are shown at figs. 3, 4, 5, and 6. 



fh Cf 



Fig. 3. 



Fig. 4. 



—X 



Analysis of the Movements of the Wing of the Wasp, Reversal of the Planes 

 of the Wing, Reciprocating Action, <$c. — In the wasp the wing commences the 

 down or forward stroke at a of figures 3 and 5 ; and it will be observed that 

 the angle which it makes with the horizon (x of fig. 5) is something like 45°. 

 At b (figures 3 and 5) the angle is slightly diminished, partly because of a rota- 

 tion of the wing along its anterior margin (long axis of wing), partly from 

 increased speed, and partly from the posterior margin of the wing yielding to a 

 greater or less extent. 



At c the angle is still more diminished from the same causes. 



At d the wing is slowed slightly, preparatory to reversing, and the angle 

 made with the horizon (x) increased. 



At e the angle, for the same reason, is still more increased; while at /the 

 wing is at right angles with the horizon. It is, in fact, in the act of reversing. 



* For specific differences between the screws formed by the wings and the propellors employed in 

 navigation, see memoir by the author, Trans. Linn. Society, vol. xxvi. pages 228, 229, 230, and 231. 



