340 



DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



likewise elevating. There is this further difference. The margins of the blades of 

 the oar are of the same thickness, the axis of rotation running midway between 

 the two ; the anterior margin of the wing, on the contrary, is much thicker than 

 the posterior one, the axis of rotation corresponding to the former. The oar, as 

 far as the margins of its blade are concerned, is as it were concentric, the wing 

 eccentric. As the downward screwing movement of the wing, in virtue of the 

 action and reaction of the wing and air upon each other, is at once converted 

 into an upward screwing movement, as shown at a! b' c d! e' '/' ' g' h' i'f k' V m' nf 

 o' p' of fig. 9, it follows that the body of the insect is rapidly but steadily elevated 

 in an almost vertical wave-line. The impulse is communicated to the wing at 

 points corresponding to the heavy portions of the line in figure 8, and the 

 corresponding upward recoil is indicated at similar points in figure 9. 



Fig. 9. 



Hoiv the Figure of 8 is Unravelled, and becomes a Waved-Track. — When the 

 insect flies in a horizontal direction, and the speed attained increases with the 

 duration of flight, the wing reciprocates less and less perfectly, because the figure 

 of 8 sweeps described by it are converted into a looped and then a waved track, 

 as represented at # & c d efg h ij k I m n o p q r s t of figure 10 (p. 341); the cor- 

 responding looped and waved track clue to recoil being shown at similar letters 

 of figure 11 (p. 341). When the horizontal speed attained by the insect is high, 



or diving wing. In the gannet, cormorant, merganser, grebe, &c, which fly under the water, it is the upper 

 or dorsal surface of the pinion which gives the effective stroke, whereas in aerial flight it is the under or 

 ventral surface. This is proved by the fact that in the penguin and great auk, which are incapable of flying 

 out of the water, and confine their efforts to diving or swimming under it, the wing is actually twisted 

 round, so that the dorsal surface of the pinion occupies the position normally occupied by the ventral surfaces 

 in all other birds. This is necessitated by the fact that a diving bird, seeing it is of lighter specific 

 gravity than the water, must always fly downwards ; in other words, it must counteract buoyancy 

 as the flying bird counteracts gravity — buoyancy forcing the diving bird to the surface of the water in 

 the same way that gravity drags the flying bird to the surface of the earth. Levity and weight are 

 therefore separate forces, and act under diametrically opposite conditions, levity being quite as useful to 

 the diving bird as weight to the flying one. The wings of diving birds are applied to the water 

 precisely in the same manner as the flippers of the seal, sea bear, walrus, turtle, porpoise, whale, manatee, 

 &c. All these animals are lighter than the water, and, as a consequence, their travelling surfaces 

 to be effective must act from below as in the case of the scull. It is the reverse in the air, the 

 travelling surfaces acting invariably from above. Eor further development of this view see footnote to 

 page 371. 



