DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 267 



ascent, the angle being greatest at the middle of the down stroke, and least at the middle 

 of the up or return stroke. The increase and diminution of the angle is necessary for 

 two reasons: — 1st, to increase the elevating-, propelling-, and sustaining-power; and 

 2ndly, to diminish the amount of slip or waftage. 



XIII. In those birds which fly under the water, the wings do not act as in ordinary 

 flight. In them the back of the pinion gives the effective stroke?, and this is delivered 

 downwards and backwards instead of downwards and forwards. This is necessary, in 

 order that the wings may make the same angle, and strike in the same direction as th<> 

 feet, with which they may or may not act in conjunction, the stroke being reversed on 

 account of the greater buoyant power of the water and the increased resistance opposed 

 to the foot by this element. In the Penguins and Auks, which cannot fly out of the 

 water, the wings are, for this purpose, completely twisted round— in faet, reversed. In 

 these curious birds, progression is effected by the employment of the wings and feet al >, 

 after the manner of inclined planes, these acting at variable angles and at different degrees 

 of obliquity. The inclined plane, therefore, however disguised and employed, fen. is an 

 essential feature in animal progression, whether on the land, in the water, or in the air. 



In the solidungulate animals, as the Horse, Ass, and Zebra, it appears as the small horny 

 hoof; in the amphibia, as the actively contracting and expanding foot ; in the fish, as the 

 tail and lower portion of the body; and in the bat, insect, and bird, as the greatly 

 expanded pinion. The size of the plane is diminished on the land, increased in the 

 water, and excessively so in the air. In either case it is the agent of a powerful muscular 

 system, and is applied in such a manner as at once to support and lever the animal 

 forwards. I say « support," because, as I have shown, the wing, which is simply a lever, 

 can sustain the bat, insect, and bird even in the thin medium of the air. 



EXPLANATION OF THE PLATES 



Bate XII. is intended to show the comparatively small extremities and travelling ^^f * 

 lan d - amma ls, their g radual inerease in sueh as take to ££Z^£~ I S 2 ' 

 ^ r^SS: S2SSL - ana for ** ano the relate 



proportions of the pinions in hats and insects. 

 Plate XIV. displays the relative proportion and size of the wing 



quick- and slow-flying ones. 

 In Plate XV. the extremities and travelling surfaces of animals 



application to the land, water, and air indicated. 



heavy-bodied 



and 



given 



figures, 78 in numb 



Plate XII 



hillingham Bull {Bos Scoticus). From Dallas. 

 tremities adapted for land transit. 



heavy 



