212 DR. PETTIGREW ON THE MECHANISM OF FLIGHT 



Flight under icater, SfG. — In birds which fly indiscriminately above and beneath the 

 atcr the wing is provided with stiffer feathers than usual, and reduced to a minimum 



o 



mots, or in conjunction with the feet, as in the Grebes *. To convert the wing into a 

 powerful oar for swimming, it is only necessary to extend and flex it in a slightly 

 backward direction, the mere act of extension causing the feathers to roll down, and 



giving to the back of the wing which communicates the effective stroke the an^le 



oblicmity necessary for sending the animal forward. This angle, I may observe, cor- 

 responds with that made by the foot during extension, so that, if the feet and wings are 

 both employed, they act in harmony. If proof were wanting that it is the back or convex 

 surface of the wing which gives the effective stroke in subaquatic flight, it would be found 

 in the fact that in the Penguin (Plate XII. fig. 5) and Great Auk, which are totally 

 incapable of flying out of the water, the wing is actually twisted round (Plate XV. fig. 57 d) 

 in order that the concave surface, which takes a better hold of the water, may be directed 

 backwards f . The thick margin of the wing when giving the effective stroke is, moreover, 

 turned downwards (Plate XV. fig. 57c), as happens in the flippers of the Walrus, Otaria 

 (Plate XV. fig. 51c), and Turtle (Plate XV. fig. 55c). This, I need scarcely remark, is 

 precisely the reverse of what occurs in the ordinary wing in aerial flight. In those 



extraordinary birds (Great Auk and Penguin) the wing is covered with short, bristly 

 looking feathers, and is a mere rudiment and exceedingly rigid, the movement which 



OV "O 



wields it emanating from the shoulder, where the articulation pa vtakes of the nat ure of 

 universal joint. The wing is beautifully twisted upon itself (Plate XV. fig. 67M), and 

 during the act of extension rolls up from the side of the bird at varying degrees of 

 obliquity till it makes a right angle with the body, when it presents a narrow or cutting 

 edge to the water, the wing, when fully extended, as in ordinary flight, making, on the 

 contrary, an angle of something like 30° with the horizon. A reverse movement takes 

 ce during flexion, when the effective or back stroke is made J. The wing of the 

 Pengum and Auk consequently acts very much after the manner of a screw; and this, 



as I shall endeavour to show, holds true likewise of the wing adapted for aerial flight 



The Guillemots in diving do not iisp tVmir &> A f . m* *v * ^i »• 

 Durnose are redno^A tn th. 11 T , i v ' ^ hten ^ fl < V Under the water ' Their win g s for this 



^:: ;;" ^Kr^sr tr: with flight - The Loons ' ° n the ° ther ^ wbi,e 



the Fro, fCnvW, . AnJi wTl T T g The suba( l» eous progression of the Grebe resembles that of 



. f In the Crocodile, Turtle, Triton, and 

 are likewise turned backwards. 



) 



to Aeronautics," Proceeding, of ZZJZ^ f r ' '?' *" V " i0US M ° deS ° f ""^ " "**" 

 man Sir George Cavlev gave the flrst nTti, 7 "'""' ""* 22 " 6 ' 186 '"' ° Ur »"** 



in ,7,6. In rhat ^eoL^r: IT iES^?, ^ " T T " ^ '° ^ * 

 irere each composed of f„„ r feather, stock in a piece of c „rk ? Tl *"™ " ^ ^^ "" "^" 



shaft or a li3 . To the shaft . .K.I.I™. .^J™ T* th ' C ° rks bem S driIkd in "» <*«'* *• ««*« • dri ™* 



which coiled round the shaft (and by which the spring 



was wound up), was affixed. 



the proper degree of tension was conferred on the snrinT 7, ^ " g ^"^ * SUffident number ° f timeS ' 



This interesting experiment is described at length Lfi '"^ ^ &mTatUS W * 3 liberate<1 ' * fleW int ° tht> ** 



at length, and the apparatus figured, in Nicholson's Journal for 1809, p. 17* 



ds size. In subaqueous flight the wings may act by themselves, as in the Guille- 



6 u.v. xu muse 



