DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 273 



ing surfaces c and cl of figs. 51, 55, and 57; with a a! of fig. 58 ; with C<*4 and fed of figs. (50 and 

 62; with fa and a f of fig. 61; with df and c b of figs. 68, 69 & 70, and with fb and d e of 

 fio-s. 73 & 74. A careful comparison of the surfaces {ca,d a) presented by this fig. (52) with 

 the corresponding surfaces of figs. 51, 55,57,60,62, 68, 69, 70, 73, 74 & 1~>, will show that the 

 anterior extremities of the Sea-Bear, Turtle, and Auk, and the win-sof insects, hats, and birds 

 are tAvisted upon themselves, and form true screws; and a comparison of thesurf; i s in qui stion 



ra 



^ 



''fio-s. 58 & 59) and bird (fig. 61) will show what is still more remarkable, ris. that the wing in 

 action rotates on its long axis, and describes a spiral or wave-track, in fact a sort of diadow <>r 

 spectre screw. This is particularly evident when a a! of fig. 52 is contra d with ftfl 1 of 

 figs. 58, 59, and 61. From this it is clear that the tails of iishes, Whales, Manatn and Dtt- 

 gongs, the posterior extremities of Seals and Walruses, the anterior extremities of Sea-Bean, 



immiii 



originally, or become such when in motion, i.e. when performing the various functioni delegated 



og and flying. They are consequently formed according to a < mmon type ; 



swimm 



iology 



Fig. 53. Swallow, in the act of flying. This bird is remarkable for the great Length of the primary 



if) 



Tin* Swallow and all 



birds apply their wings to the air by a twisting rotatory movement of the wing or. Lta lonj 



axis, the angle which the wing makes with the horizon in a direction from above downwards 

 and from behind forwards beii 

 ment taking place during; the 



Fig. 54. 



immm 



stroke, which is delivered outwards, downwards, and backwards, as represented at r of fig. } /, 

 the left foot being closed, and about to make the return stroke, which is delivered in ■ 

 forward, upward, and inward direction, as shown at * of fig. 77. The feet are .lightly rotated 

 on their long axes, or tilted, both during the effective and non-effective strokes; and as tiny 

 always move in curves, their course is accurately represented by a double sn.ral, as shown 

 at fig. 77. Compare the spirals formed by the feet of the bird in swimming with tl, e described 

 by the fish (fig. 76), and by ourselves in walking (fig. 78). 

 Fig. 55. Turtle, swimming. Here both the anterior (c) and posterior (*) extremities arc engaged in 



natation. The effective stroke is delivered when the feet are being Hexed or posted ,.. . 

 backward direction {vide arrows). What was said of the movements of the anterior extremity 



Turtle 



Fig. 56 



oi me oea-joear isee uuuer ug. <j*-> o^h^o — 



Glow-worm, in flight. Here the elytra, or horny anterior pair of w.nf . ar, dented and earn., 



forward heyond the reaeh of the posterior or membranons wings, .h*h are . true org,,, o, 



flight; the-e.ytra, which fornr enrved inclined planes, aeting as -»_. , ; a, £-*""- 



and propellers. The posterior 



descent, as in the bird (see under fig. 53). . . 



Fig. 57. Penguin, swimming. What was said of the anterior extremity of the Otana « ^Bea 



(fig. 51) applies equally to the rudimentary or swimming-wing of 0. remarkabl, bud. 1 or 



\ b ' P1 equally lu j ^ s ln0VC syn . 



the movements of its feet, see under fig. 54 1 neca omv 



chronously or together, the feet alternately. Blow-Fly, 



Fig. 08, Blur or impression produced on the eye by the rapid oscillation of th< g . 



i rou;« fi»m* represents the rotating ot tne w ing on 



when the insect is fixed. Seen from above. In* ngme rep*., o 



its long axis, and the donble cone which it forms during its ,- ,:t and tocen 



Of the con< - 



arked 



Ace, and is caused by the pinion rotating on 



in 



b 



from 



