272 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



Rg. 43. The Rook (Corvus frugilegus), with wings elevated and in tlie act of springing from the earth. 



The Rook and most other birds secure, by a vigorous leap, the altitude requisite for the effec- 

 tive play of their pinions. This figure shows how the feathers of the wing overlap each other. 



Plate XV. 



Fig. II. Extreme form of compressed foot, as seen in the Deer (r) Antelope, Ox, &c, adapted specially 



for land transit. 



I ; ig. 45. Extreme form of expanded foot, as seen in the Ornithorhynchas (r), fee., adapted more particu- 

 larly for swimming. 



Figs. 1G & 17. Intermediate form of foot, as seen in the Otter (fig. 46), Frog (fig. 47), &c. Here the foot 



is equally serviceable in and out of the water. 



Fig. 48. Foot of the Seal (~), which opens and closes in the act of natation, the organ being folded upon 



itself during the non-effective or return stroke, and expanded during the effective or forward 

 stroke. Due advantage is taken of this arrangement by the Seal when swimming, the animal 



rot 



b 



to the water during the return stroke, and the flat, or the greatest available surface of both, 

 during the effective or forward stroke. 



Fie. 19. Fi 



******** * s x X 



the beginning of the effective or forward stroke. .In this case the tail is slightly compresed, 

 the lower portion of the body presenting a concave or biting surface towards the long axis of 



the tish. A\ hen the mesial line (a) is nnssprl tin* m»^m enWV,« Q ^^^^,1 i-.^ +v,r, + n ;i nn A l™™- 



return 



The fish advances by powerful lateral strokes of the tail, the body being rotated on its long 



urface 



durin 



he forward one. The surface exposed during the 

 effective or forward stroke is in some instances augmented by the rays of the caudal fin diva- 

 ricating or expanding. 



Fig. 50. The Manatee swimming. In this instance the tail is fleshy, and consequently incapable of increase 



or diminution as regards its actual breadth. The swimming of the Manatee differs from that 

 of the fish as regards the direction of the stroke, the Manatee lashing the water with the tail and 

 the lower portion of the body vertically, or from above downwards, the tail of the fish in all in- 

 stances oscillat , g from side to side, or laterally. The Manatee, like the Fish, rotates on its long 

 ax» («) during the non-effective or return stroke, and likewise during the effective or forward 



Fig 



stroke, the partial rotation, tilting, or twisting 

 and maximum of ratface alternated 



enablir 



t rinr 7 Z * m ^ miSS - ln th ° Sca - Bcar «<• P™«P*1 organ of natation is the an- 



m t ~ T * °: PP0 I; " Partia * r ° tati0n ' ***» ™ d «*» of the body and posterior ex- 



To m » 77 S m dlreC ' ing the m °" s ™ d «***« the eonrse of the animal 



menTso tsni T""^ "^ ° f "" ^ thc «S «M. or rotatory move- 



m ?;: ;i— • e z" ' tra ;r d to the antc J ^ °- *■• {c dh 



the bhde of . *,,, „ U P e th ° fllp P er c rf ) vith d »' "S- 52, wMeh represents 



me rjiacie 01 a scrcw-nroneller Unwy,,, +1 , , ° 



from the hodv in tno a T ,, g retm ' U Stroke ' or **« *■ fli PP^ & carried away 

 irom tnc body m the direction of the line nh +h„ w , s ■< \u 



from Mow ttprart, nntil it rttrin. ft! ^ ?" ^^ * COm ' eX SUrfaC ° W "" 



wato, Dur 4 ^ eftee i oZl Z ' ^ * PreSentS a **« "* * f 



rach,a,K- n ' t 1?L ™ ard S r ° ke a " ~ent takes plaee (see arrow), the 



flipper gradual i 



Fkr B2 HUA * V 1 " M ~"*" lg a tancave °r biting surface (d). 



Fig. o2. Blade of an ordinary screw-propeller: c, Z m JJ 



- \ i rf ac 



and 



blade; da, concave or biting 

 m of this figure with correspond- 



