92 



ANIMAL LOCOMOTION. 



movement which wields it emanating, for the most part, from 

 the shoulder, where the articulation partakes of the nature of 

 a universal joint. The wing is beautifully twisted upon itself, 

 and when it is elevated and advanced, it 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, makes, on the contrary, an angle of 

 something like 30° with the horizon. When the wing is 

 depressed and carried backwards,^ the angles which its under 

 surface make with the surface of the water are gradually 

 increased. The wing of the penguin and auk propels both 

 when it is elevated and depressed. It 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. 



Differ PMce between Suhaquatic and Aerial Flight, — The difl'er- 

 ence between subaquatic flight or diving, and flight proper, 

 may be briefly stated. In aerial flight, the most efl'ective 

 stroke is delivered downwards and forwards by the under, 

 concave, or biting surface of the wing which is turned in this 

 direction; the less efl'ective stroke being delivered in an up- 

 ward and forward direction by the upper, convex, or non- 

 biting surface of the wing. In subaquatic flight, on the 

 contrary, the most effective stroke is delivered downwards and 

 backwards, the least efl'ective one upwards and forwards. In 

 aerial flight the long axis of the body of the bird and the 

 short axis of the wings are inclined slightly upwards, and make 

 a forward angle with the horizon. In subaquatic flight the 

 long axis of the body of the bird, and the short axis of the 

 wings are inclined slightly downwards and make a backward 

 angle with the surface of the water. The wing acts more or less 

 efficiently in every direction, as the tail of the fish does. The 

 difference noted in the direction of the down stroke in flying 

 and diving, is rendered imperative by the fact that a bird which 

 flies in the air is heavier than the medium it navigates, and 

 must be supported by the wings ; whereas a bird which flies 

 under the water or dives, is lighter than the water, and must 



^ The effective stroke is also delivered during flexion in the shrimp, prawn, 

 and lobster. 



