PKOGRESSION ON AND IN THE WATEK. 



71 



the precise kind of currents which afford it the greatest 

 leverage. In this respect the tail of the fish is infinitely 

 superior as a propelling organ to any form of screw yet de- 

 vised. The screw at present employed in navigation ceases to 

 be effective when propelled beyond a given speed. The 

 screw formed by the tail of the fish, in virtue of its recipro- 

 cating action, and the manner in which it alternately eludes 

 and seizes the water, becomes more effective in proportion to 

 the rapidity with which it is made to vibrate. The remarks 

 now made of the tail and the water are equally apropos of the 

 wing and the air. The tail and the wing act on a common 

 principle. A certain analogy may therefore be traced be- 

 tween the water and air as media, and between the tail and 

 extremities as instruments of locomotion. From this it fol- 

 lows that the water and air are acted upon by curves or wave- 

 pressure emanating in the one instance from the tail of the 

 fish, and in the other from the wing of the bird, the recipro- 

 cating and opposite curves into which the tail and wing are 

 thrown in swimming and flying constituting mobile helices 

 or screws, which, during their action, produce the precise 

 kind and degree of pressure adapted to fluid media, and 

 to which they respond with the greatest readiness. The 

 whole body of the fish is thrown into action in swimming ; 

 but as the tail and lower half of the trunk are more free to 

 move than the head and upper half, which are more rigid, 

 and because the tendons of many of the trunk-muscles are 

 inserted into the tail, the oscillation is greatest in the direction 

 of the latter. The muscular movements travel in spiral waves 

 from before backwards ; and the waves of force react upon the 

 water, and cause the fish to glide forwards in a series of curves. 

 Since the head and tail, as has been stated, always travel in 

 opposite directions, and the fish is constantly alternating or 

 changing sides, it in reality describes a waved track. These 

 remarks may be readily verified by a reference to the swim- 

 ming of the sturgeon, whose movements are unusually deli- 

 berate and slow. The number of curves into which the body 

 of the fish is thrown in swimming is increased in the long- 

 bodied fishes, as the eels, and decreased in those whose bodies 

 are short or are comparatively devoid of flexibility. In pro- 



