68 



ANIMAL LOCOMOTION. 



head does not move in the direction c & as stated, but in the 

 direction c h, the body of the fish describing the arc of a 

 circle, a c h. This is a matter of observation. If a fish when 

 resting suddenly forces its tail to one side and curves its 

 body, the fish describes a curve in the water corresponding 

 to that described by the body. If the concavity of the 

 curve formed by the body is directed to- th^ right side, 

 the fish swims in a curve towards that side. To this there 

 is no exception, as any one may readily satisfy himself, by 

 watching the movements of gold fish in a vase. Observation 

 and experiment have convinced me that when a fish swims it 

 never throws its body into a single curve, as represented at 

 fig. 31, p. 67, but always into a double or figure-of-8 curve, as 

 shown at fig. ?>2} 



Fig. 3"2. — Swimming of the Sturgeon. From Nature. Compare with figs. 18 

 and 19, pp. 37 and 39 ; fig. 23, p. 43 ; and figs. 64 to 73, pp. 139, 141 an.l 

 14:4:.— Original. 



The double curve is necessary to enable the fish to present 

 a convex or non-biting surface (c) to the water during flexion 

 (the back stroke of authors), when the tail is being forced 

 away from the axis of motion {a b), and a concave or biting 

 surface (s) during extension (the forward or efi'ective stroke of 

 authors), when the tail is being forced with increased energy 

 towards the axis of motion (a b) ; the resistance occasioned by 

 a concave surface, when compared with a convex one, being in 

 the ratio of two to one. The double or complementary curve 

 into which the fish forces its body when swimming, is neces- 

 sary to correct the tendency which the head of the fish has 

 to move in the same direction, or to the same side as that 



^ It is only when a fish is turning that it forces its body into a single curve. 



