SWIMMING OF FISHES. 367 



L^t US suppose, for example, that the tail has been 

 brought into the position, represented in Fig. 185 ; 

 that is, slightly inclined to the right. When it is 

 in this situation, the muscles on the left side, which 

 tend to bring the tail in a right line with the body, 

 being suddenly thrown into action, cause the tail 

 to press against the water in a direction r p, per- 

 pendicular to its surface, and by the reaction of 

 that fluid in the opposite direction p r, transfer 

 to the whole body of the fish an impulse in this 

 latter direction, causing its centre of gravity, c, to 

 move onwards in the line c b, parallel to p r. The 

 impulse thus given is not destroyed by the further 

 flexion of the tail towards the left side, for two 

 reasons. The first is, that the water 

 against which the tail has been 

 pressing, in moving from r to m, 

 having by that action been set in 

 motion in the direction r p, con- 

 tinues its motion in the same direc- 

 tion, and while thus accompanying 

 the tail, presents no resistance, or 

 scarcely any, to it while it is bent 

 from M to L, so as to occupy the 

 position directed by the dotted outline.* The 

 second reason is, that the muscles on the left side 



* In the above explanation, 1 have not thoiiglit it necessary to 

 take into account the effect resulting from the action of the tail on 

 the water when it is first bent from a straight position into that with 

 which I begun the description, lest I should have rendered it too 

 complex. It is true, indeed, that the resistance arising from this 

 action must tend to make the body of the fish recede to a certain 

 extent : but then this effect is immediately afterwards much more 

 than compensated by the forward impulse which is created by the 

 return of the tail to its straight position ; during which return, it is 



