52 STRENGTH OF FISHES. 



and consequently for swimming. The bulk of the 

 body in fishes increases from the tail to the head, 

 while the extent of surface, on the contrary, follows 

 an opposite principle ; for, the tail, in consequence of 

 its least thickness, the magnitude of its fin, and the 

 other fins of the back and the vent which act as 

 auxiliaries, has more surface, in proportion to its 

 bulk than the body. It is owing to the difference of 

 bulk, which exists between the body and the tail, 

 including the tail fin, and to the extent of surface 

 which the tail and its fins present, that fishes find 

 a point of support to direct all their strength, and yield 

 a necessary mobility to the anterior parts of their 

 bodies. 



It may be remarked, that the jointed, gristly rays of 

 the tail fin, as well as of the other fins, perform a simi- 

 lar office of spreading out or of narrowing the surface, 

 as the sticks of a fan ; and consequently, the progress 

 which is made by the fore part of the body, by the 

 spreading out of the tail fin, may, to a certain point, 

 be independent of the will, in the same way as the out- 

 stretched limb of a man, who is standing up, w r ill 

 involuntarily incline to the ground. Let us suppose 

 that a bow, the ends of which are of unequal thickness 

 and proportions, be bent and unstrung in the water, 

 there will be more influence exerted by the water upon 

 the end which has the greater surface in proportion to 

 its bulk, than upon that whose mass is more consider- 

 able, and the bow will be displaced and carried farther 

 on the heaviest side. 



It is in this way that, the water offering resistance 



