FISHES. 409 



the reflux of the water collected behind. With 

 a view to diminish friction as much as possible, 

 the surface of the body has been rendered smooth, 

 and the skin impregnated with oil, which defends 

 it from injurious impressions, and at the same 

 time prevents the water from penetrating into its 

 substance. 



The body of a fish is nearly of the same spe- 

 cific gravity as the water it inhabits ; and the 

 effect of gravity is therefore almost wholly 

 counterbalanced by the buoyant force of that 

 fluid : for the weight of a mass of water, equal 

 in bulk to the body itself, is the exact measure 

 of this buoyant force. If this weight were pre- 

 cisely the same as that of the fish, the animal 

 would be able to remain suspended in any part 

 of the fluid without the necessity of employing 

 any voluntary motion or exertion for that pur- 

 pose : but as the body of a fish is generally a 

 little heavier than the fluid medium, especially 

 if it be fresh water, it is necessary for the animal 

 to give its body some degree of motion, in order 

 to prevent its sinking. 



In land quadrupeds, the limbs have to perform 

 the double office of supporting the body, and of 

 effecting at the same time its locomotion : but as 

 nearly the whole of the weight of a fish is already 

 sustained by the element in which it is immersed, 

 its instruments of motion may be employed ex- 

 clusively for progression ; and the powerful 



