NATURAL THEOLOGY. 255 



apparatus in the body of an animal. The prin- 

 ciple of the contrivance is clear ; the application 

 of the principle is also clear. The use of the 

 organ to sustain, and, at will, also to elevate, the 

 body of the fish in the water, is proved by ob- 

 serving, what has been tried, that, when the 

 bladder is burst, the fish grovels at the bottom ; 

 and also, that flounders, soles, skates, which are 

 without the air-bladder, seldom rise in the water, 

 and that with effort. The manner in which the 

 purpose is attained, and the suitableness of the 

 means to the end, are not difficult to be appre- 

 hended. The rising and sinking of a fish in 

 water, so far as it is independent of the stroke 

 of the fins and tail, can only be regulated by the 

 specific gravity of the body. When the bladder, 

 contained in the body of the fish, is contracted, 

 which the fish probably possesses a muscular 

 power of doing, the bulk of the fish is contracted 

 along with it ; whereby, since the absolute weight 

 remains the same, the specific gravity, which is 

 the sinking force, is increased, and the fish de- 

 scends: on the contrary, when, in consequence of 

 the relaxation of the muscles, the elasticity of the 

 enclosed and now compressed air restores the di- 

 mensions of the bladder, the tendency downwards 

 becomes proportionably less than it was before, or 

 is turned into a contrary tendency. These are 

 known properties of bodies immersed in a fluid. 

 The enamelled figures, or Httle glass bubbles, in a 

 jar of water, are made to rise and fall by the same 



