144 BOOK OF NATURE LAID OPEN. 



so formed as to be easily turned with a slight stroke 

 of the tail ; for, when a fish desires to turn, a blow 

 from the tail sends it about in an instant; and when it 

 strikes both ways, the motion is progressive, and en- 

 ables it to dart forward with an astonishing velocity. 

 One distinguishing appendage peculiar to the fin- 

 ny tribe, and which is found in the bodies of all spi- 

 nous or bony fish, is the air bladder, by means of 

 which they can make themselves more or less buoy- 

 ant, and rise or fall in (he water at pleasure. " This," 

 as Dr. Paley observes, " affords a plain and direct 

 instance, not only of contrivance, but strictly of that 

 contrivance which we denominate mechanical It 

 is a philosophical apparatus in the body of an ani- 

 mal. The principle of the contrivance is clear; the 

 application is also clear " The rising and sinking 

 of a fish in the water, so far as is independent of the 

 strokes 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 

 it probably possesses a muscular power of doing, the 

 bulk of the fish is contracted along with it ; where- 

 by, since the absolute weight remains the same, the 

 specific gravity, which is the sinking force, is in- 

 creased, and the fish descends ; on the contrary, 

 when, in consequence of the relaxation of the mus- 

 cles, the elasticity of the inclosed, and now com- 

 pressed air, restores the dimensions of the bladder, 

 the fish becomes proportionally lighter, and rises in 

 the water. 



