FISHES. 211 



portion to theirs. For the same reason, while man can 

 swim with his hands and feet, he can not fly. The Fly- 

 ing Fish, Fig. 167, is enabled to fly by having fins which 

 approach in extent the wings of a bird. 



Fig. 167 Flying Fish. 



355. Besides, most fishes have a peculiar contrivance 

 for enabling them to rise and fall in the water easily. It 

 is a bladder of air which the fish has the power of com- 

 pressing or enlarging at pleasure. If the fish wishes to 

 go down rapidly in the water, it compresses this air-blad- 

 der, and so increases its specific gravity. If, now, it wish- 

 es to rise, it takes off the pressure from the air-bladder, 

 which therefore enlarges to its former dimensions, and 

 lessens the specific gravity of the fish, or, in other words, 

 makes its bulk greater, while the weight remains the 

 same. Sometimes the fish loses its power of compress- 

 ing the air-bladder, and then it is so light all the time 

 that it has no power to go down in the water. A gen- 

 tleman had a Goldfish which swam with its belly upward, 

 probably from a wrong position of the air-bladder. 



356. The chief agent in swimming in the Fish is the 

 tail, which acts like a sculling-oar, moving to the one side 

 and the other alternately. It is terminated, for this pur- 



