30 Nature of Aquatic Environment 



Mobility— Water is the most mobile of substances, 

 yet it is not without internal friction. Like molasses, 

 it stiffens with cooling to a degree that affects the 

 flotation of micro-organisms and of particles suspended 

 in it. Its viscosity is twice as great at the^ freezmg 

 point as at ordinary summer temperature (77°F.)- 



Buoyancy — ^Water is a denser medium than air; _ it 

 is 775 times heavier. Hence the buoyancy with which 

 it supports a body immersed in it is correspondingly 

 greater. The density of water is so nearly equal to 

 that of protoplasm, that all living bodies will float in 

 it with the aid of very gentle currents or of a very little 

 exertion in swimming. Flying is a feat that only a 

 few of the most specialized groups of animals have 

 mastered, but swimming is common to all the groups. 



Pressure — This greater density, however, involves 

 greater pressure. The pressure is directly proportional 

 to the depth, and is equal to the weight of the super- 

 posed column of water. Hence, with increasing depth 

 the pressure soon becomes enormous, and wholly insup- 

 portable by bodies such as our own. Sponge fishers 

 and pearl divers, thoroughly accustomed to diving, 

 descending naked from a boat are able to work at depths 

 up to 20 meters. Professional divers, encased in a 

 modem diving dress are able to work at depths several 

 times as great; but such depths, when compared with 

 the depths of the great lakes and the oceans are com- 

 parative shoals. 



Beyond these depths, however, even in the bottom 

 of the seas, animals live, adjusted to the great pressure, 

 which may be that of several hundred of atmospheres. 

 But these cannot endure the lower pressure of the 

 surface, and when brought suddenly to the surface they 

 burst. Fishes brought up from the bottom of the 

 deeper freshwater lakes, reach the surface greatly 



