168 THE WATERS OF THE EARTH 



the more it will hold suspended in it. Oils and fats, which do 

 not dissolve in water, may be suspended in water by emulsion. 



Water, like air, exerts pressure, the amount of which de- 

 pends on the depth of the water. The pressure at any given 

 depth is equal in all directions. Water also transmits 

 pressure equally in all directions. 



A submerged body displaces a volume of water equal to 

 its own volume, and loses weight exactly equal to the weight 

 of the water displaced. If a body weighs less than an equal 

 volume of water it floats ; if more, it sinks. 



Animals that live in water obtain the oxygen they need 

 from air in solution. Since an animal's energy depends 

 largely on the amount of oxygen it consumes, water animals 

 are generally less energetic than land animals. 



The oceans are the earth's water reservoirs. The seas have 

 for all time been receiving soluble contributions from the land. 

 When water evaporates, the dissolved substances are left 

 behind. Thus sea water is denser than fresh water and 

 freezes at a lower temperature. The greatest depth thus 

 far found in the ocean is more than six miles. At the depth 

 of two miles, the pressure is more than 300 times as much 

 as at the surface. The ocean floor is an almost level expanse 

 with only occasional volcanoes or gradually sloping swells. 

 Near the shore mud and sand washed from the land cover 

 the ocean floor. In deeper water the ocean floor is covered 

 with ooze, and below 18,000 feet with a peculiar reddish 

 clay, not found elsewhere. At the surface, the temperature 

 of ocean waters varies in general with the latitude. Below 

 the surface, the effect of solar heat diminishes rapidly. 

 Below 600 feet there is probably no annual change in tem- 

 perature, and at the bottom a steady temperature of 32 

 to 35 F. is maintained. 



