THE BEST-KNOWN OCEAN CURRENTS 281 



gradually sinks and creeps under the warmer water of 

 lower latitudes maintaining a temperature of 32 to 35 

 on the bottom, even at the equator. This steady creep 

 of cooled surface water along the bottom supplies the 

 animals of the deep ocean floor with the air which they 

 must have. Without it the water at great depths would 

 have its air exhausted and all life would be destroyed. 



At the surface of the ocean the temperature of the water 

 varies in a general way with the latitude ; it is over 

 80 at the tropics and about the freezing point at the 

 poles. Near the poles and near the equator there is very 

 little variation in the temperature of the surface water 

 during the year, but in the intermediate latitudes the 

 annual variation is considerable. Below the surface the 

 effect of solar heat rapidly diminishes and at a depth of 

 300 ft. it is probable that the annual variation in tempera- 

 ture is nowhere more than 2 F. Below 600 ft. there is 

 probably no annual change in temperature. 



On the surface the daily average range of temperature 

 is not more than 1 F. and the annual range does not 

 exceed fifteen degrees, except where the same surface 

 is washed at different seasons by currents of different 

 character, and near the shore, where the heat of the land 

 affects it. This contrast in temperature conditions be- 

 tween the ocean and the land is most marked. The life 

 conditions in one are uniform and unvaried while in the 

 other they are most changeable and are subject to ex- 

 tremes of temperature. That is why the land animals 

 must be much more highly organized than those of the 

 sea in order to survive these changeable conditions. 



133. The Best-known Ocean Currents. The ocean is a 

 region of never-ceasing motion. At considerable depths 

 its motion is very slow, but near the surface, where the 

 prevailing winds can affect it, the movement is consider- 



