THE OCEAN 331 



There are other reasons why the surface water of the ocean 

 does not get colder steadily from equator to poles. Rivers enter- 

 ing the sea are often warmer than the sea in summer, and colder 

 in winter. They, therefore, help to make surface temperatures 

 unequal. Enclosed or partly enclosed arms of the sea in low lati- 

 tudes are warmer than the open ocean in the same latitude. The 

 highest temperatures of the sea are found in such situations. The 

 surface temperature of the Red Sea is sometimes 90 or even 100 F. 



Temperature and movement. Water expands on being warmed. 

 Warm water is therefore lighter than cold water, if both are equally 

 salt. It follows that unequal surface temperatures cause move- 

 ment of the surface waters. The movements due to this cause 

 are always slow, but since the surface temperature is kept unequal 

 all the time by unequal heating, by the inflow of rivers, and by 

 melting ice, there must be constant though slow movement of the 

 surface waters, because of differences of temperature. 



Temperature beneath the surface. The water becomes cooler 

 with increasing depth, except where the surface is at or near the 

 freezing-point. Even where the surface water is warmest, the 

 temperature at a depth of a few hundred fathoms is below 40 F., 

 and that at the bottom still colder. The following table shows the 

 average temperature of the sea at various depths: 



Average 

 Depth Temperature 



600 feet 60.7 



1,200 feet. . ... , 50.0 



3,000 feet 40.1 



6,000 feet 36.5 



13,200 feet 35.2 



It is estimated that not more than one-fifth of the water of the 

 ocean has a temperature as high as 40 F., while its average tem- 

 perature is probably below 39 F. At the bottom of the deep sea 

 the temperature is generally below 35 F. The only parts of the 

 ocean bottom where the temperature is as high as 40 F. are 

 certain areas of shallow water, and the enclosed seas of relatively 

 low latitudes. 



The temperature of the water below the surface is known -by 



