HYDROGRAPHY 149 



west, as would be the case if the temperatures depended 

 solely on the latitude, but are frequently diverted somewhat 

 to the north or south by the influence of currents, distribu- 

 tion of land and water, and prevaiHng winds. For example, 

 in the North Atlantic the corresponding isotherms are much 

 lower on the American than on the European coast, as a 

 result of the influence of the Labrador cold current flowing 

 south from Davis Strait and the warm Gulf Stream flowing 

 north and east towards Europe. 



Throughout the oceans the surface water is generally 

 warmer than that below, and, as a rule, deep water is cold 

 water. In the tropics the temperature may be over 80° F. 

 at the surface, and at or about freezing-point (28° F.) at the 

 bottom. As a general rule, the temperature decreases con- 

 tinuously as the depth increases, as is shown in the follow- 

 ing series, extracted from Murray's table of the " Challenger " 

 results, of mean temperatures for the whole ocean : — 



100 fathoms =60-7° F. 

 200 „ = 50-1° F. 



500 „ -401°F. 



1,000 fathoms = 36-5° F. 

 1,500 „ =35-3°F. 



2,200 „ = 35-2° F. 



There may, however, be variations from this rule due to 

 layers of warmer water between colder, or the reverse. 



In some cases the temperature of the deeper water does 

 not bear the same relation to that of the surface at all times 

 of year. For example, off the Norwegian coast the surface 

 of the sea is coldest in February and warmest in August, 

 while at a depth of 200 fathoms in the same locality the water 

 is at its lowest temperature in August and at its highest in 

 February ; and Murray (in 1888) found the same seasonal 

 reversal of conditions in Upper Loch Fyne on the west 

 coast of Scotland. 



The bottom temperatures are below 30° F. in the polar seas; 

 they are between 30° and 35° F. over much of the Antarctic 

 and the Southern Ocean, the Indian Ocean, and parts of the 

 Atlantic and Pacific ; between 35° and 40° F. in the North 

 Atlantic and parts of the Pacific. In the open oceans there 



