TEMPERATURE OF SEA WATER 31 



few years have shown that the distribution of this cold 

 water, which varies greatly from season to season, 

 and from one year to another, has a profound effect 

 upon climate and upon the distribution of marine 

 life (including food-fishes), and more observations in 

 this direction are urgently needed. 



This fresh water is relatively light, notwithstanding 

 its low temperature, but, when it moves into lower 

 latitudes, its specific gravity tends to increase rela- 

 tively to the warmer water surrounding it ; hence we 

 have a zone of maximum specific gravity at the 

 surface outside the latitude of the trade winds, a sort 

 of combined effort of warm salt water on one side and 

 cold fresh water on the other. 



Lastly (c), the complete mixture of waters in the 

 oceans frequently takes place with great slowness, 

 compared to the rate of the horizontal movements 

 known as " currents " ; hence, fresh or salter water may 

 often be traced for great distances merely by the 

 difference in salinity from adjacent parts of the ocean. 

 The opportunity which a knowledge of salinity 

 affords in this way of tracing the course and volume 

 of currents gives these observations their greatest 

 value. 



Temperature of Sea Water. 



The distribution of temperature at the surface and 

 in the depths is shown in Chart IV. 



By this distribution several distinct questions are 

 raised, all of which require more material for full 

 discussion ; but we may call attention to two main 

 points : 



i. Assuming the waters of the sea to be at rest, 

 what are the main sources of gain and loss of heat 

 which determine their temperature ? The surface of 



