VARIATIONS IN DENSITY 143 



Influence of the Coast. 



Marked déviations in the courses of océan currents are 

 produced by the obstructive influence of the coasts of the 

 various continents and islands. 



Variations in Density. 



Variations in the density of the sea-water are due either 

 to différences in salinity or température, or both combined. 

 Actually variations in salinity are more commonly the cause 

 of différences of density in sea-water. Only in the greater 

 oceanic depths, where the salinity of large volumes of sea- 

 water is practically constant, is there any marked variation in 

 density due to température changes. But even hère salinity 

 changes are of greater importance in producing altérations in 

 density, and consequently setting up océan currents. 



From depths of 2,000 mètres downwards the température 

 and salinity in the whole of the Pacific and Indian Océans 

 (except in the highest latitudes) and in the south-west of the 

 South Atlantic are very similar, whereas in the North Atlantic 

 the water at similar depths has higher salinity and tempéra- 

 ture. Although our knowledge of oceanic salinities at thèse 

 depths is very defective, we can assume that in the North 

 Atlantic it is 35 per mille, in the Southern Atlantic and Indian 

 Océans 34"65 per mille, and in the Pacific 34'6o per mille. 

 The bottom waters of the North Atlantic are heavier than those 

 of the adjacent oceanic areas, and consequently there should 

 be a bottom current flowing southwards. This current, if it in 

 fact exists, has not yet been observed. 



The Polar Origin of Abyssal Waters. 



At depths of 2,000 fathoms and over the waters of ail 

 océans are very cold, and but a little above the freezing-point 

 of fresh water. Even in tropical régions the bottom tempéra- 

 tures are only 1° C. or a little over, and Murray says that the 



