VARIATIONS IN DENSITY 143 



INFLUENCE OF THE COAST. 



Marked deviations in the courses of ocean 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 differences in salinity or temperature, or both combined. 

 Actually variations in salinity are more commonly the cause 

 of differences 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 temperature changes. But even here salinity 

 changes are of greater importance in producing alterations in 

 density, and consequently setting up ocean currents. 



From depths of 2,000 metres downwards the temperature 

 and salinity in the whole of the Pacific and Indian Oceans 

 (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 tempera- 

 ture. Although our knowledge of oceanic salinities at these 

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

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

 Oceans 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 all 

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

 of fresh water. Even in tropical regions the bottom tempera- 

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



