158 FOUNDERS OF OCEANOGRAPHY 



marked at the upper ends of fjords in Norway and in some 

 of the Scottish sea-lochs, where the water from a stream may- 

 lie on the surface of the Salter sea-water, without mixing, to 

 such an extent that it is drinkable as fresh water. 



Currents may be traced in the sea for considerable dis- 

 tances by their saHnity. At the Strait of Gibraltar a strong 

 surface current of colder and less saline water flows in from 

 the Atlantic to make up for the large amount of evaporation 

 in the Mediterranean, and a return current of warmer and 

 Salter water flows out along the bottom, over the barrier at 

 a depth of about 100 fathoms, into the Atlantic, where it 

 can be traced for some distance. Similar interchanges are 

 known in other parts of the world, and the presence of these 

 currents of different temperatures and salinity has a pro- 

 found effect upon the distribution of many pelagic animals. 



In brief, it may be stated that the distribution of marine 

 organisms depends mainly upon the temperature of the water, 

 the temperature in any region depends largely upon the 

 existence of currents of different salinities and temperatures, 

 these currents are caused mainly by prevalent winds, the 

 winds are due to differences of barometric pressure, and these 

 pressures depend finally upon the action of the sun's rays. 



The origin, course, and effect of a typical warm current 

 of high saHnity (the Gulf Stream) will be dealt with in more 

 detail in the next chapter. 



Density. 



A salinity of 35 parts per thousand corresponds to a density 



or specific gravity of 1-026 (fresh water being taken as 1), and 



the increase in density (and reduction in temperature) with 



increasing depth in the ocean is seen in the following series : — 



Surface density = 1-025, 

 100 fathoms „ = 1-026 (temp. = 60-7° F.). 



300 „ „ = 1-027 ( „ =44-7°F.). 



2,000 „ „ = 1-028 ( „ =35-2°F.). 



A familiar effect of difference in specific gravity is seen in 



the increased buoyancy of a loaded vessel on entering the 



