x The Hydrosphere 165 



the continent of Europe. The density at 60 F. corre- 

 sponding to various degrees of salinity is as follows : 



Salinity o-oo i-oo 2-00 3-00 3-25 3-50 3-75 4-00 

 Density i-oooo 1-0058 1-0138 1-0220 1-0240 1-0260 1-0280 1-0300 



224. Salinity of the Ocean. As a rule the surface 

 water of the ocean is salter than that lying beneath, the 

 fresher water below being denser in its position, because its 

 temperature is much lower and the pressure upon it greater. 

 In those parts of the ocean where the rainfall is heavy the 

 surface water is always being freshened, and its salinity is 

 consequently lowered. The map (Plate VIII.) shows the 

 freshened regions by a lighter tint of pink, the figures 

 referring to the density. There is one band of compara- 

 tively fresh water in the rainy equatorial region of each 

 ocean, and fresh zones around the melting ice of the Arctic 

 and Antarctic coasts. Seas and ocean shores situated in 

 regions of great rainfall, or receiving large rivers, are also 

 usually fresher than the average. The saltest water occurs 

 in the regions of greatest evaporation and least rainfall, 

 pre-eminently the Mediterranean and Red Sea, and in the 

 trade -wind regions of the open oceans. The track of 

 fresher water along the west coast of Africa and of South 

 America is probably produced by upwelling in consequence 

 of off-shore winds ( 240, 241). The way in which the very 

 salt water extends close to shore along the coast of South 

 America, between the mouths of the rivers Amazon and La 

 Plata, is accounted for by the westward trade-wind drift of 

 surface water. All the salts dissolved and invisible in the 

 whole ocean would suffice to form a solid crust 170 feet 

 thick over the entire sea surface. J& ' 



225. Absorbed Gases in Sea- water. All atmospheric 

 gases are to some extent dissolved by sea- water. The 

 amount absorbed depends conjointly on the pressure of the 

 gas (being greater as the pressure is greater), the tempera- 

 ture of the water (being greater as the temperature is lower), 

 and the nature of the gas itself. Under the same pressure 

 oxygen is nearly twice as soluble in water as nitrogen ; but 

 nitrogen exerts on the sea surface four-fifths, and oxygen only 



