SALTNESS OF THE SEA. 47 



2. Variations in the Saltness of Sea- water. 



Water is so much the more heavy as it is more salt. 

 It is not surprising, therefore, that the saltness of the 

 sea should increase with its depth. That increase is 

 not indefinite, because water at a certain temperature 

 can only hold in solution a given quantity of mineral 

 matters. 



We must at once confess to our ignorance of the 

 quantity of salts held in solution in the profound depths 

 of the Ocean. With the apparatus of Biot we can, it 

 is true, obtain water from great depths. Still, it is 

 not possible to operate at distances of 20,000 or 

 i^0,000 feet from the surface ; or, if possible, the 

 business is at once too costly and too difficult to be 

 often attempted. 



Rain and evaporation cause the saltness of the 

 superficial waters of the sea to vary considerably. 

 Evaporation increases, rain diminishes it. The 

 effects due to these causes are, generally speaking, 

 not very apparent ; but they are very observable when 

 one of them predominates. If it rains frequently in 

 certain regions, the saltness of the surface of the 

 sea is slight in comparison with that of places where 

 the clearness of the atmosphere favours evaporation. 

 It is less at the equator than near the tropics. It is 

 greatest at the 21st parallel of north latitude, and 



