SALTNESS AND CIRCULATION. 97 



greater than the evaporation ; and it is not salter 

 than the Mediterranean, a sea into which a vast 

 number of rivers fall, bringing with them from 

 the countries through which they flow a very 

 large and perpetual supply of salts, sulphates 

 and carbonates of lime, magnesia, soda, potash, 

 and iron, chemical substances which are found in 

 solution in sea-water. Now this equality in salt- 

 ness is extremely remarkable, and can be ac- 

 counted for only on the supposition of a system 

 of circulation in the waters of the ocean, and this 

 system we know does subsist. 



But .the saltness of the sea has a special rela- 

 tion to this ' system of circulation. In the Eed 

 Sea and the Mediterranean there is an under- 

 current flowing far below the surface into the 

 Indian Ocean from the former, and into the 

 Atlantic from the latter sea. These under-cur- 

 rents may in a great measure, if not wholly, be 

 attributed to the saltness of their waters. The 

 surface-currents likewise which flow from the 

 ocean into both these inland seas, may be attri- 

 buted to the same cause. 



And this is easily explained. It is well known 

 that in the process of evaporation the vapour of 

 water which is taken up into the air is pure, for 

 the salts of the sea-water are not raised into the 

 atmosphere. Now by the abstraction of the watery 

 particles, the surface-water is rendered salter, 

 and therefore heavier, than the stratum of water 

 immediately below it. It therefore sinks down- 

 ward and gives place to a layer of water lighter 

 H 



