EVAPORATION IN THE TROPICS. 163 



deep ; and yet all this vast amount of matter makes no differ- 

 ence in the bulk of the ocean, whose waters would occupy 

 as much space if it were all taken away. It does, however, 

 affect the weight of the water, which is much heavier than 

 fresh water, and is heaviest where it is saltest 



So perfectly is the circulation of the ocean maintained, 

 by means of currents, &c., that sea-water, varies little in 

 composition all over the world ; and even before the cutting 

 of the Suez Canal, the waters of the Mediterranean and Red 

 Sea contained the same minerals, though the latter is not 

 fed by a single Triver or brook, and communicates with the 

 Indian Ocean, while the former opens into the Atlantic. 



As, however, evaporation takes place from the surface, 

 the -surface water becomes slightly salter, but as it also be- 

 comes heavier, it sinks, the lighter water from below taking 

 its place, and as a constant exchange is thus kept up, no one 

 part can be much salter than another. , 



In the tropics, where evaporation goes on rapidly, and 

 on coasts where much water is locked up in the form of ice, 

 which is always fresh, a slight increase of saltness is observ- 

 able. 



