S2 THE PHYSICAL GEOGKAPHY OF THE SEA. 



Dr.Thomassy, a French savant, who lias been extensively engaged 

 in the manufacture of salt bj solar evaporation, informs me that on 

 his passage to the United States he tried the saltness of the water 

 with a most delicate instrument : he found it in the Bay of Biscay 

 to contain 8|- per cent, of salt ; in the trade-wind region, 4y*y- per 

 cent. ; and in the Gulf Stream, off Charleston, 4 per cent., not- 

 withstanding the Amazon and the Mississippi, with all the intei-- 

 mediate rivers, and the clouds of the West Indies, had lent their 

 fresh water to dilute the saltness of this basin. 



103. Now the question may be asked, What should make the 

 Agents concerned, watcrs of the Mcxicau Gulf and Caribbean Sea Salt- 

 er than the waters in those parts of the ocean through which the 

 Gulf Stream flows? There are physical agents that are known 

 to be at work in different parts of the ocean, the tendency of 

 which is to make the waters in one part of the ocean Salter and 

 heavier, and in another part lighter and less salt than the aver- 

 age of sea water. These agents are those employed by sea-shells 

 in secreting solid matter for their structures ; they are also heat'^ 

 and radiation, evaporation and precipitation. In the trade-wind 

 regions at sea (Plate VIII.)j evaporation is generally in excess of 

 precipitation, while in the extra-tropical regions the reverse is 

 the case ; that is, the clouds let down more water there than the 

 winds take up again ; and these are the regions in which the Gulf 

 Stream enters the Atlantic. Along the shores of India, where 

 observations have been carefully made, the evaporation from the 

 sea is said to amount to three fourths of an inch daily. Sup- 

 pose it in the trade-wind region of the Atlantic to amount to only 

 half an inch, that would give an annual evaporation of fifteen feet. 

 In the process of evaporation from the sea, fresh water only is 

 taken up ; the salts are left behind. Now a layer of sea water 

 fifteen feet deep, and as broad as the trade-wind belts of the At- 

 lantic, and reaching across the ocean, contains an immense amount 

 of salts. The great equatorial current (Plate VI.) which often 

 sweeps from the shores of Africa across the Atlantic into the Car- 

 ibbean Sea is a surface current ; and may it not bear into that sea 

 a large portion of those waters that have satisfied the thirsty trade- 

 winds with saltless vapor? If so — and it probably does — have 



* According to Dr. Marcet, sea water contracts down to 28° ; my own to about 

 2a. G. 



