34 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS IVIETEOROLOGY. 



Dr. Thomassy, a Frencli savant, who lias been extensiyely engaged 

 in the manufacture of salt by solar evaporation, infonns 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 3 J- per cent, of salt ; in the trade-wind region 4^ per 

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

 withstanding the Amazon and the Mississippi, vnih. all the inter- 

 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 

 Asents concerned, wators of the Mcxican Gulf and Caribbean Sea Salter 

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

 Gulf Stream flows ? There are physical agents that are knov\-ii 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 average of sea-water. 

 These agents are those employed by sea-shells in secreting soHd 

 matter for their structures ; they are also heat* and radiation, eva- 

 poration and precipitation. In the trade-vviad regions at sea (Plate 

 VIII.), evaporation is generally in excess of precipitation, while in 

 the extra-tropical regions the reverse is the case ; that is, the clouds 

 let dovm more water there than the winds take up again; and 

 these are the regions in which the GuK Stream enters the Atlantic. 

 Along the shores of India, where observations have been made, the 

 evaporation from the sea is said to amount to three fourths of an 

 inch daily. Suppose it' in the trade-wind region of the Atlantic to 

 amount to only half an inch, that would give an annual evapora- 

 tion of fifteen feet. In the process of evaporation fi'om 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 Atlantic, and reaching across the ocean, contains 

 an immense amount of salts. The great equatorial ciuTont (Plate 

 VI,) which often sweeps from the shores of Africa across the Atlan- 

 tic into the Caribbean 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 vapom' ? If so — and it probably 

 does — have we not detected here the footprints of an aQ:ent that does 

 tend to make the waters of the Caribbean Sea Salter, and there- 

 fore heavier, than the average of sea-water at a given temperatm-e ? 



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

 25.6. 



