36 THE PHYSICAL GEOGRAPHY OF THE SEA. 



and with such velocity into the Atlantic, have not only chemical 

 affinities peculiar to themselves, but, having more salts, higher tem- 

 perature, and a high velocity, they are not so permeable to water 

 diifering from them in all these respects, and, consequently, the 

 line of meeting between them and the other water of the ocean 

 becomes marked. This is the case with almost all waters in rapid 

 motion. Where the Mississippi and Missouri rivers come togeth- 

 er, there is a similar reluctance on the part of their waters to min- 

 gle, for the line of meeting between them can be traced for miles 

 below the junction of the two rivers. 



29. The story told by the copper (§ 26) and the blue color (§ 3) 

 indicates a higher point of saturation with salts than sea water 

 generally, and the salometer confirms it. Dr. Thomassy, a French 

 savant^ who has been extensively engaged in the manufacture of 

 salt by solar evaporation, informs me that on his passage to the 

 United States he tried the saltness of the water with a most del- 

 icate instrument : he found it in the Bay of Biscay to contain 3^ 

 per cent, of salt ; in the trade- wind region, A.-^-^ per cent. ; and in 

 the Gulf Stream, off Charleston, 4 per cent., notwithstanding the 

 Amazon and the Mississippi, with all the intermediate rivers, and 

 the clouds of the West Indies, had lent their fresh water to dilute 

 the saltness of this basin. 



30. Now the question may be asked. What should make the 

 waters of the Mexican Gulf and Caribbean Sea Salter than the 

 waters of like temperature in those parts of the ocean through 

 which the Gulf Stream flows ? 



31. 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 average of sea water. These 

 agents are those employed by sea-shells in secreting solid matter 

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

 tion and precipitation. 



32. In the trade- wind 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 down more 



* According to Doctor Marcet, sea water contracts down to 28°. 



