38 THE PHYSICAL GEOGRAPHY OF THE SEA. 



35. Now that we may form some idea as to the influence which 

 the salts left hj tlie vapor that the trade-winds, northeast and 

 southeast, take up from sea water, is calculated to exert in crea- 

 ting currents, let us make a partial calculation to show how much 

 salt this vapor held in solution before it was taken up, and, of 

 course, while it was yet in the state of sea water. The northeast 

 trade-wind regions of the Atlantic embrace an area of at least three 

 million square miles ; and the yearly evaporation from it is (§ 33), 

 we will suppose, fifteen feet. The salt that is contained in a mass 

 of sea water covering to the depth of fifteen feet an area of three 

 million square miles in superficial extent, would be sufficient to 

 cover the British islands to the depth of fourteen feet. As this 

 water supplies the trade-winds with vapor, it therefore becomes 

 Salter, and as it becomes Salter, the forces of aggregation among 

 its particles are increased, as we may infer from the fact (§ 27), 

 that the waters of the Gulf Stream are reluctant to mix with those 

 of the ocean. 



36. Whatever be the cause that enables these trade-wind waters 

 to remain on the surface, whether it be from the fact just stated, 

 and in consequence of which the waters of the Gulf Stream are 

 held together in their channel ; or whether it be from the fact that 

 the expansion from the heat of the torrid zone is sufficient to com- 

 pensate for this increased saltness ; or whether it be from the low 

 temperature and high saturation of the submarine waters of the in- 

 ter-tropical ocean ; or whether it be owing to all of these influences 

 together that these waters are kept on the surface, suffice it to say, 

 we do know that they go into the Caribbean Sea (§ 34) as a sur- 

 face current. On their passage to and through it, they intermin- 

 gle with the fresh waters that are emptied into the sea from the 

 Amazon, the Oronoco, and the Mississippi, and from the clouds, 

 and the rivers of the coasts round about. An immense volume of 

 fresh water is supplied from these sources. It tends to make the 

 sea water, that the trade-winds have been playing upon and driv- 

 ing along, less briny, warmer, and lighter ; for the waters of these 

 large inter-tropical streams are warmer than sea water. This ad- 

 mixture of fresh water still leaves the Gulf Stream a brine stronger 

 than that of the extra-tropical sea generally, but not quite so 

 strong as that of the trade-wind regions (§ 29). 



