36 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Suppose the excess of precipitation in these extra-tropical regions 

 of the sea amounts to but twelve inches, or even to but tv^^o, it is 

 twelve inches or two inches, as the case may be, of fresh water 

 added to the sea in those parts, and which, therefore, tends to 

 lessen the specific gravity of sea water there to that extent ; and 

 for the simple reason, that what is taken from one scale, by being 

 put into the other, reduplicates the difference. 



35. Now, that w^e may form some idea as to the influence which 

 the salts left by the vapor that the trade-winds, northeast and south- 

 east, take up from sea water, is calculated to exert in creating 

 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 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 w^e may infer from the fact (§ 27), 

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

 of the ocesfn. 



Now, w^hatever be the cause that enables these waters to re- 

 main 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 both 

 of these influences together that these waters are kept on the sur- 

 face, suffice it to say, we do know that they go into the Carib- 

 bean Sea (^ 34) as a surface current. The trade-winds, by their 

 constant force, may assist to skim them off from the Atlantic, and 

 push them along into the Caribbean Sea, whence, for causes un- 

 known, they escape by the channel of the Gulf Stream in prefer- 

 ence to any other. 



36. In the present state of our knowledge concerning this won- 

 derful phenomenon — for the Gulf Stream is one of the most mar- 



