§ 104, 105. THE GULF STREAM. 33 



we not detected here the foot-prints of an agent that does tend to 

 make the waters of the Caribbean Sea Salter, and therefore heav- 

 ier, than the average of sea water at a given temperature ? 



104. It is immaterial, so far as the correctness of the principle 

 Evaporationandpre- ^^V^^ "^^ich this rcasoning depends is concerned, 

 cipitation. whether the annual evaporation from the trade- 

 wind regions of the Atlantic be fifteen, ten, or five feet. The 

 layer of water, whatever be its thickness, that is evaporated from 

 this part of the ocean, is not all poured back by the clouds upon 

 the same spot whence it came. But they take it and pour it down 

 in showers upon the extra-tropical regions of the earth — on the 

 land as well as in the sea — and on the land more water is let down 

 than is taken up into the clouds again. The rest sinks down 

 through the soil to feed the springs, and return through the riv- 

 ers to the sea. Suppose the excess of precipitation in these ex- 

 tra-tropical regions of the sea to amount to but twelve inches, or 

 even to but two — 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 to produce a double dynamical effect, for the 

 simple reason that what is taken from one scale, by being put into 

 the other, doubles the difference. 



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

 cuitent into the Car- thc salts left by the vapor that the trade- winds, 

 ibbean Sea. northcast and southeast, take up from sea water, is 

 calculated to exert in creating currents, let us make a partial cal- 

 culation 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 em- 

 brace an area of at least three million square miles, and the yearly 

 evaporation from it is (§ 103), 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 va- 

 por, 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 (§ 98) that the waters of the Gulf Stream are reluc° 

 tant to mix with those of the ocean. 



C 



