THE GULF STREAM. 35 



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, also of heat* and radiation, evaporation and 

 precipitation. 



32. In the trade-wind regions at sea (Plate VIII.), evaporation 

 is generally in excess of precipitation, while in the extra-tropica! 

 regions the reverse is the case ; that is, the clouds let down more 

 water than the winds take up again ; and these are the regions in 

 which the Gulf Stream enters the Atlantic. 



33. Along the shores of India, where experiments have been 

 carefully made, the evaporation amounts 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 evap- 

 oration of say fifteen feet. In the process of evaporation from the 

 sea, fresh water only is taken up, the salts are left behind. 



Now a layer of sea w^ater 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. 



34. The great equatorial current (Plate VI.) which sweeps from 

 the shores of Africa across the Atlantic 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 vapor ? If so — and it probably does — have we not 

 detected here the foot-prints of an agent that does tend to make 

 the waters of the Caribbean Sea Salter, and therefore heavier than 

 the average of sea water ? 



It is immaterial, so far as the correctness of the principle upon 

 which this reasoning depends is concerned, 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 in the same place 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 — where, as a rule, 

 more water is let down than is taken up into the clouds again, 



* According to Doctor Marcet, sea water contracts down to 28^ 



