36 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



trade-wind regions at sea (Plate VIIT.), evaporation is generally 

 in excess of precipitation, while in the extra-tropical regions the 

 reverse is the case ; that is, the clouds let do^^^l more water 

 there than the winds take np again ; and these are the regions 

 in which the Gulf Stream enters the Atlantic. Along the shores 

 of India, where observations have been made, the evaporation 

 from the sea is said to amount 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 evaporation of 

 fifteen feet. In the process of evaporation from the sea, fresh 

 water only is taken up ; the salts are left behind. Now a laj-er 

 of sea-water 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. The great equatorial current (Plate 

 VI.) which often 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 vapour ? If so — 

 and it probably does — have we not detected here the footprints 

 of an agent that does tend to make the waters of the Caribbean 

 Sea Salter, and therefore heavier, than the average of sea-water 

 at a given temperature ? 



104. Evaporation and precipitation. — 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 

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

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

 on the land as Ttell 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 returns through 

 the rivers to the sea. Suppose the excess of precipitation in 

 these extra-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 efi'ect, for 

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

 into the other, doubles the difi'erence. 



