trade-wind regions at sea (Plate VIII.), evaporation is generally in excess of precipitation, while in the extra-tropical regions the reverse is the case; that is, the clouds let down more water there than the winds take up 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 layer 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 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 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 effect, for the simple reason that what is taken from one scale, by being put into the other, doubles the difference.
