Page:Popular Science Monthly Volume 48.djvu/289

Rh occur. The lighter water will be lifted by the surrounding heavier waters till there is no difference in pressure between its lower boundary and the surrounding waters at the same depth; but, as its pressure at all levels above this lower boundary will now have become greater than that of the surrounding heavier waters, it will instantly begin to displace and overflow them. This movement of the lighter water will require considerably more time than the movement of the heavier water by which it was lifted and continues to be lifted as its level sinks by lateral diffusion, because the sum of the differences of pressure which caused the lifting of the lighter water was, in the first place, greater than the sum of the differences that caused its lateral diffusion. Secondly, the differences of pressure that caused the first movement must extend all the way to the bottom, whereas those which cause the latter extend no deeper than the lighter stratum itself, and, even within the extent of that, have their chief effect confined to the superficial strata.

On the other hand, when the equilibrium of a mass of water is disturbed by causes that do not diminish the specific gravity, the disturbance must extend down to the bottom, and the differences of pressure at all levels beneath the surface must be equal. The equilibrium is then restored by a general movement of the whole mass, which movement is sensible in inverse proportion to the mass that is set in motion. This is the essential cause for the difference in strength between the currents observed in salt and fresh waters, for, of all the current-producing causes which act in fresh waters, only the one resulting from variations of temperature can sensibly affect the specific gravity, while the specific gravity of sea water, besides being much more affected by variation of temperature, is still further influenced by the fresh water which rains upon the surface of the ocean. If the whole basin of the ocean were filled with fresh water and exposed to the most extreme meteorological influences, the currents produced would not be nearly equal either in size or strength to those now observed in the waters of the ocean.

So the saltness of the sea is involved in all the great subjects into which the ocean currents enter. Having contributed to the growth of the continents, it has in a like degree peopled them by influencing human migrations through the streams of the ocean upon which the race of man was spread to the distant archipelagoes at a time when there were only rudimentary means for struggling against the forces of Nature. Besides its influences in geology and anthropology, it is concerned to a marked extent in the climate of the earth and of the sea, and in their botany and zoölogy.