Page:Physical Geography of the Sea and its Meteorology.djvu/271

the water that remains after the evaporation takes place is, on account of the solid matter held in solution, specifically heavier than it was before any portion of it was converted into vapour. The vapour is taken from the surface water; the surface water thereby becomes more salt (§ 463), and, under certain conditions, heavier; when it becomes heavier, it sinks; and hence we have, due to the salts of the sea, a vertical circulation, namely, a descent of heavier—because Salter and cooler—water from the surface, and an ascent of water that is lighter—because it is not so salt, or, being as salt, is not so cool (§ 404)—from the depths below. This vapour, then, which is taken up from the evaporating regions (§ 293), is carried by the winds through their channels of circulation, and poured back into the ocean where the regions of precipitation are; and by the regions of precipitation I mean those parts of the ocean, as in the polar basins, where the ocean receives more fresh water in the shape of rain, snow, etc., than it returns to the atmosphere in the shape of vapour. In the precipitating regions, therefore, the level is destroyed, as before explained, by elevation; and in the evaporating regions, by depression; which, as already stated (§ 468), gives rise to a system of surface currents, moving on an inclined plane, from the poles towards the equator. But we are now considering the effects of evaporation and precipitation in giving impulse to the circulation of the ocean where its waters are salt. The fresh water that has been taken from the evaporating regions is deposited upon those of precipitation, which, for illustration merely, we will locate in the north polar basin. Among the sources of supply of fresh water for this basin, we must include not only the precipitation which takes place over the basin itself, but also the amount of fresh water discharged into it by the rivers of the great hydrographical basins of Arctic Europe, Asia, and America. This fresh water, being emptied into the Polar Sea and agitated by the winds, becomes mixed with the salt; but as the agitation of the sea by the winds is supposed to extend to no great depth (§ 468), it is only the upper layer of salt water, and that to a moderate depth, which becomes mixed with the fresh. The specific gravity of this upper layer, therefore, is diminished just as much as the specific gravity of the sea water in the evaporating regions was increased. And thus we have a surface current of saltish water from the poles towards the equator, and an under current of water Salter and heavier from the equator to the poles. This under