Page:Physical Geography of the Sea and its Meteorology.djvu/322

296 we not, therefore, entitled to regard the Red Sea as a make-weight, thrown in to regulate the proportion of cloud and sunshine, and to dispense rain to certain parts of the earth in due season and in proper quantities? Have we not, in these two facts evidence conclusive that the winds which blow over these two seas come, for the most part, from a dry country—from regions which contain few or no pools to furnish supplies of vapour?

547. Heavy evaporation.—Indeed, so scantily supplied with vapour are the winds which pass in the general channels of circulation over the water-shed and sea-basin of the Mediterranean, that they take up there more water as vapour than they deposit as rain. But, throwing out of the question what is taken up from the surface of the Mediterranean itself, these winds deposit more water upon the water-shed whose drainage leads into the sea than they take up from it again. The excess is to be found in the rivers which discharge themselves into the Mediterranean; but so thirsty are the winds which blow across-the bosom of that sea, that they not only take up again all the water that those rivers pour into it, but they are supposed by philosophers to create a demand for an immense current from the Atlantic to supply the waste. It is estimated that three times as much water as the Mediterranean receives from its rivers is evaporated from its surface. This may be an over-estimate, but the fact that evaporation from it is in excess of the. precipitation, is made obvious by the current which the Atlantic sends into it through the Straits of Gibraltar; and the difference we may rest assured, whether it be much or little, is carried off to modify climate elsewhere—to refresh with showers and make fruitful some other parts of the earth.

548. The winds that give rains to Siberian rivers have to cross the steppes of Asia.—The great inland basin of Asia, which contains the Sea of Aral and the Caspian, is situated on the route which this hypothesis requires these thirsty winds from south-east trade-wind Africa and America to take; and so scant of vapour are these winds when they arrive in this basin, that they have no moisture to leave behind; just as much as they pour down they take up again and carry off. We know (§ 267) that the volume of water returned by the rivers, the rains, and the dews, into the whole ocean, is exactly equal to the volume which the whole ocean gives back to the atmosphere; as far as our