29G PHYSIC.U. GEOORArHY OF THE SEA, AND ITS METEOROLOGY. 



we not, therefore, entitled to regard the Kcd Sea as a make- 

 Aveight, thrown in to regulate the proportion of cloud and sun- 

 shine, 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 IjIow 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 Mediter- 

 ranean, 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 t hat- 

 three* times as much water as the Mediten anean 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 Atlantie 

 sends into it through the Straits of Gibraltar; and the difference^ 

 we may rest assured, whether it be much or little, is earned off 

 to modify climate elsewhere — to refresh with showers and make 

 fruitful some other parts of the earth. 



548. TJie 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 Aial 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 vaj)Our 

 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. AVe know (§ 267) that the 

 volume of water returned b}^ 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 



* Vide article " Physical Geography," Encyclopaedia Britannica. 



