THE GEOLOGICAL AGENCY OF THE AVINDS. 285 



547. Indeed, so scantily supplied with vapour are the winds 

 Heavy evaporation, which pass lu 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,, 

 thi'owing out of the question what is taken up from the suiface 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 dis- 

 charge 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 difierence, 

 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 great inland basin of Asia, which contains the Sea of 

 The winds that give Aral and the Caspian, is situated on the route which 

 rains to Siberian ^]^g Jiypothcsis rcquires thcso tliirsty winds from 

 the steppes of Asia, soutli-oast 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 

 knowledge extends, the level of each of these two seas is as per- 

 manent as that of the great ocean itself. Therefore, the volume 

 of water discharged by rivers, the rains, and the dews, into these 

 two seas, is exactly equal to the volume which these two seas 

 give back as vapour to the atmosphere. These winds, therefore, 

 do not begin permanently to lay down their load of moisture, be 

 it gTeat or small, until they cross the Oural Mountains. On the 

 steppes of Issam, after they have suppHed the Amazon and the 

 other great equatorial rivers of the south, we find them first be- 



* V('de article "Pliysicul Geography," Encyclopaedia Britannica. 



