ON THE GEOLOGICAL AGENCY OF THE WINDS. 193 



Mediterranean receives from its rivers is evaporated from its sur- 

 face. This may be an over-estimate, but the fact that evapora- 

 tion 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 re- 

 fresh with showers and make fruitful some other part of the earth. 



400. The great inland basin of Asia, in which are Aral and the 

 Caspian Seas, is situated on the route which this hypothesis re- 

 quires these thirsty vdnds from southeast trade-wind Africa and 

 America to take ; and so scant of vapor 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 (^ 116) 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 permanent 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 vapor to the atmosphere. 



401. These winds, therefore, do not begin permanently to lay 

 down their load of moisture, be it great or small, until they cross 

 the Oural Mountains. On the steppes of Issam, after they have 

 supplied the Amazon and the other great equatorial rivers of the 

 south, we find them first beginning to lay down more moisture than 

 they take up again. In the Obi, the Yenesi, and the Lena, is to 

 be found the volume which contains the expression for the load of 

 water which these winds have brought from the southern hemi- 

 sphere, from the Mediterranean, and the Red Sea ; for in these 

 almost hyperborean river-basins do w^e find the first instance in 

 which, throughout the entire range assigned these winds, they 

 have, after . supplying the Amazon, &c., left more water behind 

 them than they have taken up again and carried off. The low 

 temperatures of Siberian Asia are quite sufficient to extract from 

 these winds the remnants of vapor which the cool mountain-tops 

 and mighty rivers of the southern hemisphere have left in them. 



402. Here I may be permitted to pause, that I may call atten- 

 tion to another remarkable coincidence, and admire the marks of 



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