ON THE GEOLOGICAL AGENCY OF THE WINDS. 
193 
Mediterranean receives from its rivers is evaporated from its sm'- 
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 winds 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 olF. 
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 brou2:ht from the southern hemi- 
.sphere, from the Mediterranean, and the Red Sea ; for in these 
almost hyperborean river-basins do we 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 
N 
