288 rnrsicAL geography of the sea, and its meteorology. 



winds for tlie lakes generally, as well as for the Mississippi 

 Valley; they are also, speaking generally, the rain winds of 

 Europe, and, I have no doubt, of extra-tropical Asia also. 



536. The influence of mountain ranges. — Now suppose a certain 

 mountain range, hundreds of miles to the south-west of the lakes, 

 but across the path of these winds, with their dew-point at 50°, 

 were to be suddenly elevated, and its crest pushed into the 

 regions of snow, having a mean temperature at its summit of 30° 

 Fahrenheit. The winds, in passing that range, would be sub- 

 jected to a mean dew-point of 30°; and, not meeting (§ 297) 

 with any more evaporating surface between such range and the 

 lakes, they would have no longer any moisture to deposit at the 

 supposed lake temperature of 50° ; for they could not yield their 

 moisture to anything above 30°. Consequently, the amount of 

 precipitation in the lake country would fall off ; the winds which 

 feed the lakes would cease to bring as much water as the lakes 

 now give to the St. Lawrence. In such a case, that river and 

 the Niagara would drain them to the level of their own beds ; 

 evaporation would be increased by reason of the dryness of the 

 atmosphere and the want of rain, and the lakes would sink to 

 that level at which, as in the case of the Caspian Sea, the pre- 

 cipitation and evaporation would finally become equal. 



537. Hoiv the level of Caspians is reduced. — There is a self- 

 regulating principle that would bring about this equality ; for as 

 the water in the lakes becomes lower, the area of its suiface 

 would be diminished, and the amount of vapour taken from 

 it would consequently become less and less as the surface was 

 lowered, until the amount of water evaporated would become 

 equal to the amount rained down again, precisely in the same 

 way that the amount of water evaporated from the sea is exactly 

 equal to the whole amount poured back into it by the rains, the 

 fogs, and the dews.* Thus the great lakes of this continent 

 would remain inland seas at a permanent level ; the salt brought 

 from the soil by the washings of the rivers and rains would cease 

 to be taken off to the ocean as it now is ; and finally, too, the 

 great American lakes, in the process of ages, would become first 

 brackish, and then briny. Now suppose the water basins which 

 hold the lakes to be over a thousand fathoms (six thousand feet) 

 deep. We know they are not more than four hundred and 



* The quantity of dew in England is abont five inches during a year.— 

 Glaisha: 



