THE GEOLOGICAL AGENCY OF THE WINDS. 277 



rence carries off to the Atlantic Ocean, is taken up by tlie south- 

 east trade-^^iQ(is of the Pacific Ocean. Suppose this to be the 

 €ase, and that the winds which bring this vapour arrive with it in 

 the lake countr}' at a mean dew-point of 50". Let us also admit 

 the south-west ^Ninds to be the rain mnds for the lakes generally, 

 iis well as for the Mississippi Valley ; they are also, speakmg gene- 

 rally, the rain ^^inds of Europe, and, I have no doubt, of extra- 

 tropical Asia also. 



536. Now suppose a certain mountain range, himdreds of miles 

 The influence of f ^hc south-wcst of the lakcs, but across the path of 

 luuuntain ranges. ^j^ggg ^yijicls, mth their dcw-pouit at 50"", were to be 

 ,suddenly elevated, and its crest pushed into the regions of snow, 

 having a mean temperature at its siunmit of 30^ Fahrenheit. 

 The winds, in passing that range, would be subjected to a mean 

 dew-point of 30^ ; and, not meeting (§ 297) with any more evapo- 

 rating surface between such range and the lakes, they would have 

 no longer any moisture to deposit at the supposed lake tempera- 

 •tm-e 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 "^inds 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 dr^Tiess of the atmosj)here and the want of rain, and 

 the lakes would sink to that level at which, as in the case of the 

 Caspian Sea, the precipitation and evaporation would finally become 

 equal. 



537. There is a self-regiilating principle that would bring about 

 itow the level of Cas- this equality ; for as the water in the lakes becomes 

 plans is reduced. lowcr, the area of its surface 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 evapo- 

 rated from the sea is exactly equal to the whole amount pom-ed 

 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 ; 

 ^nd finally, too, the great American lakes, in the process of ages, 



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



