288 THE PHYSICAL GEOGRAPHY OF THE SEA. 



the lake country at a mean dew-point of 50°. Let iis also admit 

 the southwest winds to be the rain winds for the lakes general- 

 ly, 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. Now suppose a certain mountain range, hundreds of miles 

 The influence of ^^ ^^^ southwcst of the lakcs, but across the path of 

 mountain ranges, ^-^^^q wiuds, with their dcw-polut at 50°, were to be 

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

 having a mean temperature at its summit of 80^ 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- 

 ture of 50° ; for they could not yield their moisture to any thing 

 above 30°. Consequently, the amount of precipitation in the lake 

 country would fall off; the winds whicli 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 precipitation and evaporation would finally be- 

 come equal. 



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

 How the level of caa. ^^^^ cquality ; for as the water in the lakes becomes 

 plans is reduced. lowcr, the area of its surface would be diminished, 

 and the amount of vapor taken from it would consequently be- 

 come 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 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 rivCTS 

 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 



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



