184 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
did the vapor for those rains come from ? and what has stopped 
the supply ? Surely not the elevation or depression of the Dead 
Sea basin. 
375. My researches with regard to the winds have suggested 
the probability 121) that the vapor which is condensed into 
rains for the lake valley, and which the St. Lawrence carries olf 
to the Atlantic Ocean, is taken up by the southeast trade-winds 
of the Pacific Ocean. Suppose this to be the case, and that the 
winds which bring this vapor arrive with it in the lake country at 
a mean dew-point of 50°. This would make the southwest winds 
the rain winds for the lakes generally, as well as for the Missis- 
sippi Valley ; they are also, speaking generally, the rain winds of 
Europe, and, I have no doubt, of extra-tropical Asia also. 
376. Now suppose a certain mountain range, hundreds of miles 
to the southwest of the lakes, but across the path of these winds, 
were to be suddenly elevated, and its crest pushed up into the re- 
gions of snow, having a mean temperature of 30° Fahrenheit. 
The winds, in passing that range, would be subjected to a mean 
dew-point of 30° ; and, not meeting with any more evaporating 
surface between such range and the lakes 125), they would 
have no longer any moisture to deposit at the supposed lake tem- 
perature 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 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 bed ; 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 
become equal. 
377. 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 surface would be diminished, and the amount of vapor 
taken from it would consequently become less and less as the sur- 
face 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 w^ater evaporated from the sea is 
exactly equal to the whole amount poured back into it by the 
