X84 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 w^ith regard to the winds have suggested 

 the probability (§ 121) that the vapor w^hich is condensed into 

 rains for the lake valley, and which the St. Lawrence carries off 

 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 oflf; 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 w^ould 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 water evaporated from the sea is 

 exactly equal to the whole amount poured back into it by the 



