CLIMATE AND CAUSES 203 



particularly on the shady side of the Rockies, and establish the 

 prevailing northwest winter winds which, being affected by the 

 temperature of the arctic regions on the one hand, and by the 

 mountains on the other, bring the minimum line of cold far to the 

 south. Were the American Desert an inland sea, the summers 

 of our plains would lose their exceptional character, and our 

 winters would be like those of eastern Europe. 



"In a book, like the present, however, it would be out of 

 place to discuss the climate of the eastern hemisphere; but it 

 could be shown that precisely similar causes to those which I have 

 specified, exist there and are productive of the same results. 



"Having stated what the recorded facts as to rainfall are, I 

 will give my reasons for asserting that these facts are the neces- 

 sary consequences of the physical conditions existing in the west 

 of the North American continent. 



"In the beginning of this chapter, I referred to the position 

 of the Great American Desert, and pointed out one of its effects 

 upon the air-currents rising northward from the Gulf of Mexico — 

 viz, its power to direct and draw them to itself, and to the west- 

 ward of their natural course. Another fact arises from the heat 

 given off by radiation, during the summer months. The Gulf air 

 currents, laden with moisture, when drawn over the desert, are 

 met by the rarified and heated air ascending from its surface and 

 the rain which, in the original course, they would pour down, being 

 prevented from falling, passes on and is wafted by the prevailing 

 winds in the direction of our North West. There, their land borne 

 and priceless load is given forth in the form of our summer rains. 

 As the rainfall increases northward, the bunch grass is succeeded 

 by sward after which there is copsewood and then aspen forest and 

 spruce on the water-shed. 



"Having shown cause for the summer rains, I may now state 

 that the simple suspension of those desert effects which gave the 

 summer rains, is the cause of the almost total absence of rain in 

 the autumn and winter periods. 



"It was shown, when writing of the winter temperature, that, 

 as the desert cooled, the main air-currents from the Gulf of Mexico 

 no longer pursued their westward course, but passed to the east- 



