66 WRITINGS OF JOSEPH HENRY. [1855- 



of the Alleghany Mountains however does not so much 

 affect the meteorology of the country as from the magni- 

 tude of the system we might at first suppose; and this re- 

 sults from the fact that their direction is from the southwest 

 towards the northeast, which as we shall see hereafter, is the 

 prevailing direction of the fertilizing wind of the United 

 States. They do not therefore obstruct its course ; it flows 

 on either side of them and along the valleys between them. 

 They do however in a considerable degree, modify the 

 character of the westerly winds as felt upon the coast, de- 

 priving them of their moisture. 



A reference to the map will show that the Rocky Moun- 

 tain system occupies one-third of the entire breadth of the 

 United States, and that the remaining two-thirds are divided 

 into two nearly equal portions by the Mississippi river, be- 

 ginning at its source. This great western mountain system 

 of the North American continent, which produces the most 

 important modifying influence on the climate of the United 

 States, may be described as a broad, elevated swell or plateau 

 of land, (the prolongation of the system of South Amer- 

 ica, to which the Andes belong,) extending northward 

 in the general direction of the Pacific coast, with var^ang 

 elevation and width to the Arctic circle. It occupies nearly 

 the whole breadth of Mexico, from the Rio del Norte to the 

 Pacific, and becomes still broader as it extends northward, 

 occupying at the latitude of 40°, (as has just been said,) one- 

 third of the breadth of the whole continent. Resting upon 

 this great swell of land is a series of approximately parallel 

 ridges, the principal of which are the Rocky Mountain 

 ranges on the east and the Coast ranges on the west, with 

 ridges of less magnitude between, the general direction of 

 which is north, inclining towards the west. Between these 

 ranges is a series of extensive elevated valleys of extreme 

 dryness, and, in the summer, of intense heat. 



As we proceed north from the high plains of Mexico, the 

 base of the system declines to about the 32d parallel of north 

 latitude, where its transverse vertical section presents the 

 least amount of land above the general level. It has how- 



