-1859] WRITINGS OP JOSEPH HENRY. 289 



his instinct is directed, but even these in the course of a 

 few weeks are almost reduced to a powder by the drying in- 

 fluence of the unscreened rays of a powerful sun. What 

 moisture rises from the evaporation of the rain which may 

 fell on the regions indicated by the unshaded part of the 

 map is constantly carried eastward instead of being precipi- 

 tated again on the place whence it rose. 



The direction of the several ridges of the Alleghany 

 Mountains is parallel to that of the fertilizing wind, and 

 hence these do not materially interrupt the southwestern cur- 

 rents, and are consequently sufficiently supplied with moist- 

 ure, except in the more elevated valleys which are inclosed by 

 a ridge at their southern extremities. 



From the fact, abundantly proved by observation, that 

 the vapor of the Pacific Ocean does not pass over the ele- 

 vated crests of the Rocky Mountain system, it must be evi- 

 dent that the idea that the supply of the interior of the North 

 American continent comes from the Southern Pacific by 

 ascending to the cold regions of the top of the belt of rains 

 is entirely untenable. The source from which the moisture 

 of the interior is derived is principally the Gulf of Mexico. 

 We shall endeavor to give in a subsequent Report an ac- 

 count of the climate of the several meteorological districts 

 into which the United States may be divided; the remain- 

 ing space allotted to this article will be devoted to a brief 

 exposition of the storms of the Continent. 



Storms of North America. — The two great systems of winds 

 to which we have so frequently alluded as existing over the 

 United States, present their meteorology in a simple form 

 and on a very extended scale, while the general features of 

 the phenomena of American storms are readily explicable 

 on the principles of the theory propounded by Professor 

 Espy. And first we may remark that on account of the 

 height of the Rocky Mountain system, the storms or other 

 commotions of the atmosphere which take place on its 

 western side, are seldom if ever communicated to the air on 

 the eastern; and this is a natural consequence of the prin- 

 ciple which refers these commotions to the evolution of the 



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