EEPORT OF THE SECRETARY. 49 



coast until it reaches, in diuiinislied quantity, the soutliern portion of 

 California. As the sun ascends again toward the north the rain also 

 gradually returns northward, until it leaves almost entirely, during the 

 summer, that portion of the western coast south of 50° north latitude. 



The primary currents of air we have mentioned are modified by the 

 varying- relative temi)erature of the ocean and the continents. The 

 capacity of water for heat being about six times that of land, the latter 

 becomes relatively much Avarmer h\ summer and colder in wiuter than 

 the former; and since the air at the surface of the earth tends to flow 

 from the colder to the warmer region, there must be a tendency of the 

 wind from the ocean toward the land in summer, and the contrary in 

 winter; though this may not be powerful enough to reverse the general 

 currents, it is yet sufficiently so to produce in them the modifications of a 

 very perceptible character.- In summer, the greater heat of the surface 

 of the middle portion of North America keeps the return trade-current 

 at a high elevation, and produces a surface current from the Gulf of 

 Mexico, which, on account of the motion of the earth, assumes a direc- 

 tion from southwest to northeast, bearing with it the moisture which is 

 precipitated in rain, principally throughout the region east of the Mis- 

 sissippi Eiver. Were the earth at rest the same current would How 

 over the whole of the Mississippi Valley to the base of the Eocky JVIoun- 

 tains, and the aridity of the western portion of this region would- no longer 

 exist. 



In winter, when the upper current, after sweeping across the Pacific 

 Ocean, ascends along the western slopes of the mountains, it precipi- 

 tates its moisture on their crest in the form of snow, which, melting in 

 summer, gives rise to numerous streams which, although not sufficient to 

 irrigate all the region between the Eocky Mountains and the fertile 

 country \djoining the Mississippi on the west, may yet, by well-directed 

 enterprise, serve very much to circumscribe the arid regions in the 

 Mississippi Valley, as well as to mitigate the droughts of the great in- 

 terior basins of the mountain system. 



In summer, the sun approaching, in its northern declination, a posi- 

 tion nearly vertical to the extremity of the Peninsula of Florida, heats 

 the land and produces inflowing and upward currents of air, charged 

 with moisture, which, i^erhaps more frequently in the after-part of the 

 day, falls in copious showers. In winter, on the contrary, the sun being 

 far south of the latitude of Florida, the surface currents are almost 

 neutralized, or tend to flow from the land, and, hence, the rain-fall at 

 this season is much less. 



In the region east of the Mississippi, including the whole Appalachian 

 system, the direction of the surface wind is the same as that of the trend 

 of the mountains, and hence both sides and crests of the latter and in- 

 tervening valleys are covered with vegetation. 



The region along the eastern coast of the United States is also sup- 

 plied with vapor from the Atlantic Ocean, which is borne inland in all 

 cases where an approaching storm gives rise to a wind from an easterly 

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