of Oregon and Upper California. 393 



winds come to a great extent from the northwest and west. 

 Being cooler than the land of the interior, (which is heated by 

 the sun's direct rays above the temperature of the ocean,) they 

 are gradually becoming warmer as they range along to the 

 southward and eastward over the territory: and as heated air 

 will take up more moisture than cold air, the capacity for moist- 

 ure is gradually increased. The winds therefore come from the 

 ocean with comparatively little moisture, and as they become 

 warmer in their progress, they are continually absorbing it from 



every surface or object they touch, and thus parch the open 

 lands.* 



Just the reverse condition exists upon the eastern shores of our 

 continent, where the winds blow to a great extent from a tropical 

 direction, and coming surcharged with moisture, drop it readily 

 against the cold hills r or wherever northerly airs are encountered. 

 About the hills and ridges of the interior, much moisture is 

 condensed. For besides dews, mists form in these more elevated 

 regions in three ways: — (1) either from the cooling of a moist 

 overlying atmosphere below the dew point through the influence 

 of the cooled land and foliage ; — or by means of the winds that 

 blow over the hills, which may either (2) condense by their cool- 

 ness the moisture of the air about the heights, or (3) bring in 

 moisture to be condensed by the cool hill-sides; — the results de- 

 pending somewhat upon the time of the day and the season of 

 the year. These regions are thus enabled to support a growth of 

 trees, when the plains below are bare. 



Along the shores, the ocean winds produce comparatively cool 

 weather in summer, as they have not been moderated in tempera- 

 ture by passing over any part of the land. Within thirty miles of 

 the sea, the lower airs that oscillate back and forth, as night and 

 day alternate, are usually moist and favor the formation of heavy 

 mists. The land chilled at night by winds and radiation, often 

 chills the air above, so as to cause the moisture to become visible 

 in mists. In the formation of these mists in other cases, the 

 sea-breeze encounters a cold land and cold airs, and its vapors 

 are thereby condensed ; or it meets with warmer moist airs and 

 condenses the moisture they contain. By these means, at differ- 

 ent periods, a moist climate is produced. 



Southeast and south winds usually bring clouds and rain over 

 Western America, and these are common in the winter months. 

 They are warm winds charged with moisture which is readily 





this principle in explaining the origin of deserts 



jm.1. tt » w. *»*.*-.»- w»~- ~rr r r c - 1 o — * » 



and as he has shown, it is not only sufficient, but the only one sustained by facts. 

 For it meet- the important general truth that deserts occur within the latitudes 20° 

 to 35° north or south, and principally on the western side of continents, ^rhere the 

 winds are blowing for the greater part of the year from extra-tropical latitudes, 

 towards the tropical, and are therefore drying winds. 



Second Series, VoL VII, No. 21.— May, 1849. 50 



