THEORY OF WINDS. 329 



continents and islands ; pestilential vapours dis- 

 persed ; the face of nature refreshed, and adorned 

 with living robes of surpassing beauty? By 

 sinking into the earth, they acquire the properties 

 of the various mineral beds through which they 

 pass, and again bubble up in fountains of deli- 

 cious water for the sustenance of animal and 

 vegetable life.* 



If the whole earth were of uniform surface and 

 elevation, its temperature would be the same in 

 given latitudes and seasons, and the currents of 

 the atmosphere would present a succession of 



* By the agency of warm southerly winds immense masses 

 of aqueous vapour are transported from the ocean over North 

 America, to the higher latitudes, where, meeting with a body of 

 air from the frozen regions, it is condensed into cold rains, or 

 extensive falls of snow, according to the season of the year; 

 while the northern atmosphere is thus warmed by the disengage- 

 ment of caloric from southern vapour. 



If southern winds prevail for a long time from the ocean during 

 autumn, until the atmosphere is saturated with vapour over a 

 large portion of the northern hemisphere, without meeting with a 

 mass of northern air to condense it, the commencement of winter is 

 marked by an extensive fall of snow, from a foot to eighteen 

 inches, or even three feet in depth, over which the polar winds 

 pass without being warmed, to the lower latitudes, causing 

 excessive winters. This is one reason why during some winters 

 in the United States the temperature occasionally falls from 30 

 to 40 lower than during others. After such a fall of snow the 

 wind has prevailed, for six weeks together, from the northern 

 points of the compass, as in the winter of 1779 and 80, and 

 about the same length of time during the winter of 1834 and 

 35, when the mercury fell from 20 to 40, from latitude 

 35 to 43. I shall have more to say on this subject in another 

 place. 



