Dr Forry on the Climate of the United States. 223 



elusion adverse to the vox populis which thus seems to be not 

 always the vox Dei, may be regarded as affording a confirma- 

 tion of the legitimacy of the deduction. 



We have thus attempted to answer the question, does the 

 climate of a locality, in a series of years, undergo any perma- 

 nent changes ? And we may add, that although the mean 

 temperatures, as has been ascertained by instrumental observa- 

 tions, vary from one another irregularly, either a few degrees 

 above or below the absolute mean temperature of the place ; 

 yet it has not been found that the temperature of a locality 

 undergoes changes in any ratio of progression. At the same 

 time, this series of atmospheric changes, however complicated 

 and perplexing, there is good reason to believe, is as deter- 

 minate in its nature, as the revolutions of the celestial bodies. 

 When, however, the science of meteorology shall have become 

 more advanced, we shall doubtless discover that these apparent 

 perturbations of annual temperatures are real oscillations — ■ 

 vast cycles, which will enable us to predict, no doubt with 

 some degree of certainty, the condition of future seasons. 



It now remains to advert to two collateral questions : Is 

 the climate west of the Alleghanies milder by 3^ of latitude 

 than that east? Does the climate of our north-western frontier 

 resemble that of the Eastern States on their first settlement ? 



The opinion was early entertained, that the climate of the 

 region west of the Alleghanies, is mucli milder than that of 

 the district east. Mr Jefferson estimated the difference 

 equivalent to 3^ of latitude, as similar vegetable productions 

 are found so many degrees farther north. 



These phenomena, M. Volney ascribed to the influence of 

 the southwest winds, which carry the warm air of the Gulf 

 of Mexico up the Valley of the Mississippi. As North 

 America has two mountain chains, extending from northwest 

 to southeast, neai'ly parallel to the coasts, and forming almost 

 equal angles with the meridian, Humboldt endeavoured to 

 explain the migration of vegetables toward the north, by the 

 form and direction of this great valley, which opens from 

 the north to the south, while the Atlantic coast presents 

 valleys of a transverse direction, which oppose great ob- 

 stacles to the passage of plants from one valley to another. 



