

Dr. Forry on the Climate of the United States, fyc. 231 



These phenomena M. Yolney ascribed to the influence of the 

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

 co up the valley of the Mississippi. As North America has two 

 mountain chains, extending from northwest to southeast, nearly 

 parallel to the coasts, and forming almost equal angles with the 

 meridian, Humboldt endeavored to explain the migration of ve- 

 getables toward the north, by the form and direction of this great 

 valley which opens from the north to the south ; while the At- 

 lantic coast presents valleys of a transverse direction, which oppo- 

 ses great obstacles to the passage of plants from one valley to an- 

 other. The tropical current or trade-wind, it is said, deflected 

 by the Mexican elevations, enters the great basin of the Missis- 

 sippi and sweeps over the extensive country lying east of the 

 Rocky Mountains ; and that when this current continues for some 

 days, such extraordinary heat prevails even through the basin of 

 the St. Lawrence, that the thermometer at Montreal sometimes 

 rises to 98° of Fahr. In winter, on the contrary, when the local- 

 ly of tins great circuit is changed to more southern latitudes, suc- 

 ceeded by the cold winds which sweep across the continent from 

 the Rocky Mountains or descend from h 

 becomes subject to all the rigors of a Siberian winter. 

 Upon the fallacy of these views it is deemed unnecessary to 

 e « It is proved by thermometrical data that the climate 

 w est of the Alleghany is more excessive than that on the Atlantic 

 side — a condition that would seem unfriendly to the migration 

 °' plants. Thus Jefferson Barracks, on the Mississippi, exhibits 

 a greater contrast in the seasons than Washington City; and the 

 same is true in regard to Fort Gibson and Fort Monroe, notwith- 

 standing the former is 1° 32' farther south. That the climate of 

 tne peninsula of Michigan encompassed by ocean-lakes, should 

 prove genial to plants that will not flourish in the same lati- 

 tudes in the interior of New York, is, indeed, consonant with the 

 aws °f nature ; and that the same plants should flourish 3° far- 

 ther n °rth in the valley of the Mississippi than on the eastern side 

 of l he mountain, finds a sufficient explanation in the following ex- 

 tract fro-m Murray's Encyclopaedia of Geography : "Powerful sum- 

 me r heats are capable of causing trees and shrubs to endure the 

 mos t trying effects of cold in the ensuing winter, as we find in 

 Enumerable instances ; and vice versa. Hence, in Great Brit- 

 air| j so many vegetables, fruit-trees in particular, for want of a 



dilat 



