234 Dr. Forry on the Climate of the United States, Sj-c. 



and that the meteorological phenomena of the region of the great 

 lakes do not arise from the agency of tropical winds, is apparent 

 from the single fact, that the winters are several degrees warmer, 

 and the summers at least ten degrees cooler, as regards the mean 

 temperature of these seasons, than positions one hundred miles 

 distant, notwithstanding on the same parallel or even directly- 

 south, and consequently equally exposed to the current from the 

 Gulf of Mexico. Volney's theory, in truth, bears a contradiction 

 upon its own face ; for, while he ascribes the modified climate of 

 the lakes to the agency of tropical winds, he admits that the in- 

 termediate country traversed by these winds has a much more 

 rigorous cUmate. 



The influence of predominant winds is manifest, however, 

 throughout the United States; for, one prevailing wind, the 

 southwest, blows from a warm sea, — another, the northeast, from 

 a frigid ocean, — and a third, the northwest, from frozen deserts. 



The modification in the climate of the valley of the Mississip- 

 pi, whatever may be its degree, arises from the combined agency 

 of the Gulf of Mexico and the great lakes : for if land were sub- 

 stituted for the area of the latter, (93,000 sqd^re miles,) that re- 

 gion would become, so far as the social state of man is concerned, 

 scarcely habitable. 



The opinion that the climate west of the Alleghany range is 

 milder by 3° of latitude than the east, — an opinion quoted gener- 

 ally by writers as an established fact, — arose from the circum- 

 stance that the United States present on the same parallel differ- 

 ent systems of climate — causes upon which the geographical dis- 

 tribution of plants mainly depends. 



In reference to the organic life of plants, it is well known that 

 to some entirely different constitutions of the atmosphere are adap- 

 ted. In respect to the culture of vegetables, it is necessary to 

 keep in view three objects, — the mean temperature of the sum- 

 mer, that of the warmest month, and that of the coldest month ; 

 for some plants indifferent to high summer temperature, cannot 

 endure the rigors of winter ; others, slightly sensible to low tem- 

 perature, require very warm but 'not long summers ; while to oth- 

 ers, a continuous rather than a very warm summer seems best 

 adapted. The development of vegetation in the same mean tem- 

 perature, is also retarded or accelerated, according as it is struck by 

 the direct rays of the sun, or receives the diffuse light of a foggy 



