Dr Forry on the Climate of the United States. 227 



plex theory of the winds. All thermometrical results con- 

 firm the law, that, in propoi'tion as we recede from the ocean 

 or inland seas, the climate grows more excessive ; and that 

 the meteorological phenomena of the region of the great 

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

 parent, from the single fact, that the winters are several de- 

 grees warmer, and the summers at least ten degrees cooler, 

 as I'egards the mean temperature of the 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 

 face ; for while he ascribes the modified climate of the lakes 

 to the agency of tropical winds, he admits that the inter- 

 mediate country traversed by these winds has a much more 

 rigorous climate. 



The influence of predominant winds is manifest, however, 

 throughout the United States ; for one prevailing wind, the 

 south-west, blows from a warm sea ; another, the north-east, 

 from a frigid ocean ; and a third, the north-west, from frozen 

 deserts. 



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

 sissippi, whatever may be its degree, arises from the com- 

 bined agency of the Gulf of Mexico and the Great lakes ; for 

 if land were substituted for the area of the latter (93,000 

 square miles), that region 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 

 generally by writers as an established fact, arose from the 

 circumstance that the United States present, on the same 

 parallel, different systems of climate, — causes upon which the 

 geographical distribution of plants mainly depends. 



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

 that to some entirely difi'erent constitutions of the atmos- 

 phere are adapted. In respect to the culture of vegetables, 

 it is necessary to keep in view three objects, — the mean tem- 

 perature of the summer, that of the warmest month, and that 

 of tho oolrlost month ; for some plants, indifferent to h\"\\ 



