Dr Forry on the Climate of the United States. 75 



the interior, the extreme range of the temperature increases, 

 and the seasons are violently contrasted. Having come 

 within the influence of the great lakes, a climate like that of 

 the sea-board is found ; and proceeding into the region be- 

 yond the modifying agency of these inland seas, an excessive 

 climate is again exhibited. And if we continue our route as 

 far as the Pacific Ocean, a climate even more mild and equable 

 than similar parallels in Western Europe, as will be satis- 

 factorily demonstrated, will be presented. The variations of 

 the isotheral and isocheimal curves — ^the lines of equal sum- 

 mer and of equal winter temperature, as illustrated in 

 Plate I., thus afibrd a happy illustration of the equalizing 

 tendency of large bodies of water. Hence the former divi- 

 sion of the surface of the earth into five zones, as regards its 

 temperature, has been superseded in scientific inquiries by a 

 more precise arrangement. Places having the same mean 

 annual temperature are connected by isothermal lines, and 

 the spaces between them are called isothermal zones. 



It is thus seen that, notwithstanding the mean annual 

 temperature presents little variation on the same parallels, 

 four striking inflections of the isotheral and isocheimal lines 

 are exhibited in rapid succession, constituting two systems 

 of climate, viz. that of the Atlantic Ocean and the great 

 lakes, which pertains, comparatively speaking, to the class 

 of mild or uniform, and that of the intervening tract and the 

 region beyond the lakes, characterized as climates emphati- 

 cally excessive or rigorous. The difference of climate, as the 

 mean annual temperature is neai'ly the same, is, therefore, 

 owing to the unequal distribution of heat among the seasons, 

 as is well illustrated in the accompanying Plate I. At the 

 posts on large bodies of water, the mean temperature of winter 

 is higher, and that of summer lower, than in the opposite 

 localities ; but these results are more satisfactorily evidenced 

 by comparing the difference between the mean temperature 

 of winter and summer, and the warmest and coldest month 

 in each system of climate. Thus Fort Bi'ady, at the outlet 

 of Lake Superior, shews a difference of only 42''-ll between 

 the mean temperature of winter and summer, while Hancock 

 Barracks, half a degree farther south, in the state of Maine, 



