

48 Dr. Forty on the Climate of the United States, fyc. 



two to four degrees in the annual temperature, while the interior 

 posts are only about 1° nearer the equator, which cannot, on the 

 average, cause a greater difference of temperature than 1°"50. 



Having thus shown that there is an actual increase of annual 

 temperature, or a rise of the isothermal line, on receding from the 

 Atlantic, it is deemed unnecessary to give any details proving, 

 that instead of the seasons becoming from the same causes more 

 equalized, they actually grow more contrasted, inasmuch as this 

 law has been already abundantly established. Suffice it to com- 

 pare Fort Snelling on the Mississippi and Fort Sullivan on the 

 X*\ Atlantic. \ Although the former has a mean annual temperature 



2°*88 higher than that of the latter, yet it has a contrast between 

 the mean temperature of winter and summer actually 17°*45 

 greater! Equally striking is the contrast between the results 

 given by posts on the lakes and those in the same region, not- 

 withstanding not more than one, two, or three hundred miles 

 distant. Thus, on comparing Forts Snelling and Howard with 

 positions (Forts Brady and Mackinac) in the modified climate of 

 the lakes, this relation is discovered ; for, although the mean lat- 

 itude of the latter posts is only 1° 34' north of Fort Snelling, 

 (and perhaps four hundred miles distant,) yet the mean annual 

 temperature is 4°*25 lower. Now, of this difference in annual 

 temperature, not more than one half can be accounted for by 

 difference of latitude, being an expression of the same law that 

 was revealed by the comparison with posts modified by the ocean ; 

 and we also find, that so far from the temperature of the seasons 

 being more equalized at Fort Snelling, which has a higher annual 

 temperature, the difference between the mean temperature of 

 summer and winter is in reality 12 0, 84 greater than on the lakes. 

 . Humboldt's law holds good so far as the comparison refers to 

 the eastern and western continental coasts, each being more or 

 less modified by the ocean ; but in a comparison with an interior 

 position remote from large bodies of water, a new element, aris- 

 ing from the law of the accumulation of caloric by the surface 

 of the earth, doubtless enters into the calculation. It may be 

 said, however, that this ought to be compensated by the aug- 

 mented cold of winter ; but it is found in our excessive climates, 

 compared with the modified, that the annual temperature gains 

 more by the continued elevation of the thermometer in summer 

 than it loses by its depression in winter. Besides, in excessive 



