COMPARISON OF TEMPER ATUEES. 259 



do not, however, decrease in the same ratio as we go to 

 the north ; on the contrary, the isothan-al lines nearly 

 follow the canoe route, and run to the northward and 

 westward. The elevation of the prairie slopes has less 

 influence in depressing the summer heat, than the nature 

 of the soil and other causes have in raising it. 



Experiments are still wanting whereby we may ascertain 

 the ratio of the decrease of mean heat in America with 

 the increase of altitude. In Table II. we find that, not- 

 withstanding the elevation of Franklin Malone above 

 Eastport of 645 feet, its mean temperature is greater ; its 

 interior position giving it an advantage in summer heat 

 over the sea-coast, which its greater altitude does not 

 destroy. If we refer to Dove's table, and contrast the 

 temperatures of Mont Louis, which lies near the 43d 

 parallel, with those of low country situations enumerated 

 at the foot of the preceding page, we find a mean dif- 

 ference of temperature of one degree of Fahrenheit for 

 350 feet of altitude. A similar allowance for the eleva- 

 tion of the successive steps of the St. Lawrence basin 

 would place in still greater prominence the rise of the 

 isothermal lines, and more still that of the isotha?ral ones, 

 as they recede from the Atlantic coast. There is, how- 

 ever, this difference in the climate of the summit of a 

 mountain and of an elevated plateau, that in the former 

 case we approach near the line of invariable temperature, 

 and the summer heat therefore differs less from the mean 

 of the year, and more from that of the plains, than on a 

 plateau where the depression of mean temperature pro- 

 duced by elevation is due chiefly to winter colds, and in a 

 small degree only to defect of summer heat. 



From Table II. also we may learn that the mean tem- 

 perature of the coast districts of the Pacific is greater than 

 that of the Atlantic countries, and, at the same time, more 



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