444 THE MOUNTAIN. 



cause large bodies of water modify climates by equalizing the sea- 

 sons, making the summers cooler and the winters warmer than the 

 mean of the parallel. 



" Thus, our great interior lakes modify the climate in relation to 

 temperature in their vicinity. Their summers are cooler and their 

 winters warmer ; but westward of them the same line of equal sum- 

 mer temperature, or isothermal line, rises with considerable abrupt- 

 ness ; and the winter, or isocheimal line of equal temperature, falls 

 in a similar manner; thus, the range of the thermometer, from the 

 highest elevation to the lowest depression, for the year, is very great, 

 while in the tropics the range is comparatively small. 



"From observations made at the military posts of the United 

 States, I>r. Forrey deduced summer and winter lines of equal tem- 

 perature, starting from the vicinity of Boston and running west, 

 which showed most remarkably the rise of the summer lines as in- 

 tensity increased, and the fall of the winter lines in like manner. 

 The influence of the lakes was also most obvious. The elevation of 

 the earth increases, going west, to about 700 feet at the surface of 

 the lakes, and to nearly 4000 feet at the eastern base of the Rocky 

 Mountains, and, although temperature does not decrease to as great 

 a degree when the elevation above the level of the sea is gradual, 

 yet some allowance should doubtless be made for that elevation on 

 this line. When that allowance is made, the ascent of the summer 

 line to the north, over the area of the greatest intensity, is strikingly 

 apparent." 



He farther remarks : 



"The effect of this difference of magnetic intensity upon the cli- 

 mate of Europe is marked. There, the excessive summer heat, which 

 our greater magnetic intensity and larger volume of counter-trade 

 give us, is unknown. Hence, while we can grow Indian corn (which 

 requires the excessive summer heat) over all the Eastern States, up 

 to 45, and in some localities, east of the lakes, to 47 30', and to 

 50 west of them, to the base of the Rocky Mountains, and, notwith- 

 standing the increase of elevation, they cannot grow it except over a 

 limited area, and with limited success."* 



Magnetism and Thermalism it would seem, then, are 

 intimately associated in the phenomena of the surface of 

 the earth ; and, on the point now under consideration, the 

 former exerts, according to Mr. Butler, great influence in 



* In thermometric range, the western side of the eastern continent 

 is the same as the western side of the western continent. 



