272 Prof. Bischof on the Temperatufe of 



hard, the heights correspond tolerably well for all these places. 

 On the average, then, the mean temperature would become 

 82°.0 at the height of about 6165 feet above the sea, for places 

 situated between 48° 10' and 46° 12' north latitude, and at the 

 height of 8200 feet, the supposed limit of the snowy regions in 

 the Alps, the mean temperature would be +25°.25. This nearly 

 agrees with the observations of Pictet, according to which the 

 snow first shews itself where the mean temperature is +23°. 68. 

 But here we find a difference of 9°.67 between the mean tem- 

 perature on the limits of perennial snow on the equator, as 

 taken by Boussingault from the observations of Humboldt, and 

 that of the limits of the snowy regions in the Alps. This dif- 

 ference, however, is reduced to 7°.42 if we adopt -f-32o.67 as the 

 mean temperature on the limits of perpetual snow at the equa- 

 tor, which is the value deduced from other observations of Von 

 Humboldt. 



As between the tropics the temperature of the soil shews no 

 variations at the depth of one foot, it will shew still slighter 

 variations at the height where the mean temperature is 32°. 

 Snow, which there covers the ground, will therefore be unable 

 to melt on its under surface. But on its upper surface, which 

 is exposed to the air, the snow may partially melt during the 

 summer, because the variations in the temperature of the air are 

 always more extensive than those of the soil, even in the small- 

 est depths, and the temperature of the air at that height be- 

 tween the tropics may, therefore, always rise a few degrees 

 above 32° during the summer. In the temperate zones, on the 

 other hand, the temperature of the soil still suffers considerable 

 variations at that height, in which the mean temperature be- 

 comes 32°, and, therefore, rises in summer to several degrees 

 above 32° ; supposing the soil not to be covered with glaciers, 

 or any masses of snow by which the communication of heat 

 from above M'-juld be prevented. Here, then, snow which 

 covers the ground at that height, must melt away during the 

 summer even on its under surface, and still more on its upper 

 surface, which is exposed to the air, so that it must entirely 

 disappear. This accounts for the possibility of perpetual snow 

 being met with between the tropics, where the mean tempera- 

 ture is equal to or even above S2o, whilst in higher latitudes it 



