BOOK V. 
GEOGRAPHY. 
47& 
The temperature at the surface of the earth was 
at an elevation of 2,952 feet, was 
7,288 
9,993 
11,059 
11,293 
- 74° 
- 70° 
- 72° 
- 69° 
- 45° 
38° 
The difference between the temperature of the highest eleva- 
tion and the earth's surface amounting to 36° in the space of 
twenty-seven minutes. 
The amount of the decrement of heat, as compared with 
that of latitude, has been calculated to be, in France, equal 
to one degree of retrogressive latitude for every 540 feet of 
vertical elevation ; that is to say, the temperature of a district 
of 3240 feet of elevation, in 45° N. lat., would be equal to 
the temperature of 51° N. lat. on a level with the sea. But, 
from Humboldt's computations, it appears that, nearer the 
equator, this proportion varies. He found, from careful and 
repeated observations, between 0 and 3000 feet of elevation, 
that in the middle of the temperate zone, the mean temper- 
ature of the year decreased in a degree equivalent to 2° of 
N. lat. for every 600 feet of elevation ; the mean summer heat 
1° 30'; the mean autumnal heat 1° 24^; or, on an average, 
the decrement of temperature was about 1° of latitude for 
every 396 feet of elevation. Temperature decreasing in this 
rapid ratio, it is evident that, if vegetation is affected by tem- 
perature, it will offer great differences in the ascent of a 
mountain. And accordingly it is found, as will be seen by 
the following tables, that the nature of the vegetation towards 
the upper limits at which plants grow, gradually changes from 
that of the base of the mountain, until plants entirely disap- 
pear at the limits of perpetual snow. 
