BOOK V. 
GEOGRAPHY. 
483 
Elevation 
above the Sea 
in Feet. 
Equatorial Zone, 
Lat. 0°— 21° 
Temperate Zone. 
Lat. 45°— 47° 
Mean Temperature 
of the Year. 
Difference. 
Mean Temperature 
of the Year. 
Difference. 
0 
80° 
12° 
4° 
9° 
53° 
12° 
9° 
3,000 
68° 
41° 
6,000 
64° 
32° 
9° 
9,000 
55° 
11° 
10° 
23° 
12,000 
44° 
15,000 
34° 
The diminution of the density of the air as we ascend 
produces a corresponding increase in the intensity of the light ; 
a circumstance in which high elevation has been said to cor- 
respond with high latitudes ; but this is doubtful. 
It is said that the humidity of the atmosphere decreases as 
we ascend, and that to this may be ascribed much of the 
effect produced upon vegetation by great heights. That the 
humidity of the atmosphere does much alfect vegetation is not 
to be doubted ; and if it were certain that the air became 
gradually drier as we ascend, a second cause, as powerful as 
that of temperature, would be found for the effects of elevation 
upon vegetation. But it is certain that the humidity of the 
air does not change gradually, as we ascend, with the cha- 
racter of vegetation ; on the contrary, it has been found that 
atmospheric humidity is either uniform or increased to heights 
far beyond uniformity of vegetation, and then suddenly di- 
minishes to a large amount, vegetation not suddenly altering 
with it ; so that it would seem as if the atmosphere were com- 
posed of deep beds of air, suddenly differing from each other 
in the elasticity of their aqueous vapour. 
From observations made by Capt. Sabine, with a Daniell^s 
hygrometer, at Ascension, it appears that, on that island, at 
seventeen feet above the sea, the amount of dryness was 5° ; 
and, at 2237 feet higher, was 3° 5' ; so that, in this case, the 
air became more humid as he ascended. At Trinidad the 
amount of dryness on a level with the sea was 5° ; at 1060 
I I 2 
