484 
GEOGRAPHY. 
BOOK V. 
feet higher the air was saturated v/ith moisture; in this in- 
stance, also, humidity increased with elevation. At Jamaica 
it was found that, on a level with the sea, the degree of dry- 
ness was 7° ; at 4080 feet higher the air was saturated with 
moisture ; but at 4580 feet the dryness was 16°. Hence it 
is to be inferred that, in these observations, the lower bed of 
the atmosphere was not passed through, either at Ascension 
or in Trinidad ; but that, in Jamaica, it had been left below 
at the time the third observation was taken ; and that in that 
island the lower stratum of air is something more than 4000 
feet deep. In Mr. Green's voyage the degree of dryness of 
the air, at an elevation of 9893 feet, was 5°, nearly the same 
as it was observed to be on the surface of the earth below at 
the same time; but, at 11,059 feet, it was 13°; and at 11,293 
feet, the highest point at which an observation was made, it 
was still 13° ; so that it would seem that the humidity of the 
atmosphere, at that time, did not vary through a bed of air 
rising perhaps 2000 feet beyond the highest limits of vegeta- 
tion in Europe. 
It must be confessed that these observations are by no 
means sufficiently numerous to become the foundation of any 
thing connected with the effect of elevation upon the cha- 
racters of plants ; but they, at least, answer the purpose of 
showing that, in the present state of our information, the 
effects of humidity are not appreciable in investigating the 
subject. 
Whether the increased rarity of the air, as we ascend, has 
any effect upon vegetation, is not determined. It is not easy 
to say in what way it can act, according to any yet known 
physiological laws, unless, as De Candolle remarks, in supply- 
ing an insufficient quantity of oxygen for absorption. But, 
as we find plants of the plains grow indifferently on the highest 
mountains, it does not seem that there is any such diminution 
of oxygen as interferes with the operations of vegetation. 
The diminution of atmospheric pressure, which of course 
takes place at high elevations, may facilitate evaporation ; but 
we have yet to learn in what precise way that phenomenon 
influences vegetation. 
