[300 ] 74 
found that atmospheric humidity is either uniform or increased to heights — 
beyond uniformity of vegetation, and then suddenly diminishes to a 
large oot a aie not pi test altering with it; so that it would 
Seem as if the atmosphere were composed of ‘dee eep beds oF air, suddenly 
differing from ail other in the elasticity of their aqueous 
Fron observations made by Captain Sabine, with a Daniell’s hygrometer, 
at dghenibiten it appears that on that island, at 17 feet above the sea, the 
80 nt of eee was 5°; and, at ae feet a sees was 3°5'; so eat, 
n this case, the air became more humid as he ascended. At Trinidad, 
the amount a dryness on a level with the sea was a = at 1,060 higher, 
air was saturated with moisture; in this instance, also, h midity in- 
creased with elevation. At Jamaica it was found ies ona ‘evel with the, 
sea, the degree of dryness was 7°; at 4,080 feet higher, the air was satu-~ 
‘rated with moisture; but at 4,580 feet the dryness was 16°, Hence itis 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, im Jamaica, 
“it had been left below at the time ‘the third observation was taken ; an 
that, in that island, the lower stratum of air is something more .than 
4,000 et deep. In Mr. Green’s voyage, the degree of dryness of the air, 
_at-an elevation of 9,893. eet, 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 observa ation 
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, _ 
2,000 feet beyond the highest limits of vegetation in Europe 
It must be confessed that these observations are = no means sufficiently 
fa 
ether the inereased rarity of the air, as we ascend, has an effect upon 
mesons is not  Wermined: It is not easy to say in. what way it can act, 
rding to- ees? yet known physiological laws, unless, as De Cand lolle 
remarks, in supplying an insufficient quantity of oxygen for Fagin 
Bu, as we ‘find plants « of the plains grow indifferently on the highest 
tains, it does not seem that there is any such: diminution of ox 
interferes with the operations of vegetation. The diminution of = 
eric pressure, which, of course, takes place at 
‘facilitate evaporation ; but we have yet to learn in what precise way that 
Pp ommenon influences vegetation. 
From as now been said, all that is apparent is that, as we ascend 
in the Haig vs an diminishes, and light increases an 
“stances, and. especially upon. the amount of summer heat, of which more 
will be said presently. Thus, at Pnoatckisss in Lapland, in 68° 30! north 
Jatitude, at an elevation of 1,356 feet above th ich, f 
it situation, should be seareely letped with hee Von Buch fou nd corn, 
orchards, and ar ich i cae 
Having how seen wh at differences are produced in the characters 
of vegetation by ion es the ‘Sea, let us. next take a view of the influ- 
emcee caused by latitude. In the c 
lying near = equator, the vege- 
t high elevations, may 
ptions depending upon puttanlain circum- - 
pitere sth ets hn a 
rr’ 
