48 Dr. Darwin’s f rigor if c Experiments 
was always fufpe&ed, and has of late been demonflrated by 
the aerial navigators. When large diftridts of air from the 
lower parts of the atmofphere are raifed two or three miles 
high, they become fo much expanded by the great diminution 
of the prelfure over them, and thence become fo cold, that hail 
or fnow is produced from the precipitated vapour, if they con- 
tain any : and as there is, in thefe high provinces of the atmo- 
fphere, nothing elfe for the expanded air to acquire heat from, 
after the precipitation of its vapour, the fame degree of cold 
^continues, till the air, on defcending to the earth, acquires 
again its former ftate of condenfation and of warmth. 
The Andes, almoft under the line, refts its bafe on burning 
fands ; about its middle height is a moft pleafant and tempe- 
rate climate, covering an extenfive plain, on which is built 
the city of Quito ; while its forehead is incircled with eternal 
fnow, coeval perhaps with the elevation of the mountain : 
yet, according to the accounts of Ulloa, thefe three dis- 
cordant climates l'eldom intrench much upon each other’s terri- 
tories. The hot winds below, if they afcend, become cooled 
by their expanfion, and hence cannot affedl the fnow upon the 
fummit ; and the cold winds, that fweep the lummit, become 
condenfed as they defcend, and of temperate warmth, before 
they reach the fertile plains of Quito. 
Correfpondence of the Heat of the Atmofphere 'with the Height of 
> the Barometer. 
From this principle fome of the fudden changes of our 
atmofphere from hot to cold, aud from dry to moiid, may 
likewile be accounted for. During the laid year I frequently 
obferved, that when the barometer rofe (the wind continuing 
in 
