20 
the evaporation from their numerous surfaces, is much greater 
than from the same space of land, uncovered with vegetables. 
From experiments, it appears that forests discharge one-third 
more vapour into the atmosphere, even than the same space of 
ground would do if actually covered with water. Now, as 
water, in the form of vapour, is the evident associate and con- 
ductor of heat, this forms one reason why woody countries are 
colder than those that are open and cultivated. Besides this, 
when trees are tall and close, as forests, they exclude the sun’s 
rays, and the winter snows are preserved by them, to spread 
their influence over a milder season. These combined circum- 
stances enable us to account for the amelioration of climate that 
attends agricultural cultivation. 
47 Atmosphere, influenced by Altitude. The Andes, al- 
most under the equatorial line, rest their base on burning sands. 
About their middle height is a most pleasant and temperate cli- 
mate, covering an extensive plain, on which is built the city of 
Quito; whilst their forehead is encircled with eternal snow, per- 
haps coeval with the mountains. Yet, according to the accounts 
of Don Ulloa, these three discordant climates seldom encroach 
much on each other’s territories. The hot winds below, if they 
ascend, become cooled by their expansion; hence they cannot 
affect the snow on the summit; and the cold winds, that sweep 
the summit, become condensed as they descend, and of temper- 
ate warmth before they reach the fertile plains of Quito. From 
the preceding observations, a glimpse will be caught of some 
of the secondary causes of heat and cold; but the temperature 
of the atmosphere, and the vicissitudes of its heat and cold, are 
subject to such a variety of irregularities, that no theory which 
ha^ yet been proposed, is altogether sufficient to explain. It is, 
notwithstanding, highly delightful and advantageous to the phi- 
losophic mind, to search out the cause and effect, as far as it is 
laid open to us, of the unerring progress of natural agencies, 
which are continually operating, in the works of nature, for the 
advantage of created beings. 
48 Heat and Cold. It being shown that a variety of circum- 
stances in nature, combine to prevent sudden transitions from 
one extreme of temperature to the other, the cultivator should 
assist this process of nature. In frosty weather, shade exposed 
plants from the sun; and in other cases, as reason may dictate. 
