ignorant, works in its appointed course, performing with un- 
erring certainty the precise amount of work allotted to it; 
all of them together contributing to a result so unfailing that 
it might be expressed by an algebraic formula, did we know 
perfectly the whole of the influences at work, and the laws by 
which their varying energies are regulated. 
But though we cannot hope to reach a mathematical deter- 
mination in so complicated a process, yet we may with toler- 
able certainty foretell, in general terms, the result of certain 
given conditions ; by considering one by one, the most active 
of the known causes of atmospheric condensation, and tracing 
the probable manner and amount of influence exercised upon 
each, by the conditions stated. 
The chief, if not the only way in which forests and vegeta- 
tion in general, can affect atmospheric condensation, in the 
formation of rain, must be through their influence upon the 
temperatui’e of the soil. 
. Close vegetation affects the temperature of the soil it covers 
in two ways. 
Firstly . — It causes the evaporation of moisture, and con- 
sequent cooling of the surface of the ground, to proceed slow- 
ly, and continuously ; instead of allowing the direct rays of 
the sun to first dry up such moisture suddenly, and then to 
raise the temperature of the ground to a high degree : so that 
in this respect the tendency of the vegetation is to keep the 
temperature of ground it shelters lower than that of ground 
exposed and bare. 
Secondly . — Vegetation acts precisely in the same - manner 
as a clothing of a bad conducting medium; preventing the 
communication or loss of heat ; so that it checks variation of 
temperature, and tends to keep the soil at a mean degree of 
heat. 
During the hot part of the day, ground so protected will 
be cooler than a naked surface of rock or earth, but at night 
it will in general be warmer. On the one hand, the foliage 
will prevent the full force of the sun’s rays reaching the earth ; 
the intercepted heat being returned into space, partly by re- 
flection from the surfaces of the leaves, which in tropical 
plants are often highly polished ; and partly by radiation from 
