- 70 - 
compressed air can escape, by rising and passing over the 
mountain. 
The great quantity of rain observed to fall on the Wind- 
ward side of mountain ranges, in many parts of the world, 
has been attributed to the cause described ; but it seems pro- 
bable that the actual increase of pressure has had less to do 
with the result than the circumstance that the vapour was 
forced to ascend to cold altitudes, before it could escape over 
the mountain ; also the air must frequently suffer a loss of 
heat, by contact with the mountain, which, by reason of the 
constant radiation will often be lower in temperature than the 
air. which latter maintains more nearly a mean temperature. 
ugh the idea is not without supporters, it seems to be 
n aite out of the question that the slight impediment offered 
to the wind by a forest, or by a line of trees, can cause suffi- 
cient compression to produce rain ; the forest could only af- 
fect the motion of a thin stratum of air, allowing the chief 
mass of the wind and vapour to pass onward above, altoge- 
ther unimpeded, so that under the most favorable circum- 
stances, only a few drops of rain could be deposited, near the 
border of the forest. Neither could the amount of conden- 
sation resulting from the contact of the vapour with the trees, 
(supposing the trees to be colder than the air) be great, con- 
sidering how very slowly the air is changed in a forest, even 
when a strong breeze is blowing over the tops of the trees ? 
and consequently how very small a portion of the vapour 
which supplies us with rain is ever brought into actual con- 
tact with the trees. 
The fourth cause of condensation and the last which it is 
necessary to consider, is the cooling of the clouds through 
the effects of radiation. 
When once an opaque cloud has been formed, their radia- 
tion commences, and the cloud is constantly parting with 
heat, and at the same time intercepting heat from other bo- 
dies ; the loss may often exceed the gain, and the consequent 
increase of condensation will cause the rain to fall. 
The general eff ect of forests being to impede radiation from 
the earth’s surface, the presence of the vegetation would be in 
most cases favorable to the cooling of the clouds ; but as the 
