66 
those portions of the plants which have received any heat. 
On the other hand, whatever heat which finds its way to the 
soil, is there preserved by the vegetable covering from loss by 
radiation, a process by which exposed surfaces are reduced to 
very low temperatures, when the sun is below the horizon. 
We may therefore conclude, that as a general rule, the tem- 
perature of ground protected by forests, will be within the 
extremes of temperature reached by exposed surfaces ; and 
further that the mean temperature of the sheltered land will 
be rather below that of the other. 
Now, the following are the most important of the recognized 
causes of atmospheric condensation. 
First . — The mixing together of bodies of vapour having 
different temperatures. 
Second . — The rising of vapour from low to higher aud colder 
strata of the atmosphere. 
Third . — The increase of pressure. 
Fourth . — The cooling of clouds through the effect of radia- 
tion. 
The first mentioned of these causes, namely the mixing to- 
gether of vapours of different temperatures, is no doubt a very 
common cause of rain. The condensation in this case results 
from the law by which the capacity of the air, for holding 
moisture in suspension increases and decreases more rapidly 
than the temperature ; so that when two masses of air satur- 
ated with vapour, at different temperatures, have been mixed’ 
the total quantity of vapour is necessarily greater than the 
mean temperature can support, and the surplus is thrown 
down as rain. If the masses of air are not sufficiently satur- 
ated to cause rain, the effect of the mixing will be to increase 
the degree of humidity, and perhaps to form light clouds or 
mist. 
In this operation the different masses of air are brought to- 
gether by some of the numerous causes of disturbance always 
present in the atmosphere, such as the meeting of different 
winds; sometin es perhaps differences in the electric state of 
clouds; the action of rotatory storms; and the variations in 
temperature of mountain masses, caused by the change of po- 
sition of the sun. 
