t 
— 68 — 
The second cause of condensation which has been men- 
tioned, is the rising of vapour from low to higher and colder 
strata of the atmosphere. 
In Mauritius, the formation of the ground causes frequent 
illustration of this process also. The saturated winds which 
come to us along the surface of the sea, are pushed, as it 
were, up an inclined plane, until the masses of vapour have 
reached great heights, (1400 to 2000 ft.) in passing over the 
elevated lands in the central parts of the Island. The greatly 
increased cold, due to the elevation, condenses the moisture 
and produces rain in great abundance. 
By way of example, let us suppose the air at the sea level 
to be saturated ; with the Thermometer at 85 ° and Barometer 
at 30 in : and that on the high lands of the interior the Ther- 
mometer is at /5 ° and Barometer at 28.8 in: Under these 
conditions, each cubic yard of the atmosphere at the sea level 
would contain in the form of vapour, 1.34 cubic inches of 
water ; though when expanded by the diminution of pressure 
from 30 inches to 28.8 inches, at the high level, each cubic 
yard of the atmosphere would only contain 1.31 cub. in. of 
water ; but simultaneously with the diminution of pressure at 
the high level, the temperature will be there reduced to 75 ° 
at which temperature, each cubic yard could support in a 
state of vapour only 0.995 cub. in. of water, so that the dif- 
ference, namely 0.315, or nearly ^ of a cub. in. of water must 
be thrown down, in the form of rain, from each cubic yard of 
the atmosphere which passes over the high land. 
This is no doubt a favorable instance, for the air as the sea 
level was assumed to be saturated, which will not often be the 
case ; but the example nevertheless serves to shew the depen- 
dance which may be placed upon the source of rain under 
consideration. 
It will be seen that the quantity of rain will depend upon 
the degree of humidity of the wind, and upon the difference 
of the temperatures of the lower and higher levels, and that 
it is altogether independant of the action of forests. The rain 
drops due to the increased cold, have their origin at a great 
elevation above the ground, and altogether beyond the in- 
fluence of the trees the rain drops increase in size, gradually, 
