RAIN. 
117 
Temperature. 
4o°r. 
50° 
60° 
70° 
80° 
90° 
Units of Vapour 
required for saturation. 
260 
360 
. 520 
726 
. 1012 
. 1360 
It is obvious that the increase of vapour is at a rate much more 
rapid than that of the temperature, and, consequently, that two 
distinct volumes of saturated air at different temperatures, on being 
commingled, become supersaturated; thus, a cubic foot at 40° with 
its 260 units of vapour, and a cubic foot at 60° containing 520 such 
units, will on being mixed form two cubic feet at 50° --(40+ 60) 
+ 2, each of which will have 390 units of vapour = (260+ 520) 
+ 2, that is 30 units = (390 — 360) more than is requisite for its 
saturation, and which will therefore be at once condensed into 
water, and either precipitated or formed into cloud. 
Supersaturation, however, may be produced in other ways : 
first, by the saturated air parting with its heat by radiation ; and 
second, by impinging on a mountain side. In the latter case, the 
following changes occur in succession : — first, the air ascends — that 
being the direction of least resistance ; second, being by its ascent 
partially released from the superincumbent pressure, a portion of 
the heat within it causes it to expand ; third, a portion of the heat 
by which its temperature was sustained being thus withdrawn for 
other service, it becomes cooler; and fourth, by its loss of tem- 
perature its capacity for vapour is diminished, it becomes super- 
saturated, and a cloud is formed. Mountains therefore form clouds, 
but do not attract them, as is commonly supposed. 
Clouds, which may be defined as an aggregation of particles of 
water floating in a sea of aqueous vapour and air, though composed 
of transparent matter and floating in a transparent medium, are 
more or less opaque ; for air and water having different degrees of 
refractiveness, the rays of light in passing through a cloud lose a 
large amount of light by reflexion from the numerous successive 
surfaces of air and water which they alternately encounter. 
Though consisting of comparatively hc>avy matter, each particle 
of a cloud is so minute that its weight does not exceed that of an 
equal volume of air by an amount sufficient to overcome the 
atmospheric resistance, and hence it floats. By coalescing, however, 
