— 69 — 
as they descend, partly on account of their originally low 
temperature, which enables them to condense additional moi- 
sture as they fall ; and also on account of their power of col- 
lecting by attraction the smaller watery particles amongst 
which they pass, in the same manner as a globule of mercury 
may be seen to attract to itself and agglomerate the smaller 
particles brought near to it. It is from these causes that a 
rain guage placed on the ground collects a much larger quan- 
tity of water than on top of a tower. 
Another way in which vapour is raised from low to higher 
strata, is by the ascending currents already spoken of, caused 
by the heating of mountain masses. 
An extensive conflagration, likewise, often causes such an 
upward draught, that masses of vapour, after having been 
heated and expanded, rise to great heights, where the tempo- 
rary accession of heat is lost, and much of the moisture con- 
densed. Those who have witnessed extensive fires may have 
noticed the irregular gusts of wind approaching from all quar- 
ters, often accompanied at intervals by light showers of scat- 
tered drops of rain. 
There can be little doubt that ascending currents take place 
during rotatory storms, and it is perhaps to this circumstance 
that the lowness of the Barometer in the vortex is due. 
The third cause of rain which has been referred to is in- 
crease of pressure. 
If the air be saturated, any increase of pressure, when un- 
accompanied by increase of temperature must cause conden- 
sation of moisture. 
Variation of pressure may result from atmospheric chan- 
ges, though in this case, within such small limits that it can 
be scarcely considered an active cause of rain, in itself ; though 
it may assist or retard the action of other causes. 
Increase of pressure may also take place when a strong 
wind, saturated with vapour, strikes with unbroken force 
against a mountain range, sufficiently abrupt and extensive to 
arrest the onward motion of the air. In such a case, increase 
of pressure will be caused, and maintained by the cons v/c 
motion of the wind, which continues to arrive faster than the 
