ei ie a ee 
Mr Rowell on the Phenomena of Evaporation. 55 
Extensive fires, voleanoes, &c., cause rain from the smoke 
and vapour bringing the air into a conducting state. 
Pressure is another cause of rain; thus, if a cloud be 
forming, the accumulation of vapour is from every side, but 
chiefly above, and clouds are, at times, of great depth ; now, 
every particle of vapour, on joining the cloud, would have its 
extra-charge of electricity over the particles of the cloud 
instantly dispersed through the whole mass, and would take 
its level in the atmosphere according to its density ; now, as 
all the particles in the cloud are of the same density, those 
particles of vapour which are above the mean line of density 
would press downwards, and those below that line would re- 
act on those above ; and although the electrical repulsion of 
the particles be sufficient to prevent rain at the edges and 
thinnest part of the cloud,the pressure at the greatest depths 
of the cloud may be sufficient to overcome the repulsion, and 
form rain. 
The concussion caused by a flash of lightning from such a 
cloud (that is, with its particles pressed nearly into contact) 
will easily explain the cause of the heavy dash of rain which 
follows the flash of lightning. 
Rain caused by pressure will often take place at much 
greater elevations than that caused simply by the gradual 
escape of the electricity of the vapour, which will account for 
the formation of hail: thus, a cloud is wafted from a warm 
to a colder region, and although the cold may be sufficient 
to freeze all the particles of vapour at the exterior of the 
cloud, the radiation of heat would be prevented from the cen- 
tral part, where the vapour would remain unfrozen. Rain, 
formed in the middle of such a mass of vapour, would increase 
in size in falling through the lower part of the cloud; it 
would be instantly frozen on leaving the cloud, and the drop, 
formed under such circumstances, being large, would not 
only remain frozen in falling through the warmer strata of 
air to the earth, but would also increase in size by attracting 
to itself other vapour ; but the rain or snow falling from the 
thinner parts of the cloud being in smaller drops, if frozen 
in the higher regions, would be melted in falling through the 
