56 Mr Rowell on the Phenomena of Evaporation. - 
warmer air; thus, as is often the case, there is heavy hail 
and rain falling at the same time from the same cloud. 
The successive flashes of lightning from the same cloud 
may be caused by the electricity being pressed out of the 
cloud when the electric fluid accumulated on the surface 
would strike off either to the earth or neighbouring clouds: 
or it may be caused by the formation of rain ; thus, it takes 
8,000,000 particles of vapour to form one drop of rain, but 
the capacity for electricity of the rain-drop is only equal 
to that of 40,000 particles of vapour; therefore, on the for- 
mation of every drop of rain, the electricity of 7,960,000 par- 
ticles of vapour must be dispersed through the remaining 
vapour, and thus increase the electrical charge of the cloud. 
The same reasoning will account for the dispersion of 
clouds after rain; for if the electricity does not, by some 
means, escape from the cloud in so great a proportion as the 
accumulation goes on through the formation of rain, the 
electricity must increase so as to stop the formation of rain ; 
and may disperse the cloud altogether, through the in- 
creased repulsion of the particles of vapour. 
The sinking in the barometer previous to and during rain, 
I ascribe to the rapid escape of electricity from the invisible 
vapour or clouds, thus causing a partial vacuum in the re- 
gions of the clouds, and the air, from its elasticity, rising to 
fill the vacuum, decreases the pressure on the mercury. 
Storms, in most cases, I believe, are from similar causes : 
the enormous and rapid escape of electricity from clouds 
during heavy rains, causes a rarefaction of the air in the 
clouds ; the air between the clouds and the earth rushes up- 
wards to fill the rarefied space, and the air at the earth’s 
surface rushes in from all points to gain its equilibrium ; and 
when the excessive rains, which take place at times in tro- 
pical climates, are borne in mind, I think the causes explained 
will be sufficient to account for the most terrific storm. 
