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greater than ordinary rapidity. At a considerable distance 
from the thundercloud, where the atmosphere is free from 
cloud, the air descends, acquiring temperature according to 
the law of convective equilibrium in dry air. The air then 
traverses the ground towards the region where the storm is 
raging, acquiring moisture as it proceeds, but probably 
without much diminution of temperature, on account of 
the heated ground making up for the cold of evaporation. 
Arrived under the thundercloud, the air rises, losing tempera- 
ture, but at a diminished rate, owing to the condensation of 
its vapour to form part of the immense cumulus cloud which 
overcasts the sky on these occasions. The upward current of 
air carries the cloud and incipient rain drops upwards, but 
presently, in consequence of the increased capacity of the mass 
from the presence of a large quantity of water, the refrigera- 
tion of the air, in consequence of its dilatation, will be so far 
diminished as to prevent the condensation of fresh vapour, 
and ultimately to redissolve the upper portion of the cloud. 
This phenomenon, which has been noticed by Rankine in the 
cylinder of the steam engine, will account for the defined 
outline of the upper edges of cumulus clouds. The upward 
current no doubt extends occasionally to regions below the 
freezing temperature. If cloud be carried with it, snow 
or hail will be formed, which, if sufficiently abundant, 
will pass through the cloud and fall to the ground before 
it is melted. Now, the dry cold air in which the snow 
and hail are formed is a perfect insulator. Ice has also 
been proved, by Achard, of Berlin, to be a non-conductor 
and an electric. Even water, in friction against an insulator, 
is known from the experiments of Armstrong, explained by 
himself and Faraday, to be able to produce powerful electric 
effects, and this fact has been suggested by Faraday to 
explain powerful electric effects in the atmosphere. Sturgeon 
has noted the remarkable development of electricity by hail 
showers. Few heavy thunderstorms occur without the fall 
