*
[footnote] *Arago, in the 'Annuaire du Bureau des Longitudes pour' 1838, p.
246.
Sometimes, however, the stratum of cloud from which the thunder proceeds
sinks to a distance of 5000, or, indeed, only 3000 feet above the plain.
According to Arago's investigations -- the most comprehensive that we
possess on this difficult branch of meteorology -- the evolution of light
(lightning) is of three kinds -- zigzag, and sharply defined at the edges;
in sheets of light, illuminating a whole cloud, which seems to open and
refeal the light within it; and in the form of fire-balls.*
[footnote] *Arago, op. cit., p. 249-266. (See also, p. 268-279.)
The duration of the two first kinds scarcely continues the thousandth part
of a second; but the globular lightning moves much more slowly remaining
visible for several seconds. Occasionally (as is proved by the recent
observations, which have confirmed the description given by Nicholson and
Beccaria of this phenomenon), isolated clouds, standing high above the
horizon, continue uninterruptedly for some time to emit a luminous radiance
from their interior and from their margins, although there is no thunder to
be heard, and no indication of a storm; in some cases even hail-stones,
drops of rain, and flakes of snow have been seen to fall in a luminous
condition, when the phenomenon was not preceded by thunder.
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