This electric phenomenon is unaccompanied by thunder, or too distant to
be heard: when it appears, the whole sky, but particularly the horizon,
is suddenly illuminated with a flickering flash. Philosophers differ
much as to its cause. Matteucci supposes it to be produced either
during evaporation, or evolved (according to Pouillet’s theory) in the
process of vegetation; or generated by chemical action in the great
laboratory of nature, the earth, and accumulated in the lower strata of
the air in consequence of the ground being an imperfect conductor.
Arago and Kamtz, however, consider sheet-lightning as _reflections
of distant thunderstorms_. Saussure observed sheet-lightning in the
direction of Geneva, from the Hospice du Grimsel, on the 10th and
11th of July 1783; while at the same time a terrific thunderstorm
raged at Geneva. Howard, from Tottenham, near London, on July 31,
1813, saw sheet-lightning towards the south-east, while the sky was
bespangled with stars, not a cloud floating in the air; at the same
time a thunderstorm raged at Hastings, and in France from Calais
to Dunkirk. Arago supports his opinion, that the phenomenon is
_reflected lightning_, by the following illustration: In 1803, when
observations were being made for determining the longitude, M. de
Zach, on the Brocken, used a few ounces of gunpowder as a signal,
the flash of which was visible from the Klenlenberg, sixty leagues
off, although these mountains are invisible from each other.