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During the long-continued trembling of the ground in the Piedmontese valleys
of Pelis and Clusson, the greatest changes in the electric tension of the
atmosphere were observed while the sky was cloudless. The intensity of the
hollow noise which generally accompanies an earthquake does not increase in
the same degree as the force of the oscillations. I have ascertained with
certainty that the great shock of the earthquake of Riobamba (4th Feb.,
1797) -- one of the most fearful phenomena recorded in the physical history
of our planet -- was not accompanied by any noise whatever. The tremendous
noise ('el gram ruido') which was heard below the soil of the cities of
Quito and Ibarra, but not at Tacunga and Hambato, nearer the center of the
motion, occurred between eighteen and twenty minutes 'after' the actual
catastrophe. In the celebrated earthquake of Lima and Callao (28th of
October, 1746), a noise resembling a subterranean thunder-clap was heard at
Truxillo a quarter of an hour after the shock, and unaccompanied by any
trembling of the ground. In like manner, long after the great earthquake in
New Granada, on the 16th of November, 1827, described by Boussingault,
subterranean detonations were heard in the whole valley of Cauca during
twenty or thirty seconds, unattended by motion.
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