Elastic fluids are doublessly the cause of the slight
and perfectly harmless trembling of the earth's surface, which has often
continued several days (as in 1816, at Scaccia, in Sicily, before the
volcanic elevation of the island of Julia), as well as of the terrific
explosions accompanied by loud noise. The focus of this destructive agent,
the seat of the moving force, lies far below the earth's surface; but we
know as little of the extent of this depth as we know of the chemical nature
of these vapors that are so highly compressed. At the edges of two craters,
Vesuvius, and the towering rock which projects beyond the great abyss of
Pichincha, near Quito, I have felt periodic and very regular shocks of
earthquakes, on each occasion from 20 to 30 seconds before the burning
scoriae or gases were erupted. The intensity of the shocks was increased in
proportion to the time intervening between them, and, consequently, to the
length of time in which the vapors were accumulating. This simple fact,
which has been attested by the evidence of so many travelers, furnishes us
with a general solution of the phenomenon, in showing that active volcanoes
are to be considered as safety-valves for the immediate neighborhood. The
danger of earthquakes increases when the openings of the volcano are closed,
and deprived of free communication with the atmosphere; but the destruction
of Lisbon, of Caraccas, of Lima, of Cashmir in 1554,* and of so many cities
of Calabria, Syria, and Asia Minor, shows us, on the whole, that the force
of the shock is not the greatest in the neighborhood of active volcanoes.
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