A new instrument (the Seismometer) invented for this purpose by
M. Kreil, of Vienna, consists of a pendulum oscillating in every
direction, but unable to turn round on its point of suspension; and
bearing at its extremity a cylinder, which, by means of mechanism
within it, turns on its vertical axis once in twenty-four hours. Next
to the pendulum stands a rod bearing a narrow elastic arm, which
slightly presses the extremity of a lead-pencil against the surface
of the cylinder. As long as the pendulum is quiet, the pencil traces
an uninterrupted line on the surface of the cylinder; but as soon as
it oscillates, this line becomes interrupted and irregular, and these
irregularities indicate the time of the commencement of an earthquake,
together with its duration and intensity.[30]
Elastic fluids are doubtless the cause of the slight and perfectly
harmless trembling of the earth’s surface, which has often continued
for several days. 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 vapours 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,--Humboldt has felt periodic
and very regular shocks of earthquakes, on each occasion from twenty
to thirty seconds before the burning scoriæ 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 vapours were accumulating. This simple fact, which has
been attested by the evidence of so many travellers, furnishes us
with a general solution of the phenomenon, in showing that active
volcanoes are to be considered as safety-valves for the immediate
neighbourhood. There are instances in which the earth has been shaken
for many successive days in the chain of the Andes, in South America.
In certain districts, the inhabitants take no more notice of the number
of earthquakes than we in Europe take of showers of rain; yet in such
a district Bonpland and Humboldt were compelled to dismount, from the
restiveness of their mules, because the earth shook in a forest for
fifteen to eighteen minutes _without intermission_.