HOW TO MEASURE AN EARTHQUAKE-SHOCK.
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.