The waves of an earthquake have been represented in their progress, and their propagation, through rocks of different density and elasticity; and the causes of the rapidity of propagation, and its diminution by the refraction, reflection, and interference of the oscillations have been mathematically investigated. Air, water, and earth waves follow the same laws which are recognised by the theory of motion, at all events in space; but the earth-waves are accompanied in their destructive action by discharges of elastic vapours, and of gases, and mixtures of pyroxene crystals, carbon, and infusorial animalcules with silicious shields. The more terrific effects are, however, when the earth-waves are accompanied by cleavage; and, as in the earthquake of Riobamba, when fissures alternately opened and closed again, so that men saved themselves by extending both arms, in order to prevent their sinking.
As a remarkable example of the closing of a fissure, Humboldt mentions that, during the celebrated earthquake in 1851, in the Neapolitan province of Basilicata, a hen was found caught by both feet in the street-pavement of Barile, near Melfi.
Mr. Hopkins has very correctly shown theoretically that the fissures produced by earthquakes are very instructive as regards the formation of veins and the phenomenon of dislocation, the more recent vein displacing the older formation.
RUMBLINGS OF EARTHQUAKES.
When the great earthquake of Coseguina, in Nicaragua, took place, January 23, 1835, the subterranean noise—the sonorous waves in the earth—was heard at the same time on the island of Jamaica and on the plateau of Bogota, 8740 feet above the sea, at a greater distance than from Algiers to London. In the eruptions of the volcano on the island of St. Vincent, April 30, 1812, at 2 A.M., a noise like the report of cannons was heard, without any sensible concussion of the earth, over a space of 160,000 geographical square miles. There have also been heard subterranean thunderings for two years without earthquakes.
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.
EARTHQUAKES AND THE MOON.
From a careful discussion of several thousand earthquakes which have been recorded between 1801 and 1850, and a comparison of the periods at which they occurred with the position of the moon in relation to the earth, M. Perry, of Dijon, infers that earthquakes may possibly be the result of attraction exerted by that body on the supposed fluid centre of our globe, somewhat similar to that which she exercises on the waters of the ocean; and the Committee of the Institute of France have reported favourably upon this theory.