Volcanoes are Safety-valves.

—Electric fluids are, doubtlessly, the cause of the slight and perfectly harmless trembling of the earth’s surface, which has often continued for 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 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, I have 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 were 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.

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 Caracas, of Lima, of Cashmir in 1854, 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 neighbourhood of active volcanoes.—Humboldt’s Cosmos.