Page:Popular Science Monthly Volume 2.djvu/545

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525
EARTHQUAKE-PHENOMENA.

which melts the great lava-beds, and fills cavities in the earth's crust with steam and gases, may not arise directly from the earth's central heat, but from the crushing of strata as it contracts and settles upon the cooling interior.

By a series of experiments and observations made by Mr. Mallet, it is shown that the "annual loss of heat into space of our globe at present is equal to that which would liquefy, at 32° Fahr., about 777 cubic miles of ice; and this is the measuring unit for the amount of contraction of our globe now going on."

The amount of shrinking depends, therefore, on the amount of heat lost—a view long since insisted on by Prof. Dana; and this, according to Mallet, is sufficient to account for all the phenomena. To this cause, then, we refer the never-ending oscillations of the earth's cooled exterior, and the enormous lateral strain by which it is bent and fractured, and its broken ridges made to grind and crush with terrific vibrations.

In many areas the earthquake energies of former times have been long at rest, but, according to Sir Charles Lyell, the total energy may not have diminished.

He finds evidence of convulsions as great and obvious in recent as in earlier time. Mallet, however, remarks that "seismic energy may be considered as possibly constant during historic time, but is probably a decaying energy viewed in reference to much longer periods."

Everywhere we see, in exposed portions, crevices open or filled—ejections of trap and basalt; and wall-like dikes stand out upon the slopes of mountains. These are legible and significant chapters in the earth's dynamic history.

Do earthquakes occur with any order or system, so that their coming may be foretold?

Prof. Palmieri, in his observatory on Mount Vesuvius, is able, says George Forbes, "to predict eruptions." "This is a small eruption," remarked the professor, "but there is going to be a greater one; it may be a year hence, but it will come." "In almost exactly a year," continues Mr. Forbes, "the great eruption did come."

From Mallet's catalogue of European earthquakes it appears that, during 15½ centuries, 1,157 took place during the winter, against 875 in the summer months.

Although science has cleared up some of the mystery which hung over earthquakes in less enlightened times, it has not divested them of their sublimity and terrible reality.

Their work of destruction is done in a moment. The great battles of the world have scarcely been so destructive of human life.

We read that 250,000 persons perished during the earthquake at Antioch in 526. At Lisbon 60,000 people were destroyed. During one of the Calabrian earthquakes 35,000; and during the one at Arequipa in 1868. 40,000 persons perished. Pestilence, famine, and fire,