Page:Popular Science Monthly Volume 7.djvu/755

Rh Such being some of the more prominent peculiarities attending earthquakes, let us now apply them to the theories above referred to, and endeavor to ascertain the causes of these disturbances or the agencies employed in producing them.

They do not support the theory of the foiling in of caverns being the cause of these phenomena; for they are invariably attended by an upheaval of the ground, and often with a horizontal or a circular motion. This theory, therefore, cannot be maintained, and more especially as it does not explain the electric and magnetic accompaniments.

The hypothesis that they are caused by steam or the explosion of confined gases has scarcely a better foundation. These agents might produce vertical motion and subterranean noises, but it is difficult to conceive how they could bring about circular motion at the surface; and it is quite impossible that the explosion of gases or the escape of steam could, immediately preceding a shock, attract the clouds floating in the atmosphere, so that they should remain fixed over particular spots. Other characteristics also cannot' be explained on this theory, as the lightning and disturbance of the compass.

The volcanic and igneous theory is not so easily to be disposed of; for it appears very clear that volcanic eruptions do produce earthquake. A remarkable instance is that of Santorini in 1650. Earthquake is also very common where volcanic action is extensively developed, as in South America and the neighborhood of Etna and Vesuvius.

Volcanoes produce these disturbances in two ways: 1. By their own direct motion; 2. By disturbing the electric equilibrium in their neighborhood. This electric disturbance was noticed by Pliny, who records that an officer, one of the Decuriones Municipales of Pompeii, was struck by lightning in 79, although the sky was perfectly unclouded; and these indications have been put to practical use. The presence of lightning is also a prominent feature during volcanic eruptions. When Kattleguia, in Iceland, now extinct, was last in a state of eruption, lightning proceeded from it and killed a farmer and his servant, together with some horses and cows. We cannot, therefore, exclude the consideration of volcanoes as producers of earthquake, sometimes by direct action, at others through the medium of electric disturbance.

But by far the most prominent agent seems to be electricity, and the Italians, who suffer so much from these calamities, consider it to be the only cause. The evidences of the activity of the electric fluid in this respect are so palpable that they cannot be controverted. As some may be skeptical on this point, it will not be amiss to examine a few cases in which the operation of this agent is quite apparent.

When considering this part of the subject, we must not omit to notice the frequency with which the greatest weight of these