Page:Popular Science Monthly Volume 17.djvu/474

458 and farthest point from the earth. The following table gives the results of the comparison—periods of five days being taken—in the middle of which occurs a perigee or an apogee of the moon:

A third means of ascertaining the moon's influence on earthquake phenomena consists in comparing the latter with the lunar day. There are then found to be two maxima corresponding to the moon's passage to the upper and lower meridian, or to what may be called the lunar mid-day and midnight. The minima occur near the middle of these intervals. M. Perrey has made comparisons in this way of 824 shocks felt at Arequipa from 1810 to 1845; of the journals of four observers at Monteleone, Messina, at Catanzaro and Scilla, in the years 1783 to 1785, which were marked by great eruptions of Vesuvius; and, lastly, of the journal of M. S. Arcovito, kept at Reggio from 1836 to 1854. There is manifest, more or less markedly, in all these observations, a preponderance in favor of the hours of the moon's passing the meridian.

This constant increase in frequency of the shocks at the times when the tides are strongest would seem to prove that the producing cause extends its action below the earth's surface. The increase is small, it is true, but it is constantly apparent, however the facts may be viewed.

We must not lose sight of the local perturbations to which the irregularity of the earth's internal surface may give rise. As M. Perrey has said, the lower side of this shell must consist of curves and anfractuosities, mountains whose summits project into the central liquid like gigantic stalactites, and valleys which approach the outer surface. This internal orographic system must modify the propagation of the subterranean waves. As in narrow and rapid rivers, the waves will be confined, and will gain in power between two mountains that obstruct their passage; they will spread out and lose power in a plain or valley whose configuration allows them to move more freely. Beating against cliffs and other obstructions, they will cause shocks and concussions, fissures, and a partial local falling of the internal vault, the effects of which will be felt at the surface as undulations and tremblings. All these causes combine to make of earthquakes a very complex phenomenon.

We might expect to find a species of tide-movement in the lava of active volcanoes, but data on this point are lacking. The only fact we have bearing upon it is derived from the observations of Scacchi and Palmieri during the eruption of Vesuvius in May, 1855, who noticed an