Page:Great Neapolitan Earthquake of 1857.djvu/81

Rh materials of the walls themselves—or to whatever other cause, which future research must make clear, the fact may be accepted as certain and very general—that the end wall which is first acted upon by the wave (whenever it is something near normal), has the higher velocity shown upon it, and that the fissures at that end are, cœteris paribus, found to be wider than those at the opposite one.

The fissure formed at the end $$a$$, that first reached, is frequently rather wider than what is precisely due to this difference in velocity in the two semiphases of the wave; for the end $$a$$ is first fissured, the end $$b$$ is next fissured by the second semiphase, which leaves the end wall $$b$$ broken off, behind it, but carries back with it both side walls in the direction of its own return motion, and towards the end wall $$a$$. But more or less dust and broken fragments are often intercepted in the fissure $$a$$ when first opened. These hinder the mass broken off at $$a$$ from approaching the side walls, and closing the fissures (by the inertia of the broken-off mass), so that the side walls, in this return movement, push the end wall $$a$$ before them, through the intervention of these obstacles, and so a second movement is impressed (small in extent) upon the broken-off end $$a$$, in the contrary direction to the wave transit, and in the same direction with its first movement, which ends by increasing the final width of the fissures at the end $$a$$.

The chief disturbing causes that interfere with and mask the regularity of this phenomenon are—The wave being subnormal and emergent at a considerable angle; in which case the length of the dynamic couple (as has been already generally explained) that measures the overthrowing and dislocating power at a given velocity, is greatest in the