Page:Great Neapolitan Earthquake of 1857.djvu/82

42 second semiphase of the wave, and to such an extent, as to obliterate the effect of the difference in velocity of the two semiphases, and either leave the fissures equal at both ends, or even make those at the end $$b$$ the wider.

Inequalities in the materials or masonry at the opposite ends in the line of the wave-path—want of complete or nearly approximate symmetry in the size and form of those ends—perforations of doors or windows, or such-like sudden changes of continnitycontinuity [sic] of wall—and great length of bond in the wall stones at all or at a few points—are the other conditions which chiefly perplex and interfere with the phenomenon.

Practically, however, this fact is a guide of much importance in seismic observation, inasmuch as it enables us very frequently to decide, with more or less certainty, as to the direction of wave transit, from conditions that otherwise would afford no information beyond that of the path of the wave, leaving it quite uncertain whether the seismic vertical were to be sought for towards $$a$$ or towards $$b$$.

Where the phenomena are clear, we may, on the contrary, always conclude that it lies along the wave-path, towards the end that presents the widest fissures. Very few large and massive cardinal buildings will be found that will not give, as respects a normal or slightly subnormal wave, a decisive response, from some or other of its parts, by this means.

The actual phenomena in a well-developed case are illustrated in the Photog. No. 25, which shows th front end of the church of Pertosa, looking at its N.W. end. The direction of the nearly horizontal wave that