Page:Great Neapolitan Earthquake of 1857.djvu/106

64 We are therefore enabled from what precedes, in the case of a cardinal building and abnormal wave, to infer—
 * 1st. The path of the wave.
 * 2nd. The direction of transit motion.

Measures of velocity, can scarcely ever be obtained from an abnormal wave, as the overthrown masses are quoin pieces, which fall attached at right angles, and usually defy attempts to ascertain the moments of inertia and dimensions of base.

If the fissures be clean and well defined, and the walls not too much perforated by openings, or otherwise rendered irregular, good results can be had; it is then, not important that the walls making angles with each other, from which the widths of the directing fissures are taken, should be of equal thickness, because the force acting on each is proportional to its mass, and the section of fracture for equal height is so likewise; but they must be of similar material and masonry. All such conditions, however, will be better understood after we have treated of the perturbation of phenomena produced by architectural and other features, &c.