Page:Great Neapolitan Earthquake of 1857.djvu/80

40 velocity is always proportionate to $$\mathrm{M}$$—the mass broken off or dislodged—so the extent of dislodgement after fracture (the materials being the same) is always proportionate to the velocity; and hence in any one building of like material and masonry the width of each fissure is proportionate to the velocity that has been effective in opening it; and we may compare component velocities in the directions of the planes of parallel or abutting walls by means of the widths of such fissures, the width in every case being measured with reference to an unit in length of the fissure from its origin, or where it becomes evanescent. This unit length may be arbitrary, but 10 feet in length of fissure is a very convenient unit, and the widths expressed in inches and decimals for that unit.

It is almost invariably found that in every building (with certain exceptions, to be noticed), although the masonry and form, &c., of the building may be quite or very nearly alike at both ends, the fissures $$c$$ and $$e$$, do not occur at equal distances from the respective quoins (measured along the side walls), nor are they equally opened, large, and long, at both the opposite ends.

Whether this arises—as, from other considerations respecting the vibration of pendulous lamps set in motion by shock, and to be hereafter noticed (Part III.), seems probable—from a real difference in velocity in the two semiphases of the wave itself, and that the second semiphase is described with a somewhat slower velocity than the first, owing to defect of perfect elasticity in material substances composing the earth's surface—or whether it is due to the conjoint action of the elastic wave (the earth wave) itself, and of the wave of elastic compression of the