Page:Great Neapolitan Earthquake of 1857.djvu/85

Rh these fissures having been almost parallel to the west end wall, and from the N.E. to the S.W., or from left to right of the picture. The wave of shock was of very steep emergence at Pertosa; but the whole "colline" upon which the town is perched, oscillated laterally with the wave, and horizontally, or nearly so. The horizontal fractures, and those over the door, are due to the emergent wave only.

The force producing fracture and dislocation, impressed by the shock, may be viewed as separated into two-one just sufficient to fracture the materials, the other to dislodge them more or less. Both depend upon the velocity at maximum of the wave; but the power to produce fracture depends much more upon velocity than upon the amplitude of the wave, while the energy to produce dislocation after fracture depends also upon the latter, which determines the time during which the motion of the passing wave acts upon the mass.

The flexibility and elasticity of masonry or brickwork, even of the highest quality, in masses of ordinary size is small, the limits of distortion without rupture narrow: the compressive or extending forces being due to inertia, are proportional to $$\mathrm{M V}$$, and for the same material proportionate to $$\mathrm{V}$$ only; and as the amount of extension or compression, for the unit of length due to any force suddenly applied to an elastic solid, is double that produced by the same force, if statically or slowly applied, the effect of a high velocity is to produce fracture with great facility in bodies of narrow elastic limits. A wave shock of extremely small amplitude, therefore—one so small as not to appeal at all alarmingly to our senses—may yet be