Page:Great Neapolitan Earthquake of 1857.djvu/98

56 of sufficient length, &c.) outwards, at the side first reached by the wave, and inwards at the opposite one, differing in nothing from the effects of a normal wave upon the same building except that these are all less marked, for the same velocity and amplitude, the effective velocity producing movement in the masses detached, being to that of a normal wave of equal velocity, as $$v$$: $$\frac{v}{\sec e}$$, $$e$$ being the subnormal angle, or angle of emergence.

Two other conditions require notice, however, as also affecting the dislocations produced by subnormal waves.

If such a wave, with a given value for the angle $$e$$, be resolved into a vertical and horizontal component, it is the latter that is chiefly effective in producing dislocation when $$e$$ is small, the former when it is very great: and the effects of both are modified more or less by the form of the individual blocks of stone of which the wall consists. If these are very long in their beds, they offer a most powerful resistance to fracture or dislocation by a steeply emergent wave; and when thus long bedded, close jointed and squared ashlars, prevent any indication of value being had.

In the choice of buildings, therefore, for arriving at the value of $$e$$ from subnormal fissures, those must be selected that are of large size, with walls of brick, or of rubble masonry of inferior quality, or at least of small, short-bedded stones in proportion to the size of the walls; and fortunately (for seismic researches) there is no want of such in the south of Italy.

Where, also, the value of $$e$$ is great, two other circumstances come into play to modify the widths of the fissures, and even affect their direction, more or less.