Page:Great Neapolitan Earthquake of 1857.djvu/96

54 the fractured joints in the direction $$da$$ and partly turn. When the building is large, and the angle of emergence large also, as in Fig. 32, this cross fracture is usually a curved line, more or less hollow downwards, passing out at the quoins at a bed joint as at $$t$$, and in such a direction that a chord to the hollow curve from $$n$$ to $$t$$ approaches to right angles with $$pi$$.

The end $$e$$ (Fig. 31) is projected forward towards $$d$$ by inertia in the second semi-vibration of the wave. Its fissure is more or less exactly parallel with $$pk$$ for the greater portion of its length, but it seldom runs to the base of the wall $$s$$, turning out towards $$d$$ by a horizontal joint, somewhat above that level, and at a higher point, as the angle of emergence is greater, as may be observed in Fig. 32.

The reason of this is pretty obvious. The inclined direction of the fissure causes it to reach the internal angle of the quoin before it comes down to the base of the end wall $$e$$, which therefore breaks at that level along an horizontal line, and all below that, not being detached and loaded with the mass above, is unmoved. Cross fractures may or may not, follow from this fissure towards $$c$$, dependent on the breadth of side wall, occurring between the fissures at the ends $$c$$ and $$e$$, and upon the angle of emergence, class of masonry, and other conditions.

In Fig. 32 the effects are shown of the wave when emergent at a still greater angle. The train of phenomena is quite similar to that just described, but with this addition, that where the angle of fracture, $$tpi = hia$$ = that of emergence, is great, and hence the angle $$ipm$$ great also, the overhang of the upper part of the side wall, coupled with the momentum in the direction $$ip$$, due to the small