Page:Great Neapolitan Earthquake of 1857.djvu/79

Rh opposite one, and if fracture occur the side walls fissure a short way off from the quoins, and the movement of the end wall is one of rotation round some horizontal line or lines situated along the length of its base.

Were every part of the walls of the building of perfectly equal coherence, and the rate of wave transit the same in its materials as that of the earth wave, the fracture would occur exactly at the internal angle of the wall at each quoin, breaking the side walls across in a plane coinciding with that of the internal face of the end wall. But the quoins are in practice built with larger, longer bedded, and better dressed masonry than the rest of the structure; and hence from this cause alone, without reference to others, the fissures are removed along the side walls nearer the middle, and into the less coherent masonry of the walls, and take place at $$c$$. The earth wave pushes the side walls along with it, and these push the end wall $$b$$ at either quoin before them. The end wall $$b$$ therefore cannot fall by inertia in the same way as that, $$a$$, being propped up by the side walls. The earth wave, however, having passed its first semiphase, returns through the second half vibration in the opposite direction, and, we may assume, with equal velocity. The same set of forces now operate upon the end wall, $$b$$, the movement of the whole mass being in the direction $$b$$ to $$a$$, and as described for the former end wall; so that $$b$$ tends to turn over upon its base in the contrary direction to the movement of the wave itself in its second semiphase, and the side walls are fissured as before at a distance from the quoins greater or less along them, as at $$e$$ (Fig. 21). As the force producing fracture and dislocation at any given