Page:Great Neapolitan Earthquake of 1857.djvu/78

38 from the effects their causes—from the dislocations the forces that produced them and their directions, and to point out some of the more important modifications of effect due to differences of masonry, of form, of architecture, of wall apertures, and other such accidental conditions.

Commencing with the simplest case. If a cardinal building consisting merely of four unroofed walls be exposed to a normal shock, capable of fissuring the masonry, but not completely overthrowing it, the fissures will be found as nearly vertical cracks following the joints of the masonry, and within a few feet (more or less) of each quoin, as in Fig. 21-23, and Fig. 22-24, in plan.



The fissures being widest at top, and becoming a scarce visible line at part of the way down the walls, or perhaps extending to their base, the earth wave, if in the direction $$a$$ to $$b$$, reaches the end wall, $$a$$, first. Its inertia acts as an equal and opposite force at its centre of gravity, and tends to cause it to be left behind while the remainder of the building is pushed forward. The end wall towards the direction from which the shock has come moves in the