Page:Great Neapolitan Earthquake of 1857.djvu/97

Rh motion of the wave in altitude, produce a fracture somewhere at $$m$$, or more than one, the direction of which downwards is much modified by the joints, &c. &c., of the masonry, and is generally nearer to the vertical than exactly at right angles to $$pi$$.

When the emergent wave produces a sufficient shock for complete overthrow of the end walls $$c$$ and $$e$$, they fall as in Fig. 33, leaving the fractures of the side modified, by the grind of the descending masses. It is rarely, however, that a shock of an emergent wave, sufficient to throw back one end, and forward the other, occurs without the side walls being also thrown, in or out, or both, either by transversal wave motion or by secondary actions of the falling end walls upon them.

It-was stated above that the direction of diagonal pressure through the wall was in that of the wave transit: this is perhaps not strictly true, for referring to Fig. 34, if $$a$$ to $$b$$ be the path and direction of wave transit, and the velocities of the wave itself be equal in altitude to $$st$$ and in amplitude to $$lm$$, then the resultant pressure due to its movement in the forward half of the first semi-vibration is in $$a'b'$$, which, combined with the motion of transit $$ab$$, will give a resultant pressure somewhere between $$b'$$ and $$b$$. As, however, the amplitude $$lm$$, of the earth wave, appears to be always very great with reference to $$st$$, there is no practical error (and much convenience for calculation), in considering the line of pressure as coincident with that of wave transit.

If the subnormal wave be orthogonal to that just described, and still affecting a rectangular building, so that its transit passes through the longer sides, these are bowed (if