sense; and were it sufficiently rigid along its length, it might tear off from the ends, and fissures occur as before at , , the other side wall following the movement already described for (Figs. 21-23).
The whole length of has an equal velocity impressed; it is unsupported for its whole length, except at the two extremities, where it is connected by the quoins with the end walls, and held fast by them. It bends into a curve, therefore, along its length, bowing outwards most at the top and centre of length, and receiving several fractures , approaching to vertical in direction, owing to the length of the curve being greater than that of the originally straight wall. The whole bond of the materials is more or less disturbed; but the force of the shock may only be such, as to thus curve and fracture, but not overthrow the wall. The wall in like manner is urged forward at the ends and by the connection with the quoins of the end walls; but failing also in rigidity, the central part is left behind, and bent also by inertia; differing from the first case analyzed in this also, that the direction of movement impressed is not in the same direction with the wave transit, but reverse to it; so that here both side walls and move alike in direction, but to different extents. The greater velocity is common to both, for both have their velocities impressed by the first semi-vibration of the wave; and were the nature of the connection at the quoins the same, whether the walls were forced outwards or inwards, both would, cœteris paribus, be bowed alike. From the nature of the quoin bond of masonry, however, the end walls at the quoins offer much less resistance to the wall , being forced
