Page:Great Neapolitan Earthquake of 1857.djvu/133

Rh High towers and of narrow base, fall as one mass, breaking off diagonally somewhere above the base, whatever be the direction of the wave. When, however, the angle of emergence is very steep, a certain amount of shearing force is introduced, and the angle of severance becomes very sharp likewise, so that a sharp angular "aiguille" of shattered masonry remains standing, often bearing a considerable proportion to the whole original height. A remarkable example of this form of fracture is given in Fig. 62 of the tower of the monastery of Santa Dominica, at Montemurro, sketched from the top of the Palazzo Fino, although in this instance probably due only to accidental causes, and not to steep emergence. Isolated fragments thrown from the summits of such towers, owing to their own velocity of pendulous vibration, do not always, by reference to the observed distance of projection, represent without correction, measures of the true direction or velocity of the wave. They are thrown like a stone from a sling, with a certain velocity and direction due to the shock, plus or minus, another, or perhaps a different direction and velocity, due to the proper motion of the tower; of this the observer requires to be on guard.

Unsymmetrical construction of building, always involves unsymmetrical phenomena of distribution. If compelled to adopt such a building, (in lack of better,) for observation, the first thing to be done to disentangle the phenomena, is to consider the effects, due to the want of symmetry alone. If, for example, we find the opposite walls of a cardinal church, one standing and the other prostrate, the wave transit having been abnormal, and nearly in their lines of length, the first point to be