Page:Great Neapolitan Earthquake of 1857.djvu/489

Rh limit of stability. The north gable wall had given out in the first semiphase of the shock, the south in the second semi-phase; and both appeared, from the fragments fallen into the fissures, none of which were crushed, or gripped and pressed into compacted powder, (as is not unusually the case,) to have gone out very little, if at all further, by the impressed movement, than where they stood as I examined them.

The width of this north fissure, therefore, affords an approximate measure, of the actual amplitude of the main wave of shock here; for the opening, or the actual movement of the gable, at the level of its centre of oscillation, upon the chord as axis, must have been about equal to the amplitude of the wave, taken in a horizontal line. It would be useless, upon inexact data, to pursue this minutely; we may conclude, however, that the horizontal amplitude of the main wave of shock here, did not much exceed 4 inches.

There were also immense fissures, in the 9-inch brick groining of the roof of the church, both longitudinally and transversely through the axis. The beautiful domed cupola also presented complicated fissures, but was in so tottering a condition as to prevent close examination; all these indicated a principal wave-path some degrees W. of north to south.

The urn or vase, thrown from the summit of one of the gate-piers, in the garden of the Priure, Photog. No. 230 (Coll. Roy. Soc.), and Figs. 1, 5, and 6, Diagram No. 240, presented a good example, of the high velocity with which bodies thus placed on the summits of slender vertical erections, may be projected by the elastic vibration or rocking communicated by the shock to the erection itself. This vase was thrown nearly in the same path as the balls