Page:Great Neapolitan Earthquake of 1857.djvu/122

80 effects are produced by vertical emergence. Upon these the vertical velocity produces a moment of inertia acting directly downwards, and therefore favoured by gravity. Arched roofs, groining, and that form of arched ceiling constructed of hollow pottery, then spread the walls, as they come down, and falling upon the floors below, bring them down in succession. The details of movement will, however, be best given further on.

Upon the whole, the phenomena of vertical emergence, afford little ground for exact observation, with a view to trace the elements of the shock, and their limited occurrence is not to be regretted, on this ground. When once seen they present general features by which they can almost always be recognized with tolerable certainty, but not such as will enable us to ascertain directly, the line which produced downwards, should intersect the centre of impulse beneath the central field. That must be sought for otherwise, by observations at a greater distance from the seismic vertical, where the wave movements have become more uniform, and less complicated. When ascertained by the method of intersections of wave-paths, aided, if occasion serve, by determinations of velocity of transit, applied to the values of the angle e, its correctness may be tested and controlled, within certain limits, by the coincidence or not, of the focus thus obtained, with the observed area of vertical phenomena, somewhere within which the seismic vertical is situated.