Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/308

Rh from the seismic vertical, which is one more favourable to obtaining exact results than any, either much greater or much less. The more distant the station, the less the second cause, and the more the third cause, named, of divergence, affects the observed and protracted wave-path, and vice versâ.

Apart from the perturbations that may arise, from the physical features of the surface or from the nature of the formations traversed, the indications from the most distant stations, should be the best, but the effects presented to observation from these are also the feeblest; while at the stations that cluster, very close above the focus, the phenomena, though well pronounced, are more perplexed by the violence of their production, and entangled with effects produced by the transversal movements of the wave. It may be remarked, then, with respect to the Map A, that the great preponderance, of our very best observations, as regards distance of station, give wave-paths, falling within the circle of 2½ geographical miles round the seismic vertical.

We have thus ascertained, the point of the surface vertically above the seismic focus, which we find to pass nearly through Caggiano, a village 58 geographical miles east of Naples, and 16½ geographical miles south of it. We therefore now proceed, by means of the angles of emergence observed, to determine the depth of the focus itself, beneath the surface.