Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/298

Rh not productive of any sensible error in result, all the observed wave-paths are reduced to the true azimuth, from one equal western declination of 14° 30', and are so laid down.

It was found by trial, that a difference in declination of 1°, would not make, at the mean distance of the places of observation from the seismic vertical, a change of half a mile upon the superficial position of any one of the latter.

It will be obvious at once that the great mass of the wave-paths radiate from Caggiano, or close about it, and hence that from some point beneath the neighbourhood of this village the impulses that produced the earthquake were delivered. In protracting the single line of wave-path marked red, from or through any given point of observation, where more than a single direction had been there observed, the most probable mean direction was adopted. I have taken the mean of the extremes, of observed angles of azimuths, when equal probability attached to all the observations, i.e. when the whole divergence, might be assumed as error of observation; but I have adopted the most probable, single wave-path, where obvious and à priori reasons existed for concluding, that local disturbance, due to physical configuration of the adjacent country, or other such causes, had produced partial or secondary waves whose paths should be eliminated. To the circumstances affecting such cases I shall refer again.

Outside the third isoseismic curve, and between Salerno and Naples, a number of wave-paths will be observed, which do not connect themselves directly with the seismic vertical or focus at all. These, due to waves of reflection and refraction of the direct shock, we shall pass for the present also.