Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/309



When the distance, upon the surface, from any station t the seismic vertical is known, it is obvious that a single angle of emergence of the wave-path will determine the depth of the focus, the seismic vertical itself, being in fact, the other of the two wave-paths; and if the earth's surface be viewed as a plane, which, as will be shown, we may consider it, without introducing sensible error, for a seismic region within our limits; then calling r = the distance from the seismic vertical to the station, the emergent angle at which is = e, we obtain the depth,

of the focus of that particular wave-path, below the horizonal plane passing through the station. In that way the various values of d, have been calculated, and plotted upon the Diagram Nos. 1 and 2, for the following twenty-six stations in every azimuth around the seismic vertical.

In this Diagram, in both figures, the strong horizontal line marked .0. represents the level of the sea. The fine