Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/421

Rh subordinate ones, all in directions normal to the flanks of the hills around. Numerous other examples have been remarked upon, in treating of the facts of the Val di Diano in Part II. It is but just to myself that I should state, that when observing and recording those wave-paths in the country, I had not Zannoni's map with me, nor any other that showed the directions of the mountain ranges with any correctness; and that the beautiful inosculation of cause and effect, as to those perturbations, only showed itself fully, when I was subsequently able to lay down correctly the directions of the mountain ranges, along with the wave-paths.

In the separate system of Naples, the effects of reflection and refraction are developed, on the grand scale already described. Within this area, although the general energy of the shock was so much diminished, examples of local mountain reflection are seen at both sides of the Monte St. Angelo range, and very curiously at Ottajano and Somma; which places, besides the main wave, (after its two refractions at the above range,) in the nearly south to north new wave-path, sustained the subordinate shock, of reflected waves from the N.E. flank of Vesuvius, in a path north 67° E.

Such a reflected shock may, by the accidents of surface configuration, in relation to the seismic vertical and to the place affected, reach the latter before, or after, or along with the main shock, and thus produce two closely-following successive shocks at one spot, differing in direction, though emanating from a single originating impulse, and perhaps only felt as a single shock, at another spot, a few miles away.