Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/385

320 $$t$$ and $$t'$$ the compressing force acting to its full extent at $$o\ t, o\ t'$$, and becoming evanescent at $$f$$ and$$f'$$, the

extremities of the fissure; and supposing, as already done, (Chap. V.), that the wave of shock, commenced being transmitted, when the lamellar area of the whole focal cavity, had attained half its dimensions; then the amplitude of the wave at $$d$$, would be $$d\ c$$ = 0·769 feet, assuming it the same in all directions round $$o$$, or about 9 inches. The actual dimensions of amplitude observed at Polla, a place like $$s$$, near the seismic vertical $$o\ \mathrm$$, deduced from the widths of masonry fissures was 2·5 inches, being less than the calculated result by 3·6 times. It follows, therefore, either that Tredgold's coefficient of compression, is too great, for the actual material compressed, at the focus of this earthquake, (as it most probably is,) or that the pressure was not that, due to the maximum depth, or was due to a smaller coefficient of hypogeal heat increment. If we substitute for it the pressure due to the mean focal depth, we shall obtain a result, that does not differ very widely, from the observed amplitude at Polla, and will be still nearer, if we suppose