Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/381



$$\mathrm$$ (Fig. 352) be a section vertically, at the earth's surface, $$o$$ the mean focal point, at the depth $$o\ d$$, measured

upwards upon the seismic vertical $$o\ \mathrm$$, and $$d\ c$$ be the amplitude of the wave, in its path of emergence at $$s$$, taken somewhere near $$o\ \mathrm$$. The wave is originated by the sudden compression, of the elastic materials forming the walls of the focal cavity. Admitting the temperature, as just obtained on the commonly-assumed hypogeal increment, we have obtained a measure, of the pressure producing this compression; and if we possessed the coefficient of compression