Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/331

266 surfaces, upon which the wave-path passes off into free space at their emergent flanks. In this case the portion of the wave that passes on, beneath the bottom of the valley between, and through the lateral flanking range beyond, lies so deep, that its path is not emergent to the surface beyond, until after a distance horizontally, so great, that the energy of the wave is almost expended, or greatly reduced.

This will be understood from Fig. 346; almost the whole of the wave emergent in the direction $$a\ b$$, above the level of $$c\ d$$, is extinguished at the free lying surface $$f\ r$$; but the portion that passes on, below the level of $$c\ d$$, is not emergent at the surface until it has passed through the long distance to $$e$$, when its force has become decayed.

In such a case, a city at $$t$$, may be unconscious of anything, but perhaps some tremors, emergent in nearly vertical directions, while another city, a very few miles off, at $$k$$, may have been totally destroyed, by a shock nearly horizontal in direction.

ln Part II., some local examples of this condition, acting upon a large scale, have been adverted to, as affecting the country upon the highest forks of the Tanagro, above Vietri di Potenza, and between Tito and Bella, &c. Before applying these general truths to the earthquake we are engaged with, we must consider the effects of the form and position of a focal cavity, upon the transmission of a wave of shock.