Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/371

306 The conditions developed in Chap. V., when applied to the wave-paths, lately referred to, falling to the south of the 2 mile circle, and the shallowness of focal depth, given (see Diagrams Nos. 1 and 2) by the observed emergences for all the places, very close, to the seismic vertical, and to the north, north-east, and east of it, such as Salvitello, Auletta, Villa Carusso, &c.; indicate that the vertical section of the focal cavity, was also either more or less curved, or, what is much more in accordance with the laws of fracture, was, in its general surface, inclined to the vertical; and, from the seismic vertical, sloped off upwards, towards the N.W., and the contrary way below.

We therefore finally come to this conclusion, that the focal cavity, when at its full dimensions, was a curved fissure, whose height was three geographical miles, and length along its curve of contrary flexure was nine geographical miles, while its thickness, or third dimension, between wall and wall, was probably very small, but is uncertain.

The area of the lamelliform fissure, or its height by its length, was therefore 27 geographical miles, and the depth of its central point (nearly) or mean depth of the fissure,—the true focal point, or seismic centre of force, was at 5 geographical miles below the surface. The top edge of the fissure, therefore, did not approach nearer to the surface, than 4 geographical miles, while the lower edge reached a depth of at least 7 geographical miles below the surface. Whether this were previously an open cavity, which becoming suddenly surcharged with dense steam, had its lateral dimensions suddenly enlarged, and by the impulse of which the wave of shock was generated; or whether a fissure was thus widely rent, where there was nothing