Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/315



The determination of these areas, and of the curves that limit them, must always be, to a certain extent, arbitrary because we have no sufficient means of comparing, the total mechanical effort at the surface, along the successive points of any coseismal line; and as it is certain that, apart from the perturbations due to physical configuration, this effort must be diflferent, at every different distance, from the seismic vertical, so, strictly, an isoseismal area is impossible. In order, however, to make comparison, and to obtain some clear notion, of the relative distances of propagation of equal seismic effort, in different radii, from the seismic vertical, we must attempt to sketch out, the form and boundaries of equal effort, upon conventional principles.

I have hence divided the total area, of the vast region within which the shock was in any way perceptible, into four, more or less concentric areas, each marked by a determinate though arbitrary limit, of the seismic effort that acted within it. The innermost and smallest of these, (as shown in Maps A and B,) is that within which, the greatest seismic effort has been exhibited, in boundary: it is marked