Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/307

expected, and is true to nature, and within certain limits (the second circle of 2½ geographical miles' radius round the focal vertical for example) arises from—

1st. The actual spread or dimensions, of the focal cavity of impulse.

2nd. Errors of observation, or rather of determination, from the observed effects at the several stations.

3rd. Errors introduced, by protracting all the wave-paths, (determined by magnetic bearings at each station,) to a common m£an magnetic declination.

4th. Actual small changes of direction, due to refraction, or reflection, or both, in any given path, during the propagation of the wave outwards, due to change of formation, &c

The three last causes of divergence are small, the two last very small The amount of divergence of the wave-paths grouped round the seismic vertical, from a common emanation of impulse therefrom, is therefore mainly due to the size and form of the focal cavity, and hence the diameter of the focal circle that equals such divergence, becomes an approximate measure of the horizontal dimensions of the focal cavity itself, whether it be, a rent, or suddenly blown out chamber, or whatever else.

The most distant stations of wave-path observation (for unreflectcd waves) were from the seismic vertical; Canosa, N.E., 47 geographical miles; Salerno, west, 35 geographical miles; Lauria and Sapri, south, 37 geographical miles, nearly. But the mean distance of the greater number of stations, was from 12 to 15 geographical miles