Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/393

328 from the origin to the mountain range beyond Salerno, (40 geographical miles to axis of chain,) was performed at a mean rate, between those from the origin to Monte Peloso, through the flat bedded formations, of the upper limestone and murgie rocks; and from the origin to Atella, chiefly through hard, shattered Apennine limestone, and 4 or 5 miles of volcanic rocks as hereafter to be stated, or

At this rate, the total time of transit, from the seismic vertical, to the axis of the chain at Salerno, 45 geographical miles, would require 313s; which leaves 213s (of the total time = 8m 46s) for the transit of the remaining 23 miles through the tufa. And, again, deducting the 15s·47 or 16s for the loss of time, at the point of refraction, we have 197s nett time, for the transit through the 23 miles of tufa, which gives a transit rate of 8s·6 per geographical mile, and a transit velocity = 706·6 feet per second in that formation, being less than that in the limestone, by 167·7 feet per second. There can be no doubt that a similar correction should be made, for the transit rates, given for Melfi, &c, from the fact that the last six or seven miles of the distance is also through volcanic rocks, though of much greater age, density, hiardness, and continuity, at a considerably greater depth than those of the Vesuvian area.

Proceeding now to try the transit rate, by the Naples Observatory time, we find