Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/392

Rh The distance from the origin (seismic vertical) to Naples, allowing for the change of direction, by refraction, at the Monte St. Angelo range, (but without any allowance for loss of time at the refracting point,) is 68 geographical miles, which gives 7s·735 for the transit rate per geographical mile, and a transit velocity = 781·5 feet per second.

To this we must apply two corrections. 1st. For the loss of time at the refracting point, for which we can only assume probable data, as we know nothing of the change of elasticity, of the limestone, nor of the refracting indices. If we assume a loss of time equivalent to the transit through the axial thickness of the chain, or 2 geographical miles = 15s·47 in time for the retardation at the refracting point; then the 68 miles, with a constant velocity, would have been run over, at a transit rate = 7s·514 per geographical mile, or with a transit velocity = 808·5 feet per second.

2nd. The wave emerged, after refraction from the limestone formations, to the south and S.E. of the St Angelo range, into the shattered and discontinuous volcanic formation, of the Terra di Lavoro around Naples, and principally into heavy, soft, and very inelastic tufa, (the grand buffer that has saved Naples many a time, from destruction and not the "safety valve" of Vesuvius, as popularly imagined,) which has been ascertained to have a depth of more than 1500 feet beneath Naples, and is probably immensely deeper.

The transit from St. Angelo to Naples, was therefore made at a much slower rate, than that from the (seismic vertical) origin, to the mountain range. We shall be probably very nearly correct if we assume that the transit