Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/341

276 further south, the impulse, as already observed with reference to the meizoseismal area, impinged more and more obliquely upon the axial line of this chain, which, therefore, as it went further south, became a better transmitter in that direction.

But whence comes the enormous spread out, of the greater end of this coseismal curve to the south-east? It was explained in Part II. (in treating of the main and secondary shocks at Padula), that when a transverse, or, within certain limits, an oblique impulse, impinges laterally upon a continuous mountain range, two movements of vibration are communicated; the one, a wave transmitted along and in the line of the axis, the other a transverse wave, which causes the axial line to sway laterally, and transmit a quam prox. horizontal transverse wave, along from one end to the other; like the sinuous movement which travels along a long rope when, hanging suspended between two points at the same level, it is jerked suddenly at one end, transversely to its length. Such were the two wave movements, transmitted along the chain, southwards of Tito.

If a mountain range so vibrating laterally, be quite free at both sides, the lateral wave passes along, and is finally delivered out, at the free lying surface, of the end reached by the progress of the wave; but if a number of subordinate chains spring out from one side, a large portion of the transverse wave, will be in its progress, transmitted to those abutting mountain ridges, and in succession delivered into them "end on.'' The transverse wave itself, will therefore rapidly be robbed of its volume, and very little of it will reach the outgoing end of the principal chain. Now, this is precisely what has taken place here.