Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/352

Rh It was not in superficial area, coextensive with the shock, but the form of the area within which it was heard, was closely similar to that, of the two first isoseismals generally, so that the same conditions, that were favourable, or the contrary, to the distant propagation of the wave of shock, were about equally so, to the wave of sound, but the latter was the feebler of the two, ab origine. Echoes, the disturbance of local noises at the moment, the uncertainty with which the ear judges of direction of sound, the evanescence of the phenomenon, and the difficulties inseparable from trusting to merely collected information of often incompetent observers, or unfaithful narrators, who observed under alarm, must ever deprive sound phenomena (except when heard by the physicist himself) of the unerring certainty of deduction, that belongs to the mechanical problems, presented by the phenomena left after the shock.

Still we shall find that sounds, are not without their seismic significance and response, and when more of their complex conditions shall have been submitted to careful à priori discussion, we shall be much better prepared to put the inquiries as to facts, that will be valuable in result.

No sound whatever, was heard to accompany the shock, except within a very limited portion of the central area of great disturbance.

The limits of surface, within which the shock was experienced, in its respective degrees of intensity, have been already given. Within the fiirst and second of these (the area of great disturbance) sounds were heard, along with, or nearly along with the shock, over an area equal to more