Page:Great Neapolitan Earthquake of 1857.djvu/48

12 For the same reason the observed angles of emergence will be found to vary from those that would be due to a focus of evanescent magnitude. The distinction must be clearly borne in mind between the velocity of transit of the wave—that with which the advancing form or seismal curve is transferred from point to point of the surface, and that of the earth particles moving within the limits of amplitude of each vibration. The former velocity is very great, nearly half as rapid as that of a cannon shot, and depends chiefly upon the elastic modulus of the earth's formations through which the wave transit is made; but the latter, as now measured for the first time, is very small indeed, often not greater than that which a body acquires by falling from a height of two or three feet. It is, however, to the rapidity of transit velocity, or, which is the same thing, to the great rapidity with which the proper velocity of vibration of the wave passes, from 0 to its maximum velocity, on reaching any material object, that the formidable dislocating effects of the very moderate maximum velocity of vibration are due. We need not, however, here extend these preliminary remarks.

The evidences fitted for observation after the shock, by which the conditions of earthquake motion are discoverable, may be divided into two great classes:—

1st. Fractures or dislocations (chiefly in the masonry of buildings) which afford two principal sources and sorts of information.

a. Information from the observed directions of fractures or fissures, by which the wave-path,