Page:Great Neapolitan Earthquake of 1857.djvu/169



passing to the subject of velocity of the wave, a few remarks should be made as to the methods of ascertaining its transit velocity. The transit velocity —that with which the form of the wave is transferred from point to point of the shaken surface— is so great that it can only be ascertained with the desirable precision, by means of a proper seismometer, of the self-registering class described in the author's fourth Report on the Facts of Earthquakes, ('Trans. British Association,' 1858), to be established prior to the shock; and the only known method of determination is based upon observation of the time of arrival of the wave, at each of three or more distant stations, within the shaken area.

In the facts which we can usually collect in the field, after the shock, we are limited to the casual observation by the ordinary time measurers (clocks and watches) of the moment of observed shock at several places.

Such observations are liable to multiplied sources of error; from errors in the indicated local time as shown by the time-pieces themselves, and errors of observation of the moment of true shock, as said to have been recorded by