Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/408

Rh disturbance, have also a very deep seismal focus, and also that the greatest depth of seismal focus within our planet, is probably not greater, than that ascertained for this Neapolitan earthquake, multiplied by the ratio, that the velocity of the Riobamba wave bears, to that of its wave; or, what is the same thing, by the ratio, of the altitudes, of the volcanoes of the Andes, to that of Vesuvius.

Hence the greatest probable depth of origin of any earthquake impulse occurring in our planet, is limited to 5,33 x 34,930 feet, or to 186,176 feet, or 30·64 geographical miles, and therefore only just touches the depth, which upon received notions as to the increment of hypogeal temperature, is supposed to form the upper surface, of the imaginary ocean of liquid lava, of the earth's interior.

One other point remains to be noticed, as to the velocity of the wave: most of our velocities have been deduced from overthrowing, projections, or fractures, produced by the wave in its first semiphase, or forward movement; a very few, from those of the second semiphase, or contrary one. There is reason to conclude however, that the velocity is not precisely the same, in both semiphases, and is smallest in the second. That this arises from defect of perfect elasticity, in the media through which the wave passes, seems highly probable; and this condition affords an additional ground of explanation, of the fact that in fissured buildings, (Part I. in loco,) those fissures are generally widest, that are first acted on by the wave. This inequality in velocity in the two semiphases, was first pointed out to me by my friend Professor Haughton, as deducible from the swing of the two chandeliers at Naples, to whom I submitted the facts I had obtained (Part II. in loco), in the hope that he might be