Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/387



VERY common error appears to prevail amongst writers on physical geology and seismology, in either confounding the velocity of transit, i.e. the rate of surface propagation of the wave of shock, with that of the wave itself, i.e., the rate of displacement and replacement of the particles being acted on by the wave, or in supposing them to be necessarily the same; and a want of clearness on this point, has greatly retarded the progress of earthquake investigation. A similar mistake is even found in some treatises on physical geology, in respect to the analogous case of aqueous waves of translation, in which it is assumed, that the enormous transit rate, of the tidal wave of translation upon the deep ocean, may be taken as the measure of its diluvial or drift-producing power.

The ocean tide wave may travel across the deep Atlantic, at the rate of nearly 550 miles per hour, and on reaching soundings, be reduced at the lips of the Irish Channel, to 200 or 175 miles per hour. In the Channel, a further reduction takes place, so that the observed rate of the tide stream is only 2, 3, or 4 miles per hour, and some