Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/400



power of the wave of shock, to produce overthrow, depends upon the intensity of the wave, that is, upon its amplitude, and upon the elasticity of the medium in its passing through, conjointly; and as all free displaced bodies, must be displaced to the extent, due to the greatest velocity impressed, at the centre of gravity, the measure of such displacement is always that of the velocity of the wave particle, at its point of maximum. Our ascertained velocities, are therefore those of that maximum velocity, for each separate station.

We have had the following separate determinations, which form three groups: the first being cases, in which the conditions were such, that the true wave velocity was not materially increased, nor the result interfered with, by surface oscillations; the second, where the velocity ascertained, and being in excess of these, is made up of the true wave velocity plus that of the elastic oscillation of the surface, at the displacing point; whether of the whole "colline," as at Saponara, or of any structure, such as a