Page:Great Neapolitan Earthquake of 1857.djvu/47

Rh wave, whose length is equal to the amplitude of the wave or to one half of its complete vibration.

As the coseismal curve (or crest of a wave of shock) enlarges its area, travelling outwards in all directions from the seismic vertical—that is, from the vertical line passing through the earth's surface (and centre) and the focus—every point in and upon the surface, in succession moves once forward and back in the direction of the wave-path, and to the extent of its amplitude at that point, or in two components, vertical and horizontal, that shall give such direction. We are not concerned here to consider the extent or the laws according to which the range of wave movement of each material particle, either in amplitude or attitude, or the velocity of its particles, vary with the distance from the focus.

The angle of emergence for any given depth of focus diminishes as a function of the distance of any given point of the surface from the seismic vertical; and I have shown elsewhere that the power of the shock to overthrow objects is, cæteris paribus, a maximum at a determinate distance all round the seismic vertical; and this distance would be equal all round, were the earth homogeneous, and the focus or centre of impulse confined to a mathematical point. The centre of impulse in nature, however, occupies determinate, and often large dimensions. For this reason as well as from non-homogeneity amongst others, neither the meizoseismic curve (or that of maximum overthrow) nor the isoseismic curves (or those of equal overthrow) are found to be circles or even perfectly regular closed curves, nor concentric.