Page:Great Neapolitan Earthquake of 1857.djvu/117



now arrive at the fifth and last class of waves, viz., those of vertical or very nearly vertical emergence, upon which it is necessary to make some remarks.

As a strictly normal or abnormal wave, i. e. with perfectly horizontal transit (unless by reflection) is impossible from a focus beneath the surface, so an absolutely vertical emergence is in strictness limited, to the single point of the earth's surface vertically above the focus, or to the seismic vertical itself. Inasmuch however, as in reality the disturbance producing impulse is not confined to a mathematical point, but (whatever be its nature) extends over a greater or less, and in all cases a very considerable area, and lies also at such a considerable depth, that lines extending from it to the surface make but small angles with each other; so a tolerably large area is found in the midst of every earthquake-shaken region within which the angle of emergence is so steep that it may be viewed, as respects the effects of the wave, as practically vertical.

In the preceding remarks upon the four first orders of wave-paths, we have viewed the wave itself as of one sheet and the movement of any particles in the wave to