Page:Great Neapolitan Earthquake of 1857.djvu/112

72 on one wall, to that on the other, $$w$$ to $$n$$, and the length be divided in the proportion of $$e w$$ to $$e n$$; if from the dividing point $$'p$$, a plumb-line be dropped, it will lie in the vertical plane in which the path of the wave is situate. Let now another line be stretched, or a light straight edge of wood be held, between the points $$'p$$ and $$p$$ (corresponding to the line $$o p$$, of Fig. 46); lastly, stretch a line from the point $$'p$$, so that it shall be square to the line $$'p p$$, and holding it in the hand, "sight it," to coincide visually with the plumb-line: this line or string will then be, in the path of the subabnormal wave, and its azimuth and inclination, may each be at once got, by compass and clinometer, or by two measurements, without the latter instrument. This method admits of quite sufficient accuracy, if the fractured-out pyramid be not too large, but such, that either a straight edge (a straight rafter or joist will answer, of which plenty may generally be found loose about) or a stout cord can be stretched tight across, from $$w$$ to $$n$$. The direction can be thus obtained within a degree or two at most. It is obvious that if the path of this wave, be referred to its component path, in either of the two walls, by a plane, normal to a vertical plane, and both passing through the wave-path, then the former plane, will cut the surfaces of the walls, in directions perpendicular to the fracture in each respectively, as in $$a'$$ $$b'$$ and $$a b$$ (Fig. 44), which coincides with what was stated before, as to the general. fact, that the lines of fissures from subnormal waves (i. e. those emergent in the