Page:Great Neapolitan Earthquake of 1857.djvu/109

Rh by the author (amongst very many of the class), in which from this cause a perturbation was produced, that could render the determination of the wave-path uncertain in the horizontal element, by more than 2° or 3°.

The arc corresponding to the sine of $$t e o$$, or of $$s e o$$, viz. $$\theta$$ and $$\theta'$$ of a former case, gives the abnormal angle or bearing, of the horizontal element of the wave-path, if the building be cardinal: or this, + or − the azimuth of the walls, gives it if ordinal, and the angle $$o e u = \theta''$$ is equal the angle of emergence with the horizon, to which it is alternate.

The whole of the preceding may be readily done trigonometrically, and by an observer accustomed to such operations that method will be found more advantageous, as greatly economizing time on the ground, and enabling the results to be worked out at leisure.

Let the dark lines $$m e$$, $$q e$$ be the level top of the adjacent walls (Fig. 48 bis), $$e f$$ the quoin, $$e$$ the solid angle at top, $$n$$, $$w$$, $$p$$, the points of fracture in those lines.



As $$e f$$ is plumb, $$m e f$$, and $$q e f$$, are each 90°. Let $$q e m$$, any angle be given, and also the distances, $$n e$$, $$w e$$, $$p e$$, the two former proportionate to $$f$$ and $$f'$$, the component forces