Page:Great Neapolitan Earthquake of 1857.djvu/111

Rh but as $$\text{L}+\text{M}:\text{L}-\text{M}::\tan\frac{1}{2}\rho:\tan\frac{1}{2}(\sigma-\tau)$$  and \angle$\tau=\frac{1}{2}\rho$, + arc corresponding to, $$\tan\frac{1}{2}(\sigma-\tau)$$ \angle$\sigma=\frac{1}{2}\rho$ – the same arc. Again, $$\sin\tau:\text{L}::\sin\phi:2x$$ $$2x=\frac{\text{L}\sin\phi}{\sin\tau}$$ = the distance from one fracture to the other diagonally opposite which gives the angle of emergence, or that made by the polar $$a,~b,$$ of the subabnormal wave, with the horizon. $$2\times\frac{y}{\cos\theta}$$ = R, the common resultant in the polar $$a b,$$ and, $$2y\tan\theta=f''$$ the vertical component.

An extremely easy method may be practised of finding the path of a subabnormal wave by an observer in the field.

Referring to Fig. 44. Let a line be stretched across the top of the walls (or anywhere below that, but horizontally), from the exterior or interior angle of fracture,