Page:Great Neapolitan Earthquake of 1857.djvu/432

346 we have $$b = 26$$ feet, and $$a = 16$$ feet as measured. We must estimate the deficiency of $$a$$ from the above circumstances, and, adding $1⁄16$, assume $$a = 17$$ feet.

Taking the velocity of the wave as we found it at Polla $$= 13$$ feet per second,

and solving for $$e$$ — we have

which is the angle of emergence, as given by the projection of this bell. This, owing to the disturbing conditions, can only be considered as approximatively true, to within ± or 3°. The result, however, is corroborated by the same angle having been obtained otherwise, as already given: better data were not procurable at La Sala.

The base of the church tower may be viewed as about the mean level of La Sala. The barom. reads 27.86 in. Thermo. 54° at 11$h$ 0′. A.M. Naples time, 15th February, which reduced gives the altitude of the spot = 1768.30 feet above the sea, and about 1000 feet above the piano at its northern extremity.

The cause of the disturbances in the general direction of wave-path observable at various points since leaving the north end of the valley, began now to be apparent, on examining with the maps, the relations of the lateral chains at either side, to the main direction of shock hereabouts.

Referring to Fig. 208, in which is sketched the general form of the piano, and the prevailing lines of ridge, or summits of the mountain ranges around it are marked by