Page:Great Neapolitan Earthquake of 1857.djvu/319

Rh with the blow as an elastic pendulum, so that its proper motion at top, which was quam prox. horizontal, was added to that of the emergent wave, thus reducing the direction to one of less emergence, and increasing the range and velocity of movement upon top. The great majority of the fractures at Auletta, indicate a wave-path E. and W. by compass; those of the castello, which is prostrated to the level of the first story, and stands upon the very brink of the precipitous side, when reduced give one of 115° W. of N.; but the buildings are not isolated, and the direction has been probably perturbed by this, and by longitudinal vibration in the mass of the spur on which it stands. On the whole, the wave-path here is E. and W.

The effects, of the form, elevation above its base, substance, and direction with respect to wave-path, of the collines or spurs, upon which so many of these towns are perched, in modifying the results of the shock upon them, are strikingly seen here, as well as at Castelluccio, which we have passed, and at the little village of Petina, which, from a point between this and Pertosa, I can descry with the telescope, perched high up, upon the south scarp of Monte Alburno, at least a thousand feet above me. It stands upon a level sort of short, stumpy, buttressed spur, jutting out from the steep mountain slope, which in form is like a piece of artificial earthwork, the little town standing upon the level platform on top, with a steep scarp in front of it, the mountain rising abruptly behind, and the scarp sloping in and getting lost in the mountain sides to the E. and W. of the town. The terrace upon which it stands, however, is not earth, but solid limestone—a projection, of the great horizontal strike of the beds, of the great scarp of Alburno, as