Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/76

Rh together with "lime putty" in the joints, which, from their recent formation, had, in fact, no adhesion but the friction of their mutual contacts. It was obvious at a glance, (as may be seen on studying the positions of the blocks in the Figs, and Photogs.,) that the whole of the upper part of the structure, above the line $$e\ f$$, had been subjected to rocking to and fro, in a line not far from parallel with the front face, or S.E. side; but that the large block, 6 feet x 3 feet x 1 foot, which formed this side ($$a\ b\ c\ d$$, Figs. 1 and 2) had not been so moved at all. It is unnecessary to go into details, as to how all the individual blocks assumed the positions shown, which are from careful sketches and measurements. Their movements are resolvable into rocking through small arcs as above.

The large front block ($$a\ b\ c\ d$$) acted upon edgeways, or nearly so, was not upset; but its large inertia shoved it, with its base, about 0·3 inch S. W. and opened the joints, $$t\ t\ t$$, &c. The wave-path, therefore, was nearly in its plane, and its direction was from the S.W. to N.E.

The side blocks, ($$s\ s$$, &c., were not upset either. The wave-path, however, passed through them, transverse to their broad dimensions. Assuming that the wave-path was nearly parallel to the front and rear of the structure, or nearly in direction, 42° 30' E. of north, we can calculate the limiting value for $$e$$, greater than which, the angle of emergence here must have been, so as to leave this little structure standing; for the front block, taken alone, possesses the greatest stability edgeways, of any one in the structure, to a force as above. In determining its amount, we must take into account, part of the mass of the superimposed capping, on the summit of the monument. We may