Page:Great Neapolitan Earthquake of 1857.djvu/374

298 south plane, 20° E. of north, in the same monastery, and an example, of heavy dislocated, and inclined fissures. The average angle here was 45° to 50° from the horizon. On principles explained in Part I., this is below the actual angle of emergence, as here each pier rocked more or less individually, and with the direction of the voussoir joints tended to give greater perpendicularity to the fissures. The crown of the upper tier of arches had all fallen in, and they, as well as the cross wall seen to the left of the picture broken off, had all been thrown northward, as the marks of abrasion upon the plaster of the face of the arched wall testify.

The Photog. Fig. 171 (Coll. Roy. Soc.) is an example of a frequent way in which arches are affected, indicating steep emergence. This arch is in a wall not far from north and south in plane. The ink lines $$c c$$ point out the extremes of minimum and maximum angles of emergence of the wave coming up from the northward. The dislocation of the arch produces loss of momentary support to the crown, the top ring of which has descended some inches, bringing a mass of wall along with it; but the fissure closing upon it at the second half vibration of the wave, has caught it, and the arch ring remains, nipped as shown, having descended through the space below its original intrados, at the point of intersection of the fissure, in rather less than the time occupied by one complete vibration of the wave. The data are not precise enough, however, to found calculation upon with any certainty as to this short interval of time.

The Photog. No. 172 (Coll. Roy. Soc.) looks N. W., and shows several tiled roofs fallen in, and is an example of the