Page:Great Neapolitan Earthquake of 1857.djvu/157

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ing the joists, it will be seen, here plays the part of a connecting stay to the side walls , , at the return

1857 Earthquake fig. 78-79.png

stroke or semi-vibration of the wave, tying them together by the inserted ends of the joists, whose distances they themselves fix.

When the floors have given way as described, the building is usually too far destroyed, to be of any use for seismometry; beyond this, that a clear comprehension of the mode of fall, will always enable the general direction of shock to be roughly inferred; but where the floors, being heavy and good, have not wholly fallen, nor the walls, but that these are fissured, and have given out unequally, and the floors also are fissured, but not completely displaced, very valuable indications may be obtained from them, as, for example, in Fig. 79, where the walls , are fissured and given out. We obtain excellent measures of the extent, of this in the directions , , by measurements at the edges of the concrete or tiles, and inside the walls, controlling those of the fissures, which sometimes. (though rarely) are not measurable at all, and from the directions of the fissures of the floor we may obtain evidence, of another wave movement when occurring in the direction , transverse to the principal one . In fact, the observation of the floors, is only second in importance to that of the walls. The one illustration given, however, must suffice to indicate a large and very varied class of questions to which they may be made to give response.