Page:Great Neapolitan Earthquake of 1857.djvu/150



us proceed now to the relations of floors and roofs to the shock, and the effects of both in modifying its action, immediate and final, upon the other parts of the buildings.

The common construction of provincial Neapolitan floors and roofs, has been already briefly described. Were the floors of an earthquake-shaken house perfectly homogeneous, formed, as if of a single parallel plate of cement or beton, self-supported and free from all timbering, the lines of fracture would be almost perfectly regular, and follow in accordance with those of the side and end walls; so that with a normal or subnormal wave, fractures would extend crossways, parallel to the walls, and generally uniting the fissures wherever these were situated in the latter; and with an abnormal or subabnormal wave, the fractures would be diagonal, at right angles to the line of transit of the wave, when viewed in plan upon the floor, and generally uniting the wall fissures situated in the adjacent walls, or diagonally opposite; while with a vertical or very steeply emergent wave, fractures would be found crossing each other in an horizontal plane at right angles, or nearly so, but having any degree of obliquity to the planes of the walls,