Page:Great Neapolitan Earthquake of 1857.djvu/160

112 irregularities, looseness, and twistings, of the lines of tiles.

There are two other forms of roofing, viz. arched or groined, and dome roofs, which frequently occur in the better order of churches, and which become highly instructive. Upon these some observations will be necessary. The Neapolitan churches, which are nearly all either Lombardic, Roman, or Palladian, in style of architecture, are very commonly roofed with solid brick (or more rarely stone) arching. The monasteries and public civil buildings, are also very frequently arched, both in floors and roofs, and many of the better built and more modern villas and palazzi, are so likewise, more especially in the upper stories and corridors. Semi-cylindric arching, and hemispherical domes, are the more common forms of church covering of this character, built in brick and mortar, with timbering and tiling over that, or with solid stone sloping pavement, for the outer skin of the roof. Groining of intersecting cylindric arches, is not unusual, in the monasteries and civil buildings, and hemispherical doming, intersecting and connected by cylindric arch-bands, is often met with in the more recent churches. The arching in villa architecture is of comparatively late date, and executed in hollow pottery laid in mortar.

The inertia of all these forms of roofing is very great. Intersecting arched groining universally splits along the crowns of the arches, whenever the direction of wave transit is transverse or oblique, to the line of the axis or springing, and with moderate amount of emergence. If the wave be very abnormal, and the structure pretty large, transverse fissures at right angles or more or less oblique