whole roof rocks endways upon its bearings at the level of the caves or "wall plates," and these purlins act as "battering rams" upon the gable walls, which they almost invariably carry away wholly or in part. A good illustration of this is afforded by Photog. No. 80, of the west end of the church at Picerno, where the common rafters are nearly all gone as well as the tiling, from the right hand side of the roof, and the front gable nearly all thrown down by the E. and W. rocking of the roof, and the inertia of the gables themselves. When the wave is nearly normal, and transverse to the ridge of one of those large heavy timbered roofs, with the ends of the principals resting directly upon the masonry, of the top of the side walls, or occasionally perhaps upon a heavy wall plate of ill-squared timber, the inertia of the whole is so enormous, that in almost every instance, the wall first reached by the wave was thrown outwards, by the shove from the roof and its own inertia together, and the whole roof then dropped nearly plumb down upon, the area of the building, crushing everything before it.
From this tiled roofing, however, whether carried away completely, or only disturbed upon the walls, very few seismometric observations of value, can be made. The chief of these consist, in the occasional indications afforded, by the amount of draw-out of timbers from their sockets in the walls, either longitudinal or transverse. The tiling itself, is so loose, the interstices between the overlaps so great, that it is very seldom partially disturbed, never probably in situ, unless by nearly vertical emergence of wave, it is either carried away altogether, or presents no signs of movements that can be distinguished, from the ordinary
