sending outwards the side walls at its fall. No. 84 (Coll. Roy. Soc.) is an exterior view of the apse of the church of St. Maria Maggiore at Vignola, where the wave was sub-abnormal, with moderate obliquity and emergence, and where the fragments of the tower and of parts of its conical roof, were projected on to the roof of the church.
Conical or prismatic roofs of this sort over towers, being generally of timber were not frequently disturbed; when fractured, however, they are so in ways extremely capricious and perplexing. The partial fall of the tiled roof of the apse at Vignola, was due to the drawing away of the heads of the principal rafters from their support by the movement of the curved walls consequent on the large fissures visible in them.
It may be remarked that generally no feature of architectural construction is more characterized by its destructive effects upon the remainder of the edifice when shaken than are those vaulted and domed roofs. Their inertia is enormous, the centre of gravity is high above the walls, and they are deficient in tenacity and flexibility. They therefore not only are dislocated and fractured separately, but their rocking to and fro as a whole on the tops of the walls leads to the destruction of the latter. Upon examining the gigantic ruins at Rome, of the Imperial Baths of Titus, Caracalla and Nero, &c., an eye that has become conversant with seismic observation, at once perceives that the destruction of these enormous edifices, was but little due to the feeble hand of the barbarian, and was mainly produced by the earthquakes that desolated the city between the fifth and ninth centuries, acting thus upon their massive vaults and domes of brick-work.
