Page:Great Neapolitan Earthquake of 1857.djvu/161

Rh to the former also form. When the emergence is steep, three orders of fissures, if not more, are produced in large arched vaults, one along the crown, and two others parallel to it, and distant from 40° to 50° at either side of it. Where the lateral movement amounts to even a very few inches, the detached masses descend, between those standing at each abutment, enough to destroy equilibrium and either they fall through, and more or less, from both abutments follows, or the whole comes down together. When the emergence is steep, the two lateral fissures, are further removed from that at the crown, and a very moderate vertical shock, suffices to send outwards, both side walls or abutments, and the whole vault drops between.

The gable ends or semi-tambours of such cylindric vaults, consisting (essentially) of a semicircular plate of masonry resting upon its diameter, level with the vault springing, give out at top at both ends of such vaulted roofing, when the direction of wave transit is along the line of the axis or near it, forming a large fissure transversely, at the junction of the gable and vault ring (or near it), which is usually, most open at he end first reached by the wave, from reasons obvious from what has preceded; the difference is greater as the wave is more subnormal, as gravity conspires with inertia then, to bring out the first reached gable, but acts against inertia, in the second.

When the wave is subnormal and transverse to the axis of a cylindric vault, its greatest overthrowing power is exercised, when the angle of emergence is such, that the line of transit passing through the centre of gravity of the vault, (or of its transverse section), also passes through the joint (either at one side or the other of the crown), that is