Page:Great Neapolitan Earthquake of 1857.djvu/162

114 45° from it, which, as the centre of gravity is about 0·64 $$r$$, the mean radius of the arch ring being $$r$$, gives an angle of emergence of about 15°. Hemispheric domes, are also most readily, fissured or overthrown, by a wave of small emergence. Where the emergence is steep, the fissures run in curved lines, transverse to the line of transit, and cross each other more or less, when viewed in the line of the vertical axis, and few or none of them pass through the crown. When apertures are formed (such as lights) in the dome, and especially if near its springing the fissures are directed from their upper angles, and great sector-shaped masses are dislocated. An almost vertical or completely vertical shock does not seem to affect domes at all as much, as horizontal or subnormal ones.

The directions of the fissures in any case, depend not merely upon the direction of the wave transit, but also upon the planes of the curving joints, of the structure and upon its details of construction, to such an extent, that general principles can only to a very limited extent be made available for deductive observation from domes.

The Photogs. Nos. 81 (82 Coll. Roy. Sot.), illustrate the general character of the fall of curvilinear roofing. No. 81 is of Tito Cathedral, where the emergence was steep, and the roofing a combination of cylindric vaulting and domes; it shows (see p. 99) the form of a very formidable double fissure in the crown of cylindric arching or vaulting, and the intermediate fragments given down by inertia; beyond this fragments of the lateral domes are visible, and more fully seen in No. 82 (Coll. Roy. Soc.). In No. 168 (see p. 296), the fall of the cylindric vaulting of the nave and chancel at Polla is seen, and its effects in fracturing and