form round the top of the walls commencing vertically, and masses are thrown out, each carrying down with it an ungular-shaped toe, so that the tower becomes shorn around the summit at a greater or less distance down, by the descent of several separate masses from it, in the form of Fig. 60, and, as better seen in the Photog. No. 61, page 42, at Marsiconuovo. The original fissures are here, no doubt, produced chiefly by the twisting movements, transferred quite round the walls, by transversal vibrations as referred to when treating of the wave of vertical emergence.
This applies but to a limited extent to the semi-cylindrical "apses" which form the chancel ends, of so many of the more ancient churches. These generally split Off at or near the re-entering angles of the quoins, by which they are joined on to the body of the building, if the wave transit approach a line, transverse to that diameter; if otherwise, the walls of the "apse" split vertically in the direction of the opposite diameter.
Towers of extreme altitude and very narrow base, such as slender minars, single columns, lofty campaniles, &c., involve a number of complicated and curious considerations, as respects their resistance to fracture and to overthrow by shocks.
In these the elastic modulus, density, and range of flexibility before fracture of the masonry, the time of vibration of the structure viewed as a compound elastic pendulum—together with the direction of wave transit, and the relation that may subsist between the amplitude of the wave, its maximum velocity, and the pendulum time and range of
