dency of the oscillation being, to alter the angular position of each separate mass, and to produce separation from the others in directions parallel to , to , and to .
The fissures tend to form, as in a solid wall, perpendicular to the line of transit of the wave; but as in a 'solid wall (all of horizontal courses) these must also follow the joints, so must they here where the joints above all such rectangular apertures are those of arch voussoirs at various inclinations. Hence, when the wave is normal the fissures form through the nearest vertical arch joint generally, and through to the next aperture above or below, as in Fig. 87; but when subnormal the fractures are through
the joints of the voussoir nearest square to the line of transit, as in Fig. 88; and very generally in such a case the fissures run, from the angles of the apertures. Hence the angle made by the joint, of the plate-band arch or low segment arch, above a window or doorway, through which the fractures run, with the vertical, forms an approximate measure of the angle of emergence. The precise position of the fracture is of course varied by innumerable minuter conditions, such as variation of thickness in the wall, difference of strength at different points otherwise
