similar, and many others, which must be looked out for, when the phenomena are found perplexed.
Abnormal and subabnormal waves, produce like effects here, to normal and subnormal, except that they are by the former produced in two planes of walls, meeting at an angle usually right, and accompanied by disturbances transverse to the plane of each wall. When the wave is of very steep emergence or vertical, then diagonal fissures are produced in two directions crossing each other, and often accompanied by vertical fissures also, from causes obvious on reference to the statements already made as to the fissures in solid walls due to such waves.
A remarkable example of fissures of this sort will be found in a subsequent page, occurring at Polla. Their general character is that of Fig. 89.
These figures apply to wall apertures of the usual moderate size of windows and doors. When very much larger and wider, and covered at top with plate-band arches or stone lintels, two or more fissures F often take place, by the opposite movements of the first and second semivibration of the wave, and large fragments fall out.
The usual character of fracture in arches of considerable curvature may be illustrated by the Photog. No. 90 (Coll. Roy. Soc.); but many others will occur in the succeeding pages. When the 'width of arch aperture is very considerable-eighteen, to twenty or thirty feet for example—and a large mass of wall overhead, fractures transverse