to the plane of the wall, usually occur not only at or near the crown of the arch, but (as in the case of roof vaults) 40° 50° even more, at either side of it, and the mass above probably descends more or less, and then secondary diagonal fissures are produced by its descent, which have no relation to the angle of emergence of the wave, and must not be confounded. as indicatory of it, with fissures. previously produced. An example of this will be recorded in the city of Naples.
Very steep emergence with large semicircular arches, usually produces two sets of fractures also, as in Fig. 91.
The wave emergent to , produces the fissures and , and those at and , probably in its first. semi-vibration; in the second semi-vibration the mass tends to rotate round in a direction to and to , but the instant it is displaced the weight of the hanging mass breaks the whole across at by a nearly vertical fissure; drops vertically a little, and when has resumed its state of repose, the right-hand side of the soffit of the arch is permanently a little below the left. This might readily be mistaken (alone) for a displacement due to a normal wave.
Some examples will be given in the narrative also of other singular secondary effects upon the stones and voussoirs of arches by continued oscillation produced by normal or subnormal waves.
When the angle of emergence becomes extremely steep it may occasionally be observed that little or no trace of obliquity of fissure is to be found. The wave, in fact,
