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88 vibration of the towers—all are elements. It is not requisite for our present purposes, however, here to pursue the investigation.

The effect generally, of want of symmetry in the severed masses, is to reduce them by further dislocation, prior to complete overthrow. For example: a portion of a uniform wall, severed by transverse fissures from the remainder, but having a buttress of its own or of less height somewhere along its length, is again transversely broken close to the buttress, the moment of resistance to fall being different in each. The relation of the buttress to the wall, as a support against transverse forces of a statical sort, is no longer the same, when the overthrow is produced by a force applied with the rapidity of the wave of shock; there may not be time, to transmit its own stability to the remainder of the wall.

Where the buttress is at the same time a tower rising much beyond the height of the remainder of the building, these generally tend to mutual destruction; the primary fissures occur at the junctions or near them; the walls and the tower have different times of vibration, as elastic pendulums of different lengths, and whether by chance isochronous or not, produce mutual damage, by their impulses upon each other. This is peculiarly striking, in the case of many of the meaner class, of Italian rural churches, where the belfry tower is built into one of the quoins of the main rectangular building, the two adjacent side walls are frequently completely destroyed by transverse rocking of the tower; although the latter may have only suffered fissuring at the lower portions, and that which was above the level of the church walls be overthrown.