Page:Great Neapolitan Earthquake of 1857.djvu/86

44 competent to produce considerable fractures in buildings; but in this case the fissures will be found to be close and thread-like.

Were the mass of a wall (viewed as a single parallelopiped) so circumstanced that, its integrant portions, retaining their relative positions merely, were free to oscillate, and then to remain at the points which they occupied at the moment the wave left them, having no resilience, in such case the chord of the arc of movement at the centre of oscillation would be very nearly equal to the amplitude of the wave that produced the oscillation; and this would be equally true of the width of a fissure produced in such a wall. Now, we occasionally find walls that are extremely massive in proportion to their altitude, and built of small stones or brick laid in bad masonry, and with almost bondless mortar; such walls have little or no resilience, and, when thrown more or less out of plumb, or fissured, with suitable conditions, afford a rude approximate measure of the horizontal amplitude of the earth wave, by the range of movement impressed at the level of the centre of oscillation. Some examples of this, as observed, will be found in Part II.

The adhesion of mortars and cements to stone or brick, in a direction perpendicular to the faces of the joints of the work, is always much less, with the ordinary materials employed, than the cohesion of the latter for equal sections; the exceptions being only buildings of very soft tufa, or some such stuff. Hence, although fracture and open fissures may occur occasionally, running right through some stones and breaking them across in a building which may be acted on transversely, as when very long upon their beds, and crossing a line of fissure near the axis of