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have next to refer to the abnormal wave, or that passing horizontally, or nearly so, and diagonally through a rectangular building.

In this case the fissures occur at or near the internal angles of the quoins, and are vertical or nearly so, and in so far are the same as those produced by a normal wave. In a rectangular or square building they are also (the main fissures), the same in number (four) generally, but differently disposed. If the abnormal angle (that which measures the horizontal obliquity of the path of the wave, with two of the parallel walls,) is very small, as in Fig. 37, it occasionally



happens that only four main fissures are seen, the alternate ones $$g'$$ and $$f'$$ being rather wider than the two others, when the direction of the wave is from $$a$$ to $$b$$, and the sum of $$f' + f$$, being greater than that of $$g' + g$$, on principles