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152 compression. The third column is here calculated on the mean of such data. It cannot be viewed as more than an approximation, except in the cases of mortars, which are from actual experiment, as given by Gauthey ('Sur la Construction des Ponts'), and by Rondelet ('L'Art de Bâtir').

Authorities.—T., Tredgold. W., White, 'Trans. Inst. C. E.' B., Barlow. B.G., Boistard and Ganthey, 'Const. des Ponts.' T. T., Treussart and Totten. R., Rondelet.