Alasonry dams are designed as thongh they were monolithic structures, and for this reason, as well as because of the fact that the pressure against. the face of the dam tends to rupture it both vertically and horizontally. the blocks of stone are not laid in regular courses. Portland cement mortar is used to bind the stones in one homogeneous mass, or the dam may he composed of irregularly shaped masses of stone with the intervening spaces filled with concrete, or it may be made of concrete alone. (See CEMENT.) Overturning is guarded against by giving the structure such a cross-section that the lines of pressure will be thrown within the centre third of the dam. To provide against crushing, a material with high resistance to such action is selected and the structure is so proportioned that its own weight will not crush the material in its lower part. As a general rule the pressure should never exceed fifteen tons per square foot, and with some materials it may need to be as low as six tons. Obviously the only way to pre vent excessive or crushing pressures at the bot tom of very high dams is to diminish the thick ness as the height increases. The action of ice and of the actual or possible current of over flowing water renders it necessary to make the top of the dam thicker than would be required to resist water pressure alone: otherwise the dam might lie tapered to a knife edge at the top. The common type of cross-section for high masonry dams approaches a right-angled tri angle, with the perpendicular side up-stream, but it varies from a real triangle in having both sides curved somewhat, particularly so as to give a broader base, and in having the extreme upper part built with nearly parallel sides, while the top is Hat, or perhaps more or less rounded.
While some of the early dams were quite bold in cross-section, most of thefii were far otherwise. The French engineers were the first to apply the results of theoretical study of the subject to the design of the cross-section of dams. N. Sazilly discussed the questions involved in a memoir published in Annalcs dcs Penis et Chausscs (Paris, 1853). M. Deloere carried the work on to more rational conclusions, which were made public in 1858, and on which the design of the Furens Dam, near Saint Etienne, France, was constructed (1862-66). A memoir on these stud ies was published in the journal just named in 1866. Although this dam was only 6 feet higher (170.6 against 1(14.24 feet) than the Fuentes Dam, completed in Spain in 1791, the French dam was 9.91 feet thick at the top and 161.02 at the base. while the Spanish dam was 35.73 feet thick at the top and 144.29 feet at the base. The French dam was curved in plan and the Spanish was polygonal. An Englishman, Prof. W. J. Rankine (q.v.). made the next notable study of the subject. (See his Miscellaneous Scientific Papers, London, 18,90.) A number of eminent American engineers connected with the new Croton Aqueduct and its adjuncts, for New York City, made studies which resulted in the design of the highest masonry dam yet attempted, which was put under construction in 1S92. Very extensive investigations were made in this con nection by Mr. Edward Wegmann, who published an important treatise, entitled The Design and Construction of Masonry Dams (New York, 1888), to which new matter was added and a niw title given in 1899, so as to cover all classes of dams.