The body of the work, that is, the arcades that carried the aqueduct across the valleys, is built of masses of rubble stone and cement, faced, as has been stated, with the

opus reticulatum. In this instance, this kind of work is supposed to have been formed by laying a bottom of brick, of two, three, or four layers, then a caisson of wooden sides was applied to it,- and fixed thereon. The caisson was first lined with the squared stones which were to form the face the middle was then filled with rubble stones, into which ;

a liquid cement of lime, fine gravel, and water, beaten up to a perfect degree of incorporation in its liquidness, so as to become a binding cement, was poured, and in that state entered into every interstice of the rubble work. This operation being repeated, the whole was wrought into one incrusted rock, harder than either of the materials themselves separately were. One can conceive, says a learned antiquary, how a careful beating together of these materials had the effect of creating so binding a cement, since we know from our own practice, that puddling earth, fine gravel, and water together, form a lining for a canal, that becomes impervious to water when once settled, and it was probably from this puddling, and not from any secret as to the materials of the mortar, not now known, that this ancient cement owed its cohesive strength. When this square was set, the sides of the caisson were taken off, and another layer of bricks was then laid, and so Another caisson, and so on. The bricks used in this construction were one foot nine inches