For an opening of 18 feet in height the width is 12 feet, and the piers 6

PRELIMINARY ESSAY. 37

feet, sustaining a semi -circular ; when the inequality of the surface did not allow the piers to have an equal height of 18 feet to each opening, the piers were shortened, and the other parts remained of the same general dimensions. The piers of the arches in some places, are rather less on the face than 6 feet, varying from 5 feet 8 inches to 6 feet ;

and in other cases they are 7 feet 9 inches. The arcade which conducts the water into the reservoir called St. Irenasus, is 31 feet high, its width is the half of this, and the faces of the piers are 7 feet 9 inches. As the upper part, containing the canal, is only 6 feet thick, there is an offset of 6 inches on each face at the impost of the arches. On this offset there is a projection, or pilaster, 10 inches thick, and 3 feet wide, which acts as a

counter-fort, to strengthen the sides of the water channel. The foundations of the piers having the smallest elevation, are sunk between three and four feet below the surface of the ground, and between six and eight feet for those of the greatest height. All the different supports of the aqueduct are of the same kind of masonry, formed of small, rough, squared stones, laid in a thick bed of mortar, with the apparent faces of reticulated work. This kind of masonry was bound, at every four feet of its height, by two courses of "great bricks," each brick being 22 inches square, and two inches thick. The angles of the piers, formed of small square slabs of stone, offered, in many instances, an insufficient resistance to the lozenge masses which they terminated, and their displacement has been apparently the main cause of the ruin of the greater number of the piers, for these have been formed by a sort of encasements, of the thicknesses of four feet of the