The openings are made by an exterior and interior wall, connected at every ten feet by cross walls, which are carried up to within 17 feet of the top, and there connected by a brick arch thrown from one to the other, and the spandrels between them levelled up solid, and a course of concrete put over the whole, 6 inches thick, which reaches a level 10 feet below the top, whence the exterior wail is carried up single to the top. The exterior wall has a bevel of one to six, and is uniformly four feet thick from the bottom to the top of the connecting arches the inner wall is carried up plumb, with offsets, the lower section six feet thick, the middle section five feet, and the upper section four feet thick. The space between the exterior and interior walls, at 41 feet below the top, is 14 feet, or 24 feet from the outside of exterior to inside of interior walls and the ;

space between them at the spring of connecting arches, in consequence of the bevel of the exterior wall, is reduced to 9 feet and 9 inches, and from outside of exterior to inside of interior walls 17,75 feet

The cross walls are four feet thick at bottom, and have one offset of six inches on each side at 8 feet below the spring line of connecting arches ; they have an opening 6 feet high, and 1^ feet wide, at a suitable level near the bottom, to allow a drain to be formed, to collect any water that mayleak through the work, and carry it off in sewers provided for that purpose, and also to allow persons to go in and examine the work. Some modifications in the cross walls are made to accommodate the gate chambers, and connect the corners of the work.