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 off- sets, 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 may leak 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. On each corner of the reservoir, pilasters 40 feet in width are raised, projecting four feet from the main wall, and in the centre, on the streets and 5th Avenue, there are pilasters 60 feet wide and projecting 6 feet from the wall. The pilaster in the centre, on the 5th Avenue, rises 7 feet above the main wall, and all the others 4 feet above. Doors are placed in the central pilasters on 40th and 42d streets, which give access to the pipe chambers, to work the influent and effluent stop-cocks, from which chambers, an entrance is made to the openings in the walls. In the central pilaster on the 5th Avenue, an entrance is made by a door to a stair- way that leads up to the top of the walls. On the outside walls, an Egyptian cornice is laid, which accords with the general style of the work. The pilasters are laid in courses, and well dressed ashlar face, and the main wall with coursed rubble work, rough ham- mer-dressed. Inside of the walls of masonry, a thorough puddled embankment of suita- ble earth is formed, fifty-eight and one-third feet wide at the line of reservoir bottom, and sloping on the inside face one and a half to one for 24 feet high, and one to one for the remaining 16 feet high, arid making with the walls on top a width of 17 feet ; the faces of the banks are lined with a course of rubble hydraulic masonry 15 inches thick, and coped with dressed stone. The bottom is a very impervious hard-pan, on which two feet of puddled earth is laid, and this covered by 12 inches of hydraulic concrete. The reser- voir is divided into two divisions by a wall of hydraulic masonry, at the toe of which a sloping bank of puddled earth is raised 18 feet high and covered with rubble masonry ; this wall is 19 feet thick at the bottom, six and two-third feet thick at top water line, and four feet at top. In this wall a waste weir is placed, with a well of two falls, together 52 feet, from which the waste water enters a sewer in 42d street, and passes off about one 54 £14 MEMOIR OF mile to the Hudson river. In each division there is a waste cock to draw the water from the bottom. The reservoir is designed for 36 feet of water, and, when full, will stand 115 feet above mean tide. The walls rise 4 feet above the water line. An iron railing is to be placed around the walls on top of the cornice. The capacity of this reservoir is 20,000,000 imperial gallons. GRADE LINE OF AQUEDUCT. The general declivity of the aqueduct in Westches- ter county is 0.021 foot per hundred, or a fraction over 13j inches per mile.* The top of the conduit pursuing this grade, corresponds with the top of the dam on the