The dam backs the water 5 miles, and makes a lake of an area of 400 acres, and a capacity equal to 500 millions of gallons.
The water enters a gate-house, where the quantity is regulated, before it enters the aqueduct, which is a stone structure, lined and arched with brick.
The face of the interior of the aqueduct is at the bottom an inverted arch, width 6 ft. 9 in., height 8 feet 5½ inches, area 53⅓ square feet, about large enough for an omnibus and four to pass through. The line of the aqueduct being on a regular declivity of 13¼ inches to the mile down to the Harlem River, a distance of 33 miles, it has a line of tunnels of 6841 feet, being sixteen in number, sometimes through earth and sometimes through solid rock; the deepest cut is 80 feet, and the least 25 feet. In Westchester only, the aqueduct crosses 25 streams of water, which are from 25 to 13 feet below the top of the aqueduct.
The grade line of aqueduct across the Harlem is 25 feet above tide water, and the top of the water now passes over Harlem river in one pipe of 36 inches, placed on the earthen dam made in the construction of the high bridge.
The bridge itself is now about one-third completed, and will be when finished one of the most stupendous works of the kind in the world. Its cost is estimated at one million of dollars, and its elevation is so great as not to impede the navigation of the stream. Some idea of this vast undertaking may be formed from the fact, that the excavation for one pier has been carried 34 feet below the surface of the water, and then a rock foundation not having been reached, 240 poles, from 30 to 40 feet long, were driven in for the purpose. Several piers having been already carried, by the aid of coffer-dams, from four to fifteen feet above high-water mark.
The river is 620 feet wide at water line, but the slope of the river banks adds an additional distance of 830 feet, making in all 1,480 feet.
The plan now in progress crosses the river with eight arches of 80 feet span, and on piers of 31 by 44 feet at the base, resting on the bed of the river, and 7 arches on piers on the land from the edge of the water up the two banks of the river.
The spring of one of the arches is 95 feet above the lowest foundation put down; the top of the parapet will be 149 feet from the lowest foundation. It is intended that the water shall pass over this bridge in pipes, to have it secure against the possibility of danger.
The interesting works at Clendinning Valley, being a bridge over a valley of 1,900 feet in breadth, the greatest height of the aqueduct is 50 feet from the bottom of the valley; beautiful archways are constructed for three streets, 34 feet for the carriage-way, and 10 on each side for side-walks.
Next in interest is the reservoir at Eighty-sixth Street, which might well be called the detaining or clarifying reservoir. It has two divisions, together thirty-two acres—greatest depth of water twenty-five feet, containing one hundred and fifty millions of gallons. Two lines of thirty-six inch pipes connect this with the reservoir at Fortieth-street, which has also two divisions, forming together an area of four acres—depth of water when filled thirty-six feet. From this point four and a-half miles to the Battery. Whole length of line from the Battery to the artificial lake, fifty miles. There are in this great work 55,000,000 of bricks and 700,000 cubic yards of stone-masonry.