All the culverts are of stone laid in hydraulic cement. There are 33 ventilators constructed to give free circulation of air Ventilators. through the aqueduct. Eleven of them are constructed with doors that admit an easy entrance into the aqueduct. They rise 14 feet above the surface of the ground over the aqueduct, and are constructed with well dressed stone, circular in their form, slightly beveling or tapering towards the top, where the opening is 15 inches They are in diameter.

placed at a uniform distance of one mile, except where they are rendered unnecessary by a waste weir, which serves all the purposes of a ventilator. At this distance apart, they are found to afford a free and sufficient ventilation. Waste Weirs. There are six waste weirs constructed on the line of aqueduct, so arranged as to allow the water to pass off when it rises to the proper height with gates ;

to draw the water from the aqueduct when necessary. They are constructed with well dressed stone the gates and gate frames are of cast iron, fitted to stone jambs and lintels ; ;

the frames are faced with brass for the gates to work against, and the gates are operated by a wrought iron screw rod and brass nut, working in a cast iron socket. The water from the weir or gates, falls into a well,, and is then carried off through a culvert to the outside channel. A stone building with a brick arched roof is erected, so as to inclose the waste weir and its appurtenances. Croton Dam. The southern shore of the Croton river at the point where the dam but not is located, is a gneiss rock moderately sloping up the hill from the water's edge, ;