course of well jointed stone, not less than nine inches thick is laid, forming a stone platform on which well dressed stone is laid, one or two courses high, for the abntments or side walls, and finished by a second course of well dressed flagging, not less than nine inches thick ; all the work laid in hydraulic mortar. The culverts over one and a half foot span, with the exception of three of twenty-five feet span, are con- structed as follows : A foundation of concrete masonry is formed, and in a few instances some timber and plank is used with it ; on this an inverted arch of well dressed stone is laid in regular courses, to form the bottom channel of the culvert ; side walls or abutments are thence raised to such height as required, the face being well dressed stone, laid up in courses, and backed with rough dressed work. An arch is then turned from the abutments, with well dressed stone in regular courses. Wing walls at the ends are constructed in differ- ent forms as circumstances required. The character of masonry in the large 25 feet culverts is similar to that above de- scribed, except they have no inverted arch. There are five road culverts of from 14 to 20 feet span, constructed of masonry of the same character. All the culverts are of stone laid in hydraulic cement. Ventilators. There are 33 ventilators constructed to give free circulation of air 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 bev- eling or tapering towards the top, where the opening is 15 inches in diameter. They are 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 is located, is a gneiss rock ; moderately sloping up the hill from the water's edge, but not appearing to extend far into the river. The dam required to be raised 40 feet above low CROTON AQUEDUCT. 207 water in the river ; and it was an important object to arrange the plan so as to obtain a rock foundation for the masonry. To effect this, the northern abutment was located as far in the river as the rock was supposed to extend, and from this to build up the dam to the level where the top line in- tersected the natural slope of the hill, and then to cut the hill away; making an opening sufficient for the waste weir of the dam. In prosecuting the work, it was found the rock descended more rapidly into the river than was supposed, which induced the moving of the abutment further towards the hill than at first located ; and finally, an artificial foundation had to be made for a small por- tion of it. It was intended to make the waste of the dam 100 feet, with abutments of eight feet high; but in consequence of the disappointment in regard to the extent of the rock in the river, it was found difficult to obtain the desired length of the water way, and it was concluded to raise the abutment to 12 feet at the lower end, and 15 feet at the upper end, and allow the water way to remain an average length of 90 feet for this height. The natural rock formed the southern abutment, and the aqueduct being on this side, the water was conducted to the gateway at its head, by a tunnel, cut 180 feet through the rock ; this allowed