continued two miles to the Manhattan valley, which is passed with iron pipes, descending 102 feet to the bottom of the valley, and continued rising again to its opposite side, the distance across the valley being 0-792 mile. The masonry conduit is again resumed, and crossing the Asylum ridge and the Clendinning valley, is continued 2.173 miles to the receiving reservoir at Yorkville. This basin is 1826 feet long and 836 feet wide, and including its embankments, contains an area of thirty-five acres divided into two parts; thence iron pipes are laid beneath the surface of streets 2-176 miles, to the distributing reservoir at Murray hill, three miles from the City hall.

This reservoir is 420 feet square, and covers four acres. It is divided into two equal parts, and has an average elevation of 44.05 feet above the level of the adjacent streets. The length of the aqueduct, including the iron pipes and reservoir, from the Croton dam to the receiving reservoir, is 45.562 miles; and including the elevated surface of the Croton river, and the large mains conducting the water from the distributing reservoir through the central parts of the city, the entire length is 50 miles, of which the masonry conduit constitutes 37.067 miles. The rocks through which the line of the aqueduct passes are two marble quarries in Westchester, and for the residue of the route gneiss of many varieties. A large portion of the open cutting, and nearly all the tunnel cutting, have been made through rocks, more than 400,000 feet of which have been excavated. The formation of the ground is very irregular. There are on the line sixteen tunnels, varying in length from 160 feet to 1263 feet, and being in aggregate length 6,841 feet. The height of the ridges above the great level at the tunnels, ranges from 25 to 75 feet. In Westchester county, the line of the aqueduct is crossed by twenty-five streams, at depths varying from 12 to 70 feet below the grade line. Besides these there are numerous other brooks and valleys of less depth, over which culverts are constructed. The most important valleys on the Manhattan island, over which the aqueduct passes, are the Manhattan valley, Clendinning valley, and Bowne's valley.

The bottom of the aqueduct is an inverted arch; the chord or span line is 6 feet 9 inches, and the versed sine 9 inches; the masonry of the side walls rises four feet above the springing line of the inverted arch, with a bevel of one inch to a foot rise, or

four inches on each side, which makes the width at the top of the side walls 7 feet 5 inches. These walls form the abutments of the roofing arch, which is a semicircle, having a radius of 3 feet 8 inches, or a chord line of 7 feet 5 inches. The greatest interior width of the aqueduct is 7 feet 5 inches, and the greatest height 8 feet 5 inches. The area of the interior is 53·34 square feet.

The plan, dimensions and kind of masonry, are as follows: In excavation, a bed of concrete masonry is laid down as a foundation; it is laid level across the bottom, 3 inches thick at the centre of the inverted arch, and curved on its upper surface to form a bed for the arch, which brings it 12 inches thick at the spring line, and is carried 3 inches thick under the side walls, or abutments. The abutments are 2 feet 8 inches thick at the spring line of the inverted arch, and 2 feet at the top or spring line of the roofing arch. The inverted arch is of brick 4 inches thick; the roofing arch is also of brick 8 inches thick. The abutments or side walls are of rubble stone, with a brick facing of 4 inches thick. Spandrels, of stone, are carried up solid from the exterior angle of side walls on a line that is tangent to the arch. When the bed of concrete is formed for the inverted arch, a heavy course of plastering is laid over it, on which the arch is laid. When the stone work of the side walls was up, the face that received the brick lining had its irregularities filled with successive courses of plastering, and finally a uniform course of a quarter of an inch in thickness over the whole, in front of which the brick facing was laid up. A course of plastering was also put over the roofing arch. The concrete masonry was formed by mixing one part hydraulic cement, three parts clean sand, and three parts fine broken stone. The masonry was all laid up in hydraulic cement. The mortar for the stone work was composed of one measure of cement to one of clean sharp sand; and that for the brick and plastering consisted of one part of cement to two of sand. The area of a cross section of the masonry is,

In embankment the concrete masonry is laid on foundation walls, has one foot extra thickness and three feet in extra width. The base of the side walls is also increased, and the proportion of cement to sand in concrete and mortar for stone work, is 1 to 21 feet.

The proportion of embankment to excavation on the line of the aqueduct, is about as one to eight. The aqueduct is covered with earth of sufficient depth to protect it from frost. To pass streams, there are one hundred and fourteen culverts, the aggregate length of which is 7,959 feet, and varying in span from 1 feet to 25 feet. There are five road culverts from 1 to 20 feet span. All the culverts are constructed in the most improved manner, laid in hydraulic cement.

There are thirty-three ventilators, to give free circulation of air through the aqueduct. They rise fourteen feet above the surface of the ground, tapering toward the top, and are of circular form, constructed of well-dressed stone, and have an aperture of fifteen inches in diameter: they are placed at a distance from each other of one mile.

There are six waste-weirs, constructed of well-dressed stone, having cast-iron gates and gate frames fitted to stone jambs and lintels. The frames are faced with brass for the gates to work against. The gates are operated by a wrought-iron screw rod, with a brass nut working in a cast-iron socket. The water falls from the gates into a well, and is carried off through a culvert. The waste-weirs are protected by stone buildings with brick arch roofs.

The dam in the Croton river, as first constructed, was provided with a waste-weir 90 feet wide, which, in the high flood of January, 1841, proved insufficient to pass the water, and a breach was made in the embankment about 200 feet long. This breach was then filled by a structure of hydraulic stone masonry, adopting 180 feet thereof as an additional waste-weir. The greatest height of the weir of the dam is 40 feet above the low-water mark, and 55 feet above the bed of the river. The width of masonry at lowwater line of the river is 61 feet. The form on the lower face commences on a curve described by a radius of 55 feet, and continues to within about 10 feet, of the top, when a reversed curve, on a radius of 10 feet carries the face over and meets the back line of the wall. The back line is carried up vertically, with

occasional offsets. The main body of the work is laid up of rough stone; the curve face, of large and closely-cut stone with four heavy courses at the bottom dovetailed together; the joints cut to the line of radius of curve. Above the masonry, an embankment of masonry is filled in, in width 275 feet on the bottom, with a slope of 1 to 5 on the up-stream face. The north end of the new weir is terminated by an abutment which rises 12 feet above it.

From the toe of the masonry an apron is extended 35 feet, composed of hewn timber, well-secured, and filled for 16 feet from the stone work, with concrete masonry; and the remainder with loose stone, and the whole covered with a course of six-inch white-elm plank. A second apron is made, extending 30 feet further. At 300 feet below the main dam is a second dam nine feet high, which sets the water over the apron of the main dam, and thus forms a pool to check the water as it falls over the weir. About 120 feet of the foundation of the dam is of concrete masonry, laid down on a very firm hard pan, and the remainder upon timber piers, the spaces between which are filled with concrete masonry. The dam sets the water of the river back about five miles, and forms a reservoir covering about 400 acres.

The gateway which guards the entrance to the aqueduct, is placed on the solid rock, in a situation not exposed to the floods. The gate chamber is provided with a double set of gates; one set of guard gates set in cast-iron frames; the other, a set of regulating gates made of gun metal, set in frames of the same material. The gates are all 18 by 40 inches, and there are nine in each set, and they are operated by means of wrought-iron screw-rods. The gate-chamber and bulkheads are constructed of well-dressed masonry laid in hydraulic cement. The water is conducted from the reservoir into the gate-house by a tunnel cut 180 feet through the rock, and flows into the bulkhead at the upper end of the tunnel from a level averaging 10 feet below the surface of the reservoir. The builders of this dam were M'Cullough, Black, M'Manus and Hepburn.

The Sing-Sing kill, the bottom of which is 66 feet below the grade line of the aqueduct, is crossed by a bridge resting on a single arch, of 88 feet span and 33 feet rise. The form of the arch is an oval drawn from five centres. The bridge is con

structed of well-dressed masonry laid in hydraulic cement. The builder was Andrew Young, of Philadelphia.

The width of the Harlem river, where the aqueduct crosses it, is 620 feet at ordinary high-water mark. The shore on the southern side is a rock rising from the water's edge, at an angle of about thirty degrees, to a height of 220 feet. On the northern side a strip of table-land forms the shore, and extends back from the river four hundred feet to the foot of the rocky hill, which rises at an angle of about twenty degrees, to the level of the aqueduct. The table-land is elevated about 30 feet above the river. The channel of the river to which the water is reduced at very low tides, is 300 feet wide, and the greatest depth is 16 feet.

The bridge which is now in progress of construction, crosses this valley on eight arches, each of 80 feet span, resting on piers that are (at each extremity and in the centre) twenty feet wide at the spring line of arches, with intermediate piers that are 14 feet wide at the spring line. On the south of this range of large arches, there is one, and on the north, there are six arches, each of 50 feet span, resting on piers seven feet wide at the spring line, and two abutments that terminate the arch work of the bridge. From the abutments a continuous line of wall of dry stone work is extended to the gate chambers on each side. The length of the bridge is 1450 feet. The height of the river pier above highwater line, is 60 feet to the spring of the arches, and 95 feet above the lowest foundation. The arches are semicircular, and the height 100 feet to the soffit or under side at crown; to the top of the parapets 114 feet above ordinary high water, and 149 above the lowest foundation. The width on the top of the parapets is 21 feet. The space between the parapets is arranged to receive and protect from frost two cast-iron pipes, each four feet in diameter, and lying 12 feet below the grade line of the aqueduct, and connected at each end of the bridge with the masonry aqueduct by gate-chambers. To make the capacity of the pipes for conveying water equal to that of the aqueduct, an extra fall of two feet has been given across the bridge, and the aqueduct on the southern side is depressed two feet below the grade to accommodate this arrangement. The utmost care and skill have been bestowed in securing durable foundations for the piers. The material of the bridge is well-dressed granite. While the bridge remains unfinished, the water is conveyed in iron pipes in the