CULVERTS.

Where streams intersect the line of Aqueduct, culverts are built to allow them to pass under it. They are simply a stone channel-way built under the Aqueduct of such form and dimensions as will allow the stream to pursue its natural direction without causing injury to the work. The foundation of these culverts is formed by laying down concrete, upon which an inverted arch of cut stone is laid forming the bottom of the water-way: side walls of stone are built and surmounted by an arch of stone. The span, or width of water way, of the culverts built, varies from 1½ foot to 25 feet. Those of 1½ foot span have a square form for the water-way, and are constructed by making a foundation of concrete, upon which a flooring of well dressed stone is laid forming the bottom of the water-way, and from this, side walls are built and covered by a course of thick stone flagging well dressed and closely fitted. At each end of the culvert a deep wall is built underneath so as to prevent the water from doing injury by undermining it. Buttresses and wing walls are built at each end of the culvert to guide the water to and from the channel-way, and a parapet wall is built over the top of the channel-way at each end to sustain the embankment of earth over the culvert. These wing walls and parapets have various forms; sometimes the parapet is built across the top of the culvert, and the wing walls built at right angles to it, and sloping down to the buttresses, and sometimes the wing walls and parapet form one continuous wall of a semi-circular form, the top sloping up from the buttresses in a plane parallel with the slope of the embankment covering the Aqueduct above. These culverts are permanently constructed, and in preparing the plans for them much skill has been displayed in adapting the form and size which the circumstances required, and much taste displayed in the design for their construction.

[Plate VII]. is an isometrical drawing of one of the culverts with rectangular wings and parapets; the body of the culvert is cut in two in the drawing, showing that it may be of any length, according to the width of the embankment through which it is constructed. The length is generally arranged so that the slope of the embankment may intersect the rear of the top of the parapet and pursue a direction down, parallel with the slope of the top of the wing walls.

VII

Scale of 4 feet to one inch

F. B. Tower.

Gimber.

Gate Chamber at the Head of the Aqueduct and Grade of the Water-way of the Aqueduct.

[Plate VIII]. is a longitudinal section through the tunnel and gate chamber at the head of the Aqueduct showing its connection with the Fountain Reservoir. This gate chamber is not in any way connected with the dam itself, but stands some distance from it, and the water reaches it by means of the tunnel which leaves the Reservoir above the dam and passes through the solid rock of the hill against which the masonry of the dam is built, a distance of over 200 feet. This tunnel descends into the Reservoir, so that the centre of it at the mouth is about 12 feet below the surface of the water; any floating substance cannot enter it, and during the winter season when the water is frozen over no obstruction can take place to the flow into the Aqueduct, and during the summer season the water will be drawn from a level where it is cooler than at the surface.

The gate chamber has two ranges, or sets of gates; one called regulating gates, and the other guard gates: the regulating gates are made of gun metal, and work in frames of the same material which are fitted to stone jambs and lintels: the guard gates are made of cast iron, and work in cast iron frames also attached to stone jambs and lintels. The gates are all managed by means of wrought iron rods attached to them, having a screw formed on the upper part on which a brass nut works, being set in a cast iron socket-cap.

The bottom of the water way, of the Aqueduct, where it leaves the gate chamber is 11.40 feet below the surface of the Fountain Reservoir, and 154.77 feet above the level of mean tide at the city of New-York. The following table shows the length of the Aqueduct as it is divided into different planes of descent, from the gate chamber at the Croton dam to the gate chamber at the Receiving Reservoir on the Island of New-York. Commencing at the south side of the gate chamber at the Croton dam,

The 1st plane of Aqueduct extends	26099.72 ft. or 4.943 miles, and the descent	2.94 ft.
The 2d plane of Aqueduct extends	148121.25 ft. or 28.053 miles,	30.69 ft.
Length of pipes across Har. River,	1377.33 ft. or 0.261 miles.
Diff. of level betw’n extremes of pipes		2.29 ft.
The 3d plane of Aqueduct extends	10733.14 ft. or 2.033 miles,	2.25 ft.
Length of pipes across Manhat. valley,	4105.09 ft. or 0.777 miles.
Diff. of level betw’n extremes of pipes		3.86 ft.
The 4th plane of Aqueduct extends	10680.89 ft. or 2.023	1.60 ft.
	201117.42 ft. — 38.090 miles	43.63 ft.

Making the whole distance from the gate chamber at the Croton dam to the gate chamber at the Receiving Reservoir 201117.42 feet, or 38.09 miles, and the whole descent 43.63 feet.

The descent on the first plane is about 7⅛ inches per mile.

The descent on the second and third plane is about 13¼ inches per mile.

The descent on the fourth plane is about 9½ inches per mile.

In crossing Harlem River there is a fall of 2 feet more than there would have been had the Aqueduct continued across with its regular inclination: this extra fall will afford an opportunity to adjust the number and capacity of the pipes (which descend below the level of the Aqueduct and rise again) to discharge the full quantity of water as freely as the Aqueduct, or channel-way of masonry, would have done had it continued its regular inclination across the valley.

In crossing Manhattan Valley there is an extra fall of 3 feet for the same reasons as before stated for that at Harlem River. In both cases, by using the pipes, there is a loss of the head of water for the City Reservoirs, equal to the amount of this extra fall; but this small loss of head was not considered of such importance as to induce the building of structures across these valleys up to the plane of Aqueduct grade.

VIII

F. B. Tower.

Gimber.

The bottom of the water-way of the Aqueduct at the gate chamber where it enters the Receiving Reservoir, is 7.86 feet below the level of top water line in the Reservoir, thus when the Reservoir is full the water will rise to within 7¼ inches of the top of the interior of the Aqueduct at that place, and the height from top water to the top of the interior will increase, going northward according to the inclination of the plane of Aqueduct grade, until it reach the surface level of the flow of water in the Aqueduct.

The height of the interior of the Aqueduct is 8 feet 5½ inches, and the greatest width is 7 feet 5 inches. The sectional area of the interior is 53.34 square feet. On the first plane, the Aqueduct is larger; being 2.05 feet higher at the gate chamber, 2.31 feet higher at 2244. feet from the chamber, and then diminishing, to the head of the second plane, where it assumes the size above mentioned and continues of that size throughout the remainder except in tunnels, where it assumes the forms before described. Where the Aqueduct on the first plane is larger, the width across the interior at the spring line of the roofing arch is the same as the general width, but the increase takes place only in the height of the side walls, and the slope of the inner face of the walls being the same, the width across at the spring line of the inverted arch will be less according to the increased height of walls. The original design was to continue the inclination which the second plane has, up to the Fountain Reservoir; but it was considered desirable to draw from this Reservoir at a lower level, and the head of the Aqueduct was depressed for that purpose, and a less inclination adopted for the length of the first plane. The roofing arch was left on the same inclination as was originally designed, except for the distance of 2244. feet from the gate chamber, where it was built on a level.

The curves which are used to change the direction of the line of the Aqueduct are generally formed with a radius of 500 feet; some have a radius of 1000 feet, and in a few instances larger ones are adopted, but the majority of them are of 500 feet radius.

The velocity of the water in the Aqueduct has been ascertained to be about one mile and a half an hour when it is 2 feet deep; this was determined by floating billets of wood from the Croton Dam to Harlem River and noting the time of their passage. Such an experiment would express the surface velocity and would give a greater velocity than it would be proper to attribute to the whole body of water in the Aqueduct; but the depth of water in the Aqueduct will be probably 4 feet as soon as it is brought into general use, and then there will be a corresponding increase in the velocity of the body of water. This velocity of a mile and a half an hour may be taken in general terms as the velocity of the water in the Aqueduct.

F. B. Tower.

W. Bennett.

VIEW ABOVE THE CROTON DAM.