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 differ- ent 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 do. extends 148121.25ft.or28.053miles, . . . .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 acrossManhat.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 10630.89 ft. or 2.023 ...... 1.60 ft. 201117.42 ft.— 38.090 miles . . . .43.63 ft. 92 Making the whole distance from the gate chamber at the Croton dam to the gate chamber at the Receiving Reser- voir 201117.42 feet, or 38.09 miles, and the whole descent 43.63 feet. The descent on the first plane is about 71 inches per mile. The descent on the second and third plane is about 13} inches per mile. The descent on the fourth plane is about 9| 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 con- sidered of such importance as to induce the building of structures across these valleys up to the plane of Aqueduct grade. 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 7} inches of the top of the interior of the Aqueduct at that 93 place, and the height from top water to the top of the inte- rior will increase, going northward according to the inclina- tion of the plane of Aqueduct grade, until it reach the sur- face level of the flow of water in the Aqueduct. The height of the interior of the Aqueduct is 8 feet 5Jr inches, and the greatest width is 7 feet 5 inches. The sec- tional 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 cham- ber, 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