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