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_. [Illustration: IX _F. B. Tower._ _W. Bennett._ VIEW ABOVE THE CROTON DAM.] DESCRIPTION OF THE LINE OF AQUEDUCT. The dam, built to form the Fountain Reservoir, is about six miles above the mouth of the Croton River. The reservoir forms a beautiful sheet of water in the lap of the hills in the wild region of the Croton, and has received the name of the “Croton Lake.”