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 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 in- clination 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 24 94 500 feet ; some have a radius of 1000 feet, and in a few in- stances larger ones are adopted, but the majority of them are of 500 feet radius. The velocity of the water in the Aqueduct has been as- certained 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 velo- city 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, £ i a; 321 95 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 re- servoir 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." Pine's Bridge over the Croton River, which is mentioned in the early history of the country, occupied a position which is now about the middle of this Reservoir, and there is at that place a bridge over the Reservoir resting upon piers and abutments. The hills which bound the Croton Valley where the Re- servoir is formed are so bold as to confine it within narrow limits : for about two miles above the dam the average width is about one eighth of a mile ; at this distance from the dam the valley opens so that for the length of two miles more the width is about a quarter of a mile ; here the valley con- tracts again and diminishes the width until the flow line reaches the natural width of the River at the head of the lake. The country immediately contiguous to the shore has been cleared up, and all that would be liable to impart any impurity to the water has been removed. This gives a pleasing aspect to the lake, showing where the hand of art has swept along the shores leaving a clean margin. Retir- ing from the water are the richly cultivated slopes with the 96 neat farm houses overlooking the lake, or the hills crowned with forest trees, while at intervals a valley or ravine opens and empties in its tribituary stream. Plate IX. is a view taken above the dam showing the position of the entrance to the tunnel which leads from the Reservoir to the gate chamber at the head of the Aqueduct. The entablature which is seen on the left against the rock, is built directly over the mouth of the tunnel, and from this the tunnel extends through the rock to the