At the place where it was determined to build the dam across the Croton River, the surface of the natural flow of water was about 38 feet below the elevation required as a head for the water to flow into the Aqueduct leading to the city. By going farther up the river the dam would have been of less height, and a point might have been found where it would be only necessary to build a dam to turn the water, and not form a pond of much extent above it, but for such purpose it would have been necessary to go above where some important tributaries enter the river, and would have required a considerable extension of the Aqueduct. It was

perhaps desirable to form this Fountain Reservoir, so that it would afford a supply of water to draw from, should there at any future time, in a season of drought, be more required for the use of the city than would be flowing in the river.

No essential change occurs in the form of the channelway from the Fountain Reservoir on the Croton, to the Receiving Reservoir on the island of New- York a distance ;

of thirty-eight miles, except in crossing Harlem River to reach the island, and in passing a deep valley on the island, where iron pipes are used instead of the channel-way of masonry to provide for the pressure consequent upon a depression from the regular plane. At these points the iron pipes descend and rise again, so that when the water is flowing in the channel-way they will