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

be constantly full. Thus it will be perceived that the channel-way of masonry will never be filled entirely, so as to occasion a pressure on all its interior surface. The surface of the Fountain Reservoir is 166| feet above the level of mean tide at the city of New-York and the ;

difference of level between that and the surface of the Receiving Reservoir on the island of New-York, (a distance of thirty-eight miles) is 47| feet, leaving the surface of this reservoir 119 feet above the level of mean tide. From the Receiving Reservoir the water is conducted (a distance of two miles) in iron pipes to the Distributing Reservoir, where the surface of the water is 115 feet above the level of mean

tide. This last is the height to which the water may generally be made available in the city.

GENERAL CONSTRUCTION OF THE AQUEDUCT.

Plate I. is a section of the Aqueduct showing the form of the masonry used in earth excavations. The foundation is formed with concrete ; the side walls of stone ; the bottom