Croton River where a dam could be constructed that would turn the water into a channel having a gradual descent to the required elevation at the city. So that it may easily be conceived, it is only diverting the water into another channel where it will flow on unob- structed. The manner in which water is conducted from its natural channel, for the purpose of propelling the machinery of manufacturing establishments, by a race-way or other channel, is a simple illustration of the operation of this great work. 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 channel- way from the Fountain Reservoir on the Croton, to the Re- ceiving Reservoir on the island of New- York ; a distance 80 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 de- pression 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 chan- nel-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 Re- ceiving Reservoir on the island of New-York, (a distance of thirty-eight miles) is 47| feet, leaving the surface of this re- servoir 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 gene- rally be made available in the city. 81 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 and sides of the interior being faced with brick, and the top covered with an arch of brick. In forming the concrete a mortar is made by mixing three parts of sand with one of hydraulic lime, and then mixing about three parts of stone, broken to a size allowing them to pass through a ring an inch and a half in diameter. Having thoroughly mingled the broken stone and mortar, the con- crete is placed in its proper position and form, and brought into a compact state by using a pounder ; and is then suf- fered to remain until it set, or become indurated, before any work is commenced upon it. The object should be to mix as many stones or pebbles as will thoroughly bed in the mortar, allowing none of them to come in contact, but all to be enveloped in mortar. This forms a body which becomes indurated and makes a foundation under the whole length of the Aqueduct like one continuous stone. It attains a degree of hardness which gives it the appearance of the conglome- rate bearing the name of Pudding-stone, and is an article of the greatest importance in forming foundations for walls of 21 82 great weight ; superseding in many instances, where the soil is soft, the use of piles or other timber foundation. Though we have evidence that concrete was used by the Ancient Romans in the foundations of some of their struc- tures and even in the formation of their roads — such as the