foundation and side walls are to be laid, to receive the pipes entering the effluent pipe CROTON AdUEDUCT. chamber. The foundation walls, extending from the arcade of arches, and from the abut- ment of the channel arch, are to be formed of dry masonry, except two feet, constituting the face, and two feet across the top, to form the bed for the iron pipes, all of which to be laid in cement mortar. A parapet wall will be laid on each side of the bed of the pipes, to support the earth covering, which is to be four feet deep above the pipes, to protect the water from the effects of frost. There will be an influent pipe chamber at the termination of the aqueduct on the north, or Westchester side of the river, in which the pipes are to be inserted. Commenc- ing on this side of the river, at the influent pipe chamber, the pipes descend nearly with the slope of the hill, forming an angle near its base, and thence to the lowest level ; which determines the top of the foundation wall for the pipes, at four feet above flood tide. This level is continued to the angle, before rising to the channel arch, from whence an inclined plane carries the pipes to the effluent pipe chamber, on the New York side of the river. It is proposed to arrange the structure of the chambers, and foundation for the pipes, to accommodate four 36 inch cast iron conduits, whenever so many shall be required. Four three feet pipes, according to the calculation of the engineers, will deliver 49,843,984 gallons of water every twenty-four hours ; which is about the quantity calculated to be delivered by the aqueduct, and nearly the average quantity running in the Croton. River. It is proposed, therefore, to insert only two of these pipes at present ; they being deemed more than sufficient to supply the city with water for many years to come. This estimate is reported in detail likewise, and amounts in the aggregate as follows : GENERAL ESTIMATE for plan by inverted syphon and iron pipes - - $426,027 The following is a comparison between the two plans as to the estimated cost of both of them. 1st. THE HIGH BRIDGE, maintaining its established inclination over the river, $935,745 2nd. IRON PIPES, supported by a low bridge, - 426,027 Excess of expense for the high bridge, - - $509,718 In addition to the report of the chief engineer, the Commissioners directed a con- tracted plan of the two methods of crossing the Harlem River, to be prepared by Charles B. Pearson, Esq., an architectural draftsman, now in the employ of the Commissioners, which are herewith submitted for the inspection of your honorable body. The chief engineer expresses a decided opinion in favor of the plan by inverted syphons or pipes ; and the Board of Commissioners, after due deliberation, have adopted said plan, as, in their opinion, far preferable to that for crossing by a high bridge and inclined plane. The reasons which have governed the Commissioners in this decision, are as follows : 1st. The difference in the cost of carrying an aqueduct over the river, on a bridge of 163 feet in height, on seven immense stone piers, sunk in the water and mud, on an average of 25 feet below tide, with 80 feet span of arches, and that of conducting the water over a low bridge, requiring only one pier in the river, with an abutment, is suffi- client of itself, in the opinion of the Commissioners, to decide the question. 152 MEMOIROFTHE 2d. All the purposes and objects to be obtained by the project, will as well be at- tained by the plan of a low bridge as by that of the high bridge, and at an expense of more than half a million of dollars less. 3d. The experience in sinking piers in so great a depth of water and mud, in order to reach the rock bottom, being very limited, and their great number and immense height from the rock to the spring of the arches, although their construction is practicable, it must be attended with many unforeseen difficulties and casualties ; and should the least variation from plumb occur, or the least settlement on the foundation take place, it would carry with it very injurious results to the work ; while with the low bridge and iron pipes, the same occurrence would be attended with but trifling injury, and could very soon be repaired. 4th. The water in the large aqueduct of masonry would, with much more uncer- tainty, be protected from frost on the high bridge, from the greatly elevated exposure and difficulty in surrounding it with a sufficient quantity