enable them to effect their object, more money would be required — especially 38 150 MEMOIR OF THE if, as seemed to be the wish of many citizens, the plan of Major Douglas, of passing the Harlem on a high bridge, maintaining the level of the aqueduct, be adopted. We quote the reasoning of the report on this subject, and as to the conveyance of the water over York Island, together with a new estimate of the cost of the whole work : With a view of deciding the question, as to the best manner of bringing the Croton water across the Harlem River, the chief engineer was instructed to furnish the Commis- sioners with an estimate of the cost of crossing said river by an aqueduct bridge, on an inclined plane ; and also by an inverted syphon, with iron pipes, on a low bridge. The following is a synopsis of so much of the report alluded to, as relates to the plan of con- struction, and some other matters connected with the subject. It appears the width of the river, on the high water level, was found to be 620 feet ; and the distance across the valley of Harlem River, from the grade of aqueduct in the county of Westchester, to the grade of the same in the county of New York, is 1450 feet. The depth of the river, to the rock bottom, was found to be 32 feet below high water line, near the south shore ; and only 20 feet on the north shore. The aqueduct bridge Avill have an elevation of 163 feet above the rock at the bottom of the river, or an average of about 138 above tide. The span of the arches over the river must be 80 feet, and will regularly diminish to 50 feet span, for those to be built on the land. All the piers are to be constructed hollow, except those of 50 feet span, which are to be built up solid. The piers to be built of large stone, of uniform thickness in each course, and the joints not to exceed two and a half eighths of an inch. The work gene- rally to be performed in the most approved manner practicable. In making an estimate of the cost of this structure, the chief engineer observes, that he had been governed by the value of work of much similitude, estimated by several of the most competent men in this department of masonry; that there is no work under con- tract precisely similar, or of the same magnitude, or which, from its elevation and incon- venience of access, will be so expensive in laying up, or requires so great a portion of large stone, or the same exactness of execution ; at the same time, there is sufficient resemblance to constitute a guide ; which, with careful application, will not lead astray materially in computing the expense. The estimate is given in the report in detail, and amount in the aggregate, as follows : Estimate for a high bridge, maintaining a uniform inclination of aqueduct $935,745 The plan of carrying the water across Harlem River by an inverted syphon, is next considered. It is proposed to erect a semi-circular arch, of 80 feet span, resting on abut- ment piers. The total height of the arch from the level of flood tide, to the under side of it, will be 50 feet. This arch is placed on the New York side of the river, and will form a sufficient channel way for navigation. From the north abutment of the arch, to the Westchester side of the river, will be con- structed an embankment of stone, by casting them into the river until a sufficient bed shall be formed to support the foundation wall of the aqueduct. From the south abutment pier of the main arch, on the New York side of the river, an arcade, of three arches, will be built, one of 35, one of 30, and one of 25 feet span ; and, connected with this, a foun- dation wall will be carried up to the ascent, until it reaches the grade level, where the 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