1839. To the Honorable the Water Commissioners of the City of New York : GENTLEMEN : — The resolution of your Board, requiring the undersigned to prepare plans and estimates for crossing Harlem River, by a bridge, the arches of which shall be 164 MEMOIR OF THE elevated 100 feet above common high water mark, and also for a tunnel under the channel of the river, has been under consideration. The law passed at the recent session of the Legislature provides, in relation to a tun- nel under Harlem river, that the top of the tunnel shall not be above the natural bed of the river, and that it shall extend across the channel of the same. This will require the abutments to be placed about 300 feet apart, and the top of the masonry of the tunnel about 18 feet below the high water level of the river. In relation to the bridge, the law prescribes that the arches in the channel shall be 100 feet at the under side of the crown, above common high water mark of the river, and not less than 80 feet span — conforming in these respects, we are at liberty to make the plans, in all others, without restriction from the law. In the report of the undersigned, made to your Board on the 12th December, 1837, a plan for abridge of sufficient height and stability to support the regular inclination of the aqueduct grade was presented. This plan provided for an aqueduct of masonry, to be rendered more impervious by a cast iron lining; but as it was supposed the Commis- sioners had power, and no objection having previously been made, on the ground of inter- ference with the the navigation, a structure, conforming nearly to the shape of the valley, and rising to a moderate elevation above the surface of the river, on which iron pipes were to be laid and secured, was recommended, as best adapted for economy, permanence, and more ready execution, to carry the aqueduct across this heavy depression from the grade line. Although adopted by your Board, and put in progress of execution, the law before mentioned compels its abandonment. We are therefore with only the alternatives of the Act of the Legislature, as before mentioned. The arches of the bridge, originally designed to maintain the grade of the aqueduct, were elevated 112 feet above the high water mark of the river, which is 12 feet higher than the Act requires. It is obvious, therefore, that 100 feet will not be sufficient to main- tain an aqueduct of masonry, but will require iron pipes as conduits for the water. This I do not consider an objection, as I am fully satisfied, iron pipes will make the most suita- ble conduit for the water on such a bridge, and therefore have had a plan prepared, with a view to comply with the law, arid avail of the economy and greater permanence from a less elevated structure. Owing to the less height required for the arches, and by adopting iron pipes for the conduit, the top of the parapets will be 114 feet above high water mark, which is 17 feet lower than the original plan. The superstructure being lighter than ne- cessary for an aqueduct of masonry, a diminished thickness of arch stone may with equal safety be adopted. The plan of piers and arches, also the hydraulic foundations, and manner of con- structing coffer dams, and the character of the masonry in general, are proposed to be simi- lar to that designed for the original plan. Some modifications in the construction and securing of the coffer dams, will be provided for in the estimates. For greater security, the expense of the coffer dams will be increased, by a more extensive excavation of the mud than was originally contemplated, and a course of plank to be put on the same before sinking. This plan, it will be perceived, has, in its general appearance, a great degree of similarity to what has, for distinction, been called the high bridge. It will, however, in consequence of its being of less height, and adapted to support a lighter superstructure, be less expensive. In the first instance, it is proposed to put down two three-feet pipes, which will probably be sufficient to supply the city for at least fifty years ; but to adapt the work to receive two four-feet pipe, which will be sufficient when the full CROTON AaUEDUCT. 165 capacity of the aqueduct shall be required. As there will be only a head of about 15 feet, no difficulty will be experienced in the use of pipes of this dimension. To guard more effectually against the influence of frost, the parapet walls are de- signed to be hollow. The