any water that might leak from the conduit, and prevent its being absorbed by the outer or parapet wall. The conduit over this bridge is constructed of stone and brick masonry, so arranged as to receive a lining of cast iron ; the iron being set back so as to receive a facing of brick over the bottom and sides, which is further secured by three courses of hydraulic cement plastering. It is not probable that any considerable quantity of water would have escaped, if there had been no other security than the stone and brick masonry ; but it is hardly possible to make masonry so perfectly impervious, that it will not show wet, or a sweating appear- ance on the exterior wall, and the action of frost in this climate, will produce in such cases a deterioration, that may be slow, but will eventually destroy the work ; hence the importance of using every precaution to guard against the smallest leakage. This bridge was erected under the contract of Young and Scott of Philadelphia, and the work con- ducted by Andrew Young (of the firm), who was the first contractor to commence work on the aqueduct. It is due to Mr. Young to say, this work was thoroughly executed ; the arch proved this, when on striking the centres, it did not settle over ^ of an inch, and has not changed since. This I consider a very small settlement for so large an arch. HARLEM RIVER BRIDGE. The width of the river at the place where the aqueduct line crosses it, is 620 feet at ordinary high water mark ; as has been before stated, the shore on the southern side is a bold rock rising from the water's edge, at an angle of about 30 degrees, to a height of 220 feet ; on the northern side, a strip of table land forms the shore, and extends back from the river 400 feet to the foot of a rocky hill, which rises at an angle of about 20 degrees to a few feet above the level of the aqueduct ; the table-land is elevated about 30 feet above the river ; the channel of the river to which the water is reduced at very low ebb tides, is 300 feet wide, and the greatest depth 16 feet ; each side of the channel the bed is a deep mud, covered from three to four feet at ordinary flood tide ; next below the mud there is a thin stratum of sand, and this is followed by a stratum of sand and large boulders inter- mixed ; below the stratum of boulders, or detached rock, there has been found in the cof- fer dams for two piers, Nos. 8 and 9, a compact marble rock, and in the coffers for Nos. 7 and 10, a stratum of clay and sand, that is quite impervious to water, and affords a good medium for piling. The general plan of the bridge now in progress of construction, is as follows : Across the river there are 8 arches, each of 80 feet span, resting on piers that are at each extremity and in the centre 20 feet wide at the spring line of the arches, with intermedi- ate piers that are 14 feet wide at the spring line ; on the south of this range of large arches there is one arch, and on the no«rth 6 arches, each of 50 feet span, resting on piers 7 feet wide at the spring line, and two abutments that terminate the arch work of the bridge. From the abutments a continuous foundation wall of dry stone work is extended to the gate chambers on each side. 53 210 MEMOIR OF THE" The whole length of the bridge is 1450 feet ; the height of the river piers above high water line is 60 feet to the spring of the arches, and 95 feet above the lowest foundation that has been put down ; the arehes are semicircular, and the height 100 feet to the soffit. or under side, at crown — to the top of the parapets 114 feet above the ordinary high water line of the river, and 149 feet above the lowest foundation of the piers that have been commenced. The width across, on the top of the parapets, is 21 feet ; the exterior of piers, spandrels and parapets has a bevel of 1 to 48, and have openings in the interior walls. The space between the parapets is arranged to receive and protect from frost two cast iron pipes, each four feet in diameter, which are to lie 12 feet below the grade line of the aqueduct, to which they will descend from the gate chambers at the