miles from the Croton Dam. This valley is 1900 feet across, and the Aqueduct is supported upon a foundation wall of dry stone work having the face laid in mortar, except over three streets where bridges are built, having an arch of 30 feet span for the carriage-way and one on each side of 10i feet span for the side walks. These bridges are over 98th, 99th, and 100th streets. Plate XXI. is a view of a portion of the work at Clendin- ning Valley showing the three bridges ; and comprises a length of about 700 feet. The greatest height from the foundation to the top of the work is 50 feet, and the width at the bottom of the Aqueduct is 30 feet. Parapet walls are built on the sides of the wall above the bottom of the Aqueduct to support a covering of earth over it. 29 114 Plate XXII. is an enlarged view of one of the bridges and a portion of the foundation wall and Aqueduct adjacent to it. The Aqueduct has a cast iron lining over the bridges like that described at the Sing Sing Kill. These bridges are beautiful specimens of mechanical work ; indeed the whole structure across this valley has a degree of neatness, finish, and taste, not surpassed by any on the line of Aqueduct. To visit this structure and follow along its whole extent, gives one an idea of the magnitude of the work which the City of New-York has accomplished ; particularly when it is considered that this is only one of the parts which make up the whole. From Clendinning Valley the Aqueduct soon reaches the Receiving Reservoir which is thirty-eight miles from the Croton Dam. This Reservoir occupies an elevated part of the island between 79th and 86th streets and between the 6th and 7th Avenues. It covers seven of the city blocks ; is divided into two divisions, one covering three and the other four of these blocks. It is 1826 feet long and 836 feet wide from outside to outside of the top of the exterior walls of the embankment, making an area of thirty-five acres. The situation was chosen as one affording the proper elevation : but its formation was such as to present difficulties in the way of making the Reservoir perfectly water-tight ; the surface, in tracing it from 79th to 86th street, was quite undulating, a portion of it in the southern division of the Re- servoir falling below the proposed bottom, and that portion of the surface which was earth, forming only a covering to X kg -i < 033 =1 69 §1 @ 115 the rock, which over the whole island, presents a singularly broken and uneven formation. In almost every instance of excavation, the rock was found above the proposed bottom of the Reservoir, and the difficulty of preventing leakage along the surface of this rock may easily be conceived ; but considering that measures are taken to prevent such an occurrence, another difficulty is still presented in the forma- tion of the rock : the veins and fissures which are frequent in this gneiss formation would possibly afford courses for the water to escape ; the rock being unsound in many in- stances, would render such an occurance still more liable. A Reservoir has however, been constructed here which proved, when it was filled with water, that sufficient precau- tion was used to prevent leakage, and that the difficulties which presented themselves before the commencement of the work were no longer to be feared. The embankments forming the Reservoir are made of good assorted earth, and a portion of the bank is puddled, or made compact and impervious by wetting the earth and using a spade to force it into a compact state. They are about 20 feet wide on the top, and increase in thickness to- wards the base by a slope on both sides : the outside face of the Reservoir bank has a slope of 1 foot horizontal to 3 feet vertical : the inside has a slope of li foot horizontal to 1 foot vertical. The outside face is protected by a stone wall 4 feet thick having the face laid in mortar : the inside face is protected by a slope wall of stone laid without mortar, If foot thick. The top of the bank is 4 feet above top water line, and the inside slope wall terminates at 2 feet above top water line, leaving the remainder of the face to be covered 116 with grass, so as to present a belt of green above the water on the bank entirely around the Reservoir. A neat fence bounds the outside and the inside of the top bank, forming a walk