140 INTRODUCTION. 



upon timber piers, the spaces between which are filled with concrete masonry. 

 The dam sets the water of the river back about five miles, and forms a reservoir 

 covering about 400 acres. 



The gateway which guards the entrance to the aqueduct, is placed on the 

 solid rock, in a situation not exposed to the floods. The gate chamber is pro- 

 vided with a double set of gates ; one set of guard gates set in cast-iron frames ; 

 the other, a set of regulating gates made of gun metal, set in frames of the same 

 material. The gates are all 18 by 40 inches, and there are nine in each set, and 

 they are operated by means of wrought-iron screw rods. The gate chamber 

 and bulkheads are constructed of well-dressed masonry laid in hydraulic cement. 

 The water is conducted from the reservoir into the gate-house by a tunnel cut 

 180 feet through the rock, and flows into the bulkhead at the upper end of the 

 tunnel from a level averaging 10 feet below the surface of the reservoir. The 

 builders of this dam were McCullough, Black, McManus and Hepburn. 



The Sing-Sing kill, the bottom of which is 66 feet below the grade line of the 

 aqueduct, is crossed by a bridge resting on a single arch of 88 feet span and 33 

 feet rise. The form of the arch is an oval drawn from five centres. The bridge 

 is constructed of well dressed masonry laid in hydraulic cement. The builder 

 was Andrew Young, of Philadelphia. 



The width of the Harlem river, where the aqueduct crosses it, is 620 feet at 

 ordinary high water mark. The shore on the southern side is a rock rising from 

 the water's edge, at an angle of about thirty 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 four hundred feet to the foot of the roeky hill, which rises at an 

 angle of about twenty degrees, to 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 tides, is 300 feet wide, and the greatest depth is 

 16 feet. The bridge which is now in progress of construction, crosses this valley 

 on eight arches, each of 80 feet span, resting on piers that are (at each extre- 

 mity and in the centre) twenty feet wide at the spring line of arches, with in- 



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