56 TECHNOLOGY. 
aqueduct is placed on a solid gneiss rock, through which the aqueduct passes 
by a tunnel of 108 feet in length. The gate chamber is provided with a 
double set of gates, one set of guard gates of iron, the other a set of regulat- 
ing gates made of gun metal. The gates are all 18 by 40 inches, and there 
are nine in each set ; they are operated by means of wrought-iron screw rods. 
The Harlem bridge | is represented on pl. 11, jig. 9; it crosses the valley 
of the Harlem river with eight arches of 80 et frat and seven of 50 feet 
span ; they are semicircular, and the height to the top of the parapets is 114 
feet above ordinary high water ; the width on top of the parapets is 21 feet. 
The material of the bridge is dressed granite. The water is conveyed across 
the bridge in three iron pipes of 3 feet diameter, having an extra fall of 2 
feet in order to make their capacity for conveying water equal to that of the 
aqueduct. 
The greatest depression of the Clendenning valley is 50 feet below the 
top of the aqueduct, and the valley is 1,900 feet across. Streets cross the 
line of the aqueduct in this valley at right angles, and archways are con- 
structed over them. /. 11, fig. 10, represents the aqueduct, and jig. 11is 
a section of the same. 
The receiving reservoir is 1,826 feet long and 836 wide, and covers with 
its embankments an area of thirty-five acres. It is divided into two parts, 
having respectively the depths of 20 and 30 feet ; its present capacity is 150 
millions of gallons. It is formed by earth-banks, the interior having 
regular rubble walls; the outside is protected by a stone wall on a slope of 
one horizontal to three vertical, the face laid in cement mortar, and the 
inside dry. 
The distributing reservoir at Murray hill is 420 feet square, and covers 
four acres; it is 86 feet deep, and holds twenty millions of gallons. The 
walls are of hydraulic stone masonry, constructed with openings made by an 
interior and exterior wall, connected every 10 feet by cross walls, in order 
to give an enlarged base and reduce the quantity of masonry. At 17 feet 
from the top the cross-walls are connected by brick arches; the exterior 
wall, 4 feet thick, is then carried up single to the top, where it has an 
Egyptian cornice surmounted by an iron railing. On each corner of the 
reservoir pilasters 40 feet in width are raised, projecting 4 feet from the main 
wall, and in the centre of the street-facades are pilasters 60 feet wide ; they 
have doors and stairways leading to the top of the walls and to the pipe 
chambers, in which the supply of water can be regulated by stop-cocks. 
The reservoir is divided by a wall of hydraulic masonry into two divisions, 
from both of which the city is supplied, and in each there is a waste-cock to 
draw the water from the bottom. The level of the reservoir is 45 feet 
above that of the adjoining streets, and higher than any part of the city of 
New York. 
Ef. Canal Bridges. 
On canals which are not navigated by vessels carrying masts, the foot 
and road bridges crossing them are built like other structures of the 
kind; care should be taken to give them sufficient height to allow persons 
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