MEANS OF COMMUNICATION. | 59 
across the valley of the Harlem River, after which the aqueduct of masonry 
is resumed, and continues two miles to the Manhattan valley, which is 
passed with four iron pipes, descending 102 feet to the bottom of the valley, 
and rising again to its opposite side, forming a syphon of 4100 feet in 
length. The masonry conduit is again resumed, and crossing the Asylum 
ridge and Clendenning valley, is continued two miles to the receiving 
reservoir at Yorkville, whence iron pipes laid beneath the surface of the 
ground conduct the water a distance of two miles to the distributing reser- 
voir at Murray Hill, three miles from the City Hall. 
The length of the aqueduct from the Croton dam to the receiving reser- 
voir is 454 miles. Its general declivity is 13+ inches to the mile. The 
form of the masonry conduit is seen in pl. 11, fig. 11; the bottom is an 
inverted arch, the chord of which is 6 feet 9 inches, and the versed sine 9 
inches ; the side-walls rise 4 feet from the springing line of the bottom arch, 
with a batter of 1 inch to a foot rise, making the width at the top of the 
side-walls 7 feet 5 inches. The roof-arch is a semicircle, making the area 
of the interior 53.59 feet. The supply of water furnished daily is about fifty 
millions of gallons, which is more than the aggregate of all the London 
water-works, and more by ten millions of gallons than the quantity fur- 
nished by the fourteen aqueducts which supplied Rome in the days of her 
greatest splendor. 
There are on the line sixteen tunnels, driven chiefly through gneiss and 
marble, the aggregate length of which is 6841 feet. The streams encoun- 
tered are crossed te 114 culverts with spans from 12 to 25 feet, at depths 
varying from 12 to 70 feet below the grade. There are also five road-cross- 
ings of from 14 to 20 feet span. The aqueduct is covered with earth 
to a sufficient depth to protect the water from frost. There are thirty-three 
ventilators placed at a distance of one mile from each other, to give free 
circulation of air through the aqueduct; they rise 14 feet above the surface 
of the ground, and have a circular aperture of 15 inches diameter; eleven 
of them are provided with doors by which the aqueduct may be entered. 
There are also six waste-weirs to allow the water to run off when it reaches 
a certain height, and to allow the aqueduct to be emptied should it become 
necessary. They are constructed of well dressed stone, with cast-iron gates 
and frames. 
The Croton reservoir, which has received the name of Croton Lake, is 
available for 500 millions of gallons above the level that would allow the 
aqueduct to discharge thirty-five millions per day. The greatest height of 
the weir of the dam above the bed of the river is 55 feet. The width of 
masonry at low-water line of the river is 61 feet ; the form on the lower face 
is a curve described by a radius of 55 feet, which continues to within 10 
feet of the top, when a reversed curve of 10 feet radius carries the face over 
to meet the back line of the wall. The back line is carried up vertically 
with occasional projections. The waste-weir is 270 feet in width. At 300 feet 
below the main dam is a second dam 9 feet high, which sets the water back 
over the apron of the main dam, and thus forms a pvol to check the water 
as it falls over the weir. The gateway which guards the entrance to the 
635 
