are each 13 ft. 7 in., and the diameter of the
circular form (with the exception of two short lengths) is 12 ft. 3
in. The reason for constructing the aqueduct in tunnel for so long a
distance was the enhanced value of the low-lying ground near the old
aqueduct. The tunnel deviates from a straight line only for the
purpose of intersecting a few transverse valleys at which it could be
emptied. For 25 m. the gradient is 0.7 foot per mile; the tunnel is
then depressed below the hydraulic gradient, the maximum depth being
at the Harlem river, where it is 300 ft. below high water. The depth
of the tunnel varies from 50 to 500 ft. from the surface of the
ground. Forty-two shafts were sunk to facilitate driving, and in four
cases where the surface of the ground is below the hydraulic gradient
these are closed by watertight covers. The whole of the tunnel is
lined with brickwork from 1 to 2 ft. in thickness, the voids behind
the lining being filled with rubble-in-mortar. The entry to the old
and new aqueducts is controlled by a gatehouse of elaborate and
massive design, and the pipes which take up the supply at the end of
the tunnel are also commanded by a gate-house. The aqueduct, where it
passes under the Harlem river, is worthy of special notice. As it
approaches the river it has a considerable fall, and eventually ends
in a vertical shaft 12 ft. 3 in. in diameter (where the water has a
fall of 174 ft.), from the bottom of which, at a depth of 300 ft.
below high-water level, the tunnel under the river starts. The latter
is circular in form, the diameter being 10 ft. 6 in., and the length
is 1300 ft.; it terminates at the bottom of another vertical shaft
also 12 ft. 3 in. in diameter. The depth of this shaft, measured from
the floor of the lower tunnel to that of the upper tunnel leading away
from it, is 321 ft.; it is continued up to the surface of the ground,
though closed by double watertight covers a little above the level of
the upper tunnel. Adjoining this shaft is another shaft of equal
diameter, by means of which the water can be pumped out, and there is
also a communication with the river above high-water level, so that
the higher parts can be emptied by gravitation. The cost of the Old
Croton Aqueduct was $11,500,000; that of the new aqueduct is not far
short of $20,000,000.

Nadrai.

The _Nadrai Aqueduct Bridge_, in India, opened at the en