he
pipes were 40 in. in diameter, the discharge was diminished by 7% in
ten years. An account of the state of two cast-iron mains supplying
Boston with water is given in the _Trans. Am. Soc. C.E._ vol. xxxv. p.
241. These pipes, which were laid in 1877, are 48 in. in diameter and
1800 ft. long. When they were examined in 1894-1895, it was estimated
that the tubercles of rust covered nearly one-third of the interior
surfaces, the bottom of the pipe being more encrusted than the sides
and top. They had central points of attachment to the iron, at which
no doubt the coating was defective, and from them the tubercles spread
over the surface of the surrounding coating. In this case they were
removed by hand, and the coating of the pipes was not injured in the
process. Cast-iron pipes must not be laid in contact with cinders from
a blast furnace with which roads are sometimes made, because these
corrode the metal. Mr Russell Aitken (_Proc. Inst. C.E._ vol. cxv. p.
93) found in India that cast-iron pipes buried in the soil rapidly
corroded, owing to the presence of nitric acid secreted by bacteria
which attacked the iron. The large cast-iron pipes conveying the water
from the Tansa reservoir to Bombay are laid above the surface of the
ground. Cast-iron pipes of these large diameters have not been in
existence sufficiently long to enable their life to be predicted. A
main, 40 in. in diameter, conveying soft water, after being in
existence fifty years at Manchester, was apparently as good as ever.
In 1867 Mr J.B. Francis found that no apparent deterioration had taken
place in a cast-iron main, 8 in. diameter, which was laid in the year
1828, a period of thirty-nine years (_Trans. Soc. Am. C.E._ vol. i. p.
26). These two instances are probably not exceptional.

Methods of laying.

Pipes in England are usually laid with not less than 2 ft. 6 in. of
cover, in order that the water may not be frozen in a severe winter.
Where they are laid in deep cutting they should be partly surrounded
with concrete, so that they may not be fractured by the weight of earth
above them. Angles are turned by means of special bend pipes, the curves
being made of as large a radius as convenient. In the case of the
Thirlmere aqueduct, double socketed castings about 12 in. long
(exclusive of the sockets) were used, the sockets being inclined to each
other at the required angle. They were made to vari