ons, where the
pressure is very great, or where they are for use abroad, when
considerations of weight are of importance, and when they are made of
rolled steel with riveted or welded seams. It is frequently necessary to
lay them in deep cuttings, in which case cast-iron is much better
adapted for sustaining a heavy weight of earth than the thinner steel,
though the latter is more adapted to resist internal pressure. Mr D.
Clarke (_Trans. Am. Soc. C.E._ vol. xxxviii. p. 93) gives some
particulars of a riveted steel pipe 24 m. long, 33 to 42 in. diameter,
varying in thickness from 0.22 in. to 0.375 in. After a length of 9 m.
had been laid, and the trench refilled, it was found that the crown of
the pipe had been flattened by an amount varying from 1/2 in. to 4 in.
Steel pipes suffer more from corrosion than those made of cast-iron, and
as the metal attacked is much thinner the strength is more seriously
reduced. These considerations have prevented any general change from
cast-iron to steel.

Mr. Clemens Herschel has made some interesting remarks (_Proc. Inst.
C.E._ vol. cxv. p. 162) as to the circumstances in which steel pipes
have been found preferable to cast-iron. He says that it had been
demonstrated by practice that cast-iron cannot compete with
wrought-iron or steel pipes in the states west of the Rocky Mountains,
on the Pacific slope. This is due to the absence of coal and iron ore
in these states, and to the weight of the imported cast-iron pipes
compared with steel pipes of equal capacity and strength. The works of
the East Jersey Water Company for the supply of Newark, N.J., include
a riveted steel conduit 48 in. in diameter and 21 m. long. This
conduit is designed to resist only the pressure due to the hydraulic
gradient, in contradistinction to that which would be due to the
hydrostatic head, this arrangement saving 40% in the weight and cost
of the pipes. For the supply of Rochester, N.Y., there is a riveted
steel conduit 36 in. in diameter and 20 m. long; and for Allegheny
City, Pennsylvania, there is a steel conduit 5 ft. in diameter and
nearly 10 m. long. The works for bringing the water from La Vigne and
Verneuil to Paris include a steel main 5 ft. in diameter between St.
Cloud and Paris.

Cast-iron pipes rarely exceed 48 in. in diameter, and even this
diameter is only practicable where the pressure of the water is low.
In the Thirlmere aqueduct the greatest