pressure is nearly 180 lb. on
the square inch, the pipes where this occurs being 40 in. in diameter
and 1-3/4 in. thick. These large pipes, which are usually made in
lengths of 12 ft., are generally cast with a socket at one end for
receiving the spigot end of the next pipe, the annular space being run
with lead, which is prevented from flowing into the interior of the
pipe by a spring ring subsequently removed; the surface of the lead is
then caulked all round the outside of the pipe. A wrought-iron ring is
sometimes shrunk on the outer rim of the socket, previously turned to
receive it, in order to strengthen it against the wedging action of
the caulking tool. Sometimes the pipes are cast as plain tubes and
joined with double collars, which are run with lead as in the last
case. The reason for adopting the latter type is that the stresses set
up in the thicker metal of the socket by unequal cooling are thereby
avoided, a very usual place for pipes to crack under pressure being at
the back of the socket. The method of turning and boring a portion,
slightly tapered, of spigot and socket so as to ensure a watertight
junction by close annular metallic contact, is not suitable for large
pipes, though very convenient for smaller diameters in even ground.
Spherical joints are sometimes used where a line of main has to be
laid under a large river or estuary, and where, therefore, the pipes
must be jointed before being lowered into the previously dredged
trench. This was the case at the Willamette river, Portland, Oregon,
where a length of 2000 ft. was required. The pipes are of cast-iron 28
in. in diameter, 1-1/2 in. thick, and 17 ft. long. The spigots were
turned to a spherical surface of 20 in. radius outside, the inside of
the sockets being of a radius 3/8 in. greater. After the insertion of
the spigot into the socket, a ring, 3 in. deep, turned inside to
correspond with the socket, was bolted to the latter, the annular
space then being run with lead. These pipes were laid on an inclined
cradle, one end of which rested on the bed of the river and the other
on a barge where the jointing was done; as the pipes were jointed the
barge was carefully advanced, thus trailing the pipes into the trench
(_Trans. Am. Soc. C.E._ vol. xxxiii. p. 257). As may be conjectured
from the pressure which they have to stand, very great care has to be
taken in the manufacture an