ous angles, and for
any particular curve several would be used connected by straight pipes 3
ft. long. As special castings are nearly double the price of the regular
pipes, the cost was much diminished by making them as short as possible,
while a curve, made up of the slight angles used, offered practically no
more impediment to the flow of water in consequence of its polygonal
form, than would be the case had special bend pipes been used. In all
cases of curves on a line of pipes under internal fluid pressure, there
exists a resultant force tending to displace the pipes. When the curve
is in a horizontal plane and the pipes are buried in the ground, the
side of the pipe trench offers sufficient resistance to this force.
Where, however, the pipes are above ground, or when the curve is in a
vertical plane, it is necessary to anchor them in position. In the case
of the Tansa aqueduct to Bombay, there is a curve of 500 ft. radius near
Bassein Creek. At this point the hydrostatic head is about 250 ft., and
the engineer, Mr Clerke, mentions that a tendency to an outward movement
of the line of pipes was observed. At the siphon under Kurla Creek the
curves on the approaches as originally laid down were sharp, the
hydrostatic head being there about 210 ft.; here the outward movement
was so marked that it was considered advisable to realign the approaches
with easier curves (_Proc. Inst. C.E._ vol. cxv. p. 34). In the case of
the Thirlmere aqueduct the greatest hydrostatic pressure, 410 ft.,
occurs at the bridge over the river Lune, where the pipes are 40 in. in
diameter, and in descending from the bridge make reverse angles of
31-1/2 deg. The displacing force at each of these angles amounts to 54
tons, and as the design includes five lines of pipes, it is obvious that
the anchoring arrangements must be very efficient. The steel straps used
for anchoring these and all other bends were curved to fit as closely as
possible the castings to be anchored. Naturally the metal was not in
perfect contact, but when the pipes were charged the disappearance of
all the slight inequalities showed that the straps were fulfilling their
intended purpose. At every summit on a line of pipes one or more valves
must be placed in order to allow the escape of air, and they must also
be provided on long level stretches, and at changes of gradient where
the depth of the point of change below the hydraulic gradient is less
than that at both sides, causing