Mile 156, and running up to Mile 166, and again commencing at the Luna
pumps, at Mile 171, and extending up to Mile 179, the minimum pressure
on those portions of the pump main is more than the 130 lb. per sq. in.
allowed for wood pipe, and the final estimated maximum pressures run up
to 310 lb.

The selection of iron pipe for these pressures was, first, as between
steel and cast-iron; and, second, as between the lead joint of the
standard bell and spigot pipe and the machined iron joint of the
universal joint pipe. Again, the choice was as between lead and leadite
for the bell and spigot pipe.

Cast iron was selected because of the certainty of its long life, and
the bell and spigot pipe was selected on the basis of comparative costs
for pipe laid. The standard lead joint was chosen on the result of
tests. This cast-iron pumping main has a diameter of 12 in. throughout.

_Pipe Weights._--Makers of standard bell and spigot pipe urged the usual
heavy weights selected for municipal service and heavy water-hammer.
Three pressures, _viz_., 217, 260, and 304 lb., were used for the
division of pipe weights, on which the standard pipe-makers specified
shell thicknesses of 0.82, 0.89, and 0.97 in. Eliminating water-hammer
and adopting a working stress of 2,400 lb., the thicknesses are reduced
to 0.54, 0.65, and 0.76 in. To make the latter conform to the
specifications of the New England Water-Works Association, the pipe was
cast to 0.57, 0.65, and 0.77 in. The reduction in cost amounts to
$52,811.

By the provision of air-cushions, hereafter described, the writer's
anticipation of no water-hammer on the pumping main has been fully