ream of water
one inch depth, runs very sluggishly, while the same water running through
a 12-inch pipe, laid on the same inclination, forms a rapid stream,
carrying away the heavy silt which was deposited in the broad sewer. As a
consequence of this, it has been found, where pipe sewers are used, even
on almost imperceptible inclinations, that silt is very rarely deposited,
and the waste matters of house and street drainage are carried immediately
to the outlet, instead of remaining to ferment and poison the atmosphere
of the streets through which they pass. In the rare cases of obstruction
which occur, the pipes are very readily cleansed by flushing, at a tithe
of the cost of the constant hand-work required in brick sewers.

For the first six or seven hundred feet at the head of a sewer, a six inch
pipe will remove all of the house and street drainage, even during a heavy
rain fall; and if the inclination is rapid, (say 6 inches to 100 feet,)
the acceleration of the flow, caused partly by the constant additions to
the water, pipes of this size may be used for considerably greater
distances. It has been found by actual trial that it is not necessary to
increase the size of the pipe sewer in exact proportion to the amount of
drainage that it has to convey, as each addition to the flow, where
drainage is admitted from street openings or from houses, accelerates the
velocity of the current, pipes discharging even eight times as much when
received at intervals along the line as they would take from a full head
at the upper end of the sewer.

For a district inhabited by 10,000 persons, a 12-inch pipe would afford a
sufficient outlet, unless the amount of road drainage were unusually
large, and for the largest sewers, pipes of more than 18 inches diameter
are rarely used, these doing the work which, under the old system, was
alloted to a sewer 6 feet high and 3 feet broad.

Of course, the connections by which the drainage of roads is admitted to
these sewers, must be provided with ample silt-basins, which require
frequent cleaning out. In the construction of the sewers, man-holes, built
to the surface, are placed at sufficient intervals, and at all points
where the course of the sewer changes, so that a light placed at one of
these may be seen from the next one;--the contractor being required to lay
the sewer so that the light may be thus seen, a straight line both of
inclination and direction is secured.

The rules which