" 1 " "

The pipes are made in two- and three-foot lengths, with spigot, and
socket ends." (Gerhardt.)

Sewer pipes are laid in trenches at least three feet deep, to insure
against the action of frosts.

=Construction.=--The level of the trenches in which sewers are laid
should be accurate, and a hard bed must be secured, or prepared, for
the pipes to lie on. If the ground is sandy and soft, a solid bed of
concrete should be laid, and the places where the joints are should be
hollowed out, and the latter embedded in cement.

=Joints.=--The joints of the various lengths must be gas-tight, and
are made as follows: into the hub (the enlargement on one end of the
pipe) the spigot end of the next length is inserted, and in the space
left between the two a small piece, or gasket, of oakum is rammed in;
the remaining space is filled in with a mixture of the best Portland
cement and clean, sharp sand. The office of the oakum is to prevent
the cement from getting on the inside of the pipe. The joint is then
wiped around with additional cement.

=Fall.=--In order that there should be a steady and certain flow of
the contents of the sewer, the size and fall of the latter must be
suitable; that is, the pipes must be laid with a steady, gradual
inclination or fall toward the exit. This fall must be even, without
sudden changes, and not too great or too small.

[Illustration: FIG. 15.

A BRICK SEWER.]

The following has been determined to be about the right fall for the
sizes stated:

4-inch pipe 1 foot in 40 feet
6 " " 1 " " 60 "
9 " " 1 " " 90 "
12 " " 1 " " 120 "

=Flow.=--The velocity of the flow in sewers depends on the volume of
their contents, the size of the pipes, and the fall. The velocity
should not be less than 120 feet in a minute, or the sewer will not be
self-cleansing.

=Size.=--In order for the sewer to be self-cleansing, its size must be
proportional to the work to be accomplished, so that it may be fully
and thoroughly flushed and not permit stagnation and consequent
decomposition of its contents. If the sewer be too small, it will not
be adequate for its purpose, and will overflow, back up, etc.; if too
large, the velocity of the flow will be too low, and stagnation will
result. In the separate system, where there is a separate provision
for rain