water, the size of the sewer ought not to exceed six inches
in diameter. In the combined system, however, when arrangements must
be made for the disposal of large volumes of storm water, the size of
the sewer must be larger, thus making it less self-cleansing.

=Connections.=--The connections of the branch sewers and the house
sewers with the main sewer must be carefully made, so that there shall
be no impediment to the flow of the contents, either of the branches
or of the main pipe. The connections must be made gas-tight; not at
right angles or by T branches, but by bends, curves, and Y branches,
in the direction of the current of the main pipe, and not opposite
other branch pipes; and the junction of the branch pipes and the main
pipe must not be made at the crown or at the bottom of the sewer, but
just within the water line.

=Tide Valves.=--Where sewers discharge their contents into the sea,
the tide may exert pressure upon the contents of the sewer and cause
"backing up," blocking up the sewer, bursting open trap covers, and
overflowing into streets and houses. To prevent this, there are
constructed at the mouth of the street sewers, at the outlets to the
sea, proper valves or tide flaps, so constructed as to permit the
contents of the sewers to flow out, yet prevent sea water from backing
up by immediately closing upon the slightest pressure from outside.

=House Sewers.=--Where the ground is "made," or filled in, the house
sewer must be made of cast iron, with the joints properly calked with
lead. Where the soil consists of a natural bed of loam, sand, or rock,
the house sewer may be of hard, salt-glazed, and cylindrical
earthenware pipe, laid in a smooth bottom, free from projections of
rock, and with the soil well rammed to prevent any settling of the
pipe. Each section must be wetted before applying the cement, and the
space between each hub and the small end of the next section must be
completely and uniformly filled with the best hydraulic cement. Care
must be taken to prevent any cement being forced into the pipe to form
an obstruction. No tempered-up cement should be used. A straight edge
must be used inside the pipe, and the different sections must be laid
in perfect line on the bottom and sides.

Connections of the house sewer (when of iron) with the house main pipe
must be made by lead-calked joints; the connection of the iron house
pipe with the earthenware house sewer must be made with cement, and