es claimed for the separate system are as follows:

(1) Sewers may be of small diameter, not more than six inches.

(2) Constant, efficient flow and flushing of sewage.

(3) The sewage gained is richer in fertilizing matter.

(4) The sewers never overflow, as is frequently the case in the
combined system.

(5) The sewers being small, no decomposition takes place therein.

(6) Sewers of small diameter need no special means of ventilation, or
main traps on house drains, and can be ventilated through the house
pipes.

On the other hand, the disadvantages of the separate system are:

(1) The need of two systems of sewers, for sewage and for rain water,
and the expense attached thereto.

(2) The sewers used for sewage proper require some system for
periodically flushing them, which, in the combined system, is done by
the occasional rains.

(3) Small sewers cannot be as well cleaned or gotten at as larger
ones.

The separate system has been used in Memphis and in Keene, N. H., for
a number of years with complete satisfaction. Most cities, however,
use the combined system.

CHAPTER V

=Sewers=

=Definitions.=--A sewer is a conduit or pipe intended for the passage
of sewage, waste, and rain water.

A _House Sewer_ is the branch sewer extending from a point two feet
outside of the outer wall of the building to its connection with the
street sewer, etc.

=Materials.=--The materials from which sewers are manufactured is
earthenware "vitrified pipes."

Iron is used only for pipes of small diameter; and as most of the
sewers are of greater diameter than six inches, they are made of other
material than iron.

Cement and brick sewers are frequently used, and, when properly
constructed, are efficient, although the inner surface of such pipes
is rough, which causes adherence of sewage matter.

The most common material of which sewers are manufactured is
earthenware, "vitrified pipes."

"Vitrified pipes are manufactured from some kind of clay, and are
salt-glazed inside. Good vitrified pipe must be circular and true in
section, of a uniform thickness, perfectly straight, and free from
cracks or other defects; they must be hard, tough, not porous, and
have a highly smooth surface. The thicknesses of vitrified pipes are
as follows:

4 inches diameter 1/2 inch thick
6 " " 1/16 " "
8 " " 3/4 " "
12 "