the-ordinances-of-the-legislative-counci-1890v2 — Page 409

PUBLIC IIEALTH . [ 24 of 1887. ] 999

Tlmus ; if aa 12" and aa 3" pipe have the same inclination the velocity in the
12" pipe would be about twice as great as in the 3 " pipe, provided that both
were half full. But to fill the two pipes to this extent, the quantity of water
passing through the 12" pipe, would be about thirty -two times that passing
through the 3 " pipe. But if the same quantity flowed through both pipes,
then the current in the 3" pipe, being more contracted, would flow more
rapidly than that in the partially filled, 12 " pipe.
G. These consilerations would lead to the conclusion that the best size, to
be used for any house-drain, would be that which would just suffice to carry
off the sewage with the pipe not less than half full. Within certain limitations,
this is the case .
H. In the case of street -sewers draining large districts, from which the
flow of sewage is comparatively regular and capable of estimation, the proper
size of sewer may be calculated with some approach to precision. In the
case of house -drains, however, such precision is impracticable. The rate of
flow is irregular and the quantity of sewage depends upon the habits of the
inmates. Were the size of aa house -drain calculated to carry off even a most
liberal water supply, which is , for practical purposes the measure of the
sewage, it would be found, in the vast majority of cases, to be much less than
that which experience has shewn, to be applicable in practice. It has been
found that aa house-drain less than a certain size, is inconvenie :tly liable to
stoppages, caused by extraneous matters, such as rags , paper and the like,
which occasionally find their way into the best managed house-urains.
1. The minimum size of house- lrain is usually fixed at from four to six
inches in diameter.A four- inch drain , constructed in the insual manner, is
probably somewhat small as a minimum ; and a six-inch somewhat large.
A diameter of five inches, were this size readily obtainable, would be a better
minimum .
J. Whenever the minimum size will suffice it should be used. Indeed ,
subject to the limit laid down in the preceiling paragraph, and to certain
conditions to be set forth later on , the smaller the house-drain the better.
The use of unnecessarily large house -drains amounts to an abandonment of
the self-cleansing principle. If a drain is so large that the normal flow of
sewage cannot fill it to a sufficient deptli to establish a self- cleansing velocity,
deposit takes place and goes on , either till the pipe is choked and the sewage
escapes, through leaks, to the sub -soil : or, until the deposit has accumulated
sufficiently to reluce the area of the sewage -way to that which will establish
a current sulliciently rapid to prevent further deposit.
K. In order that the size of house - rains may be reluced to the minimum
practicable, in each instance, rain -water should be excluded from them as far
as possible. Rain -water cannot, however, be wholly excluded from house
drains. Many uncovered surfaces such as back -yards, receive slops and foul
water in the course of daily use and this must be carriel off by the house
drains. A duplicate set of Irains for such areas, one for sewage and the
other for rain -water, would be a costly complication and there would be no
security that cach set would be el for its proper purpose only. Some
sewage would almost inevitably find its way into the rain -water drain, whichi,
being of large size, would be imperfectly flushed and therefore offensive.
L. Rain -water should be diverted from honse -drains to the full extent that
is possible by the use of surface -channels. As a general rule, when surface
chamels cannot be used to divert rain -water, it must be admitted to the
house - rain . Hence, in the majority of cases, the amount of rain -water to be
carried off determines the size of the house -drain . The table, hereunto
attached , gives the area from which 4" of rain, falling in one hour, will be
carried off by pipes of various sizes, laiil at different inclinations. In providing