1000 ( 24 of 1887. ] PUBLIC HEALTH .
for the removal of rain -fall, from a given surface, it is to be remembered that
it is not sufficient to provide a pipe of ample size to carry off the rain -fall.
Traps and gratings must be provided in sufficient numbers and of sizes to
admit the rain -water freely to the drain and the surface must have a fall to
carry it to the openings. In many cases neglect of these precautions has
caused flooding. This has been put down to the size of the pipes, whereas
the iulets were really at fault.
Rain -water cannot be looked upon as a legitimate agent for cleansing
house-drains from deposit. Firstly, there should be no deposit to remove
from well constructed and properly managed drains ; and secondly, because there
are long intervals in which no rain falls so that this flushing agent fails when
most wanted.
M. The irregularity and uncertainty of the flow of sewage, which precludes
an accurate determination of size, makes it impracticable, to calenlate
definitely the proper inclination of house-vrains. Experience shews that,
under the vormal conditions of use, an inclination of one in thirty is desirable
to ensure a self-cleansing drain ; especially when the drain conveys the
sewage from a single trap or inlet. An inclination of one in thirty, therefore,
should , as a rule, be given to house -drains. Drains laid at much flatter
gradients can be made to work perfectly, with care, and provided that the
flow of sewage is copious i. l. sufficient to fill the pipe nearly half full. The
designer of house-ulrains having no control over their subseqnent usage, will
do well to give them , in every case, the greatest practicable fall.
X. The ordinary “ trap ” or syphon is not a perfect preventive against the
escape of sewage -tainted air from house drains, although it is the only
appliance available for the purpose. The water which seals the trap may
evaporate during disuse, or a piece of fibrous matter may act like a syphon
and draw off the water suficiently to leave a free passage for the escape of
sewage -tainted air. Again , if there be a series of inlets connected to a drain
which is un provided with any free outlet ; a sudden rush of water through
one trap will force the air contained in the drain tl:rough the other traps or
unseal them by drawing off their contents, and open a passage for the escape
of sewage -tainted air into the dwelling, or into some contined space in its
neighbourhood . Consequently , house- draius must be “ ventilated ,” or rather
provided with vents, to prevent the accumulation of sewaye -tainted air therein ,
and to secure the efficiency of the traps. To effect this thoroughly, the drain
must have, at or near its extremities, openings giving a free escape to the air
within it , and between these openings, there should be no trap or other
obstruction to the free passage of air.
0. These ventilating -openings should be so placed that any sewage-taintel
air which may escape from them shall be as little offeasive as possible. With
this object the imer ventilating -opening should be placed above the roof of
the dwelling. This is usually done by carrying up a pipe, having a free
opening at the top, above the roof ; and at a distance from any window . The
second opening is usually made on the house -side of the trap which severs
the direct communication between the drain and the public sewer. If, bow
ever, there be no trap between the house - rain and the sewer then there is no
necessity for a second opening , the sewer itself serving the purpose. With
a well constructed system of self-cleansing street -sewers this arrangement
may be adopted withi safety, even with advantage. Then every house -drain
ventilator serves as a sewer ventilator also ; and when this is done wiversally,
no accumulation of sewer -tainted air can take place in any part of the system .
P. Care must be taken in selecting the position of the lower ventilating
opening, because the elevated ventilating pipe by no means secures a per
manent upward current of air. According to the relative temperature of the
air within and without the drain , to the direction and force of the wind, so an
upward or downward current will obtain .