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this current be at all rapid it tends to draw air into the sewer through
the openings on a lower level, and to discharge it at a higher level, so
that the highest openings are most liable to act as outlets and the
lowest as inlets. There is, however, no constancy in action, even in
the same sewer, and all street openings may at different times act as
inlets or outlets.
To obviate the nuisance arising from foul-smelling grids, various
plans have been adopted, with more or less (generally less) success.
In some cases it has been advocated to reduce the smell from one grid
by placing another on each side of it, thus turning the sewer into a
condition approximating an open channel. In many cases charcoal
baskets are used under the grids for the purpose of deodorizing the
mephitic vapours as they ascend. These baskets act efficiently while
the charcoal is dry, that is to say, until the first shower of rain, and
have consequently fallen into disfavour. Another device is to construct
ventilating shafts, in substitution for the street openings, which shafts
are carried up the side of an adjacent house, up the trunk of a tree, or
up special columns in the street. They have proved useful as an aid
to ventilation at the dead ends of sewers, but they are never of sufficient
calibre to prove of value in any considerable length of sewer. To be of
any use their size and number must be of such proportions as to entail
enormous cost in construction. Practical experience, however, shows
that, even where freely provided, they fail to act just at the time when
they are most needed, that is to say, where the temperature inside
the sewer is higher than that of the air outside, at which time the
warm sewer air finds a much readier vent through the road opening
than through the high ventilator. Again, ventilating pipes have been
passed into stokeholes or chimney shafts. This method creates a
strong draft, but its effect is very local, tending to draw air
into the sewer from all surrounding openings. It has also been
known to draw the traps of house drains and thus allow sewer
air to pass into houses while the chimney flue was not working,
while some years ago in Southwark it caused an explosion
from ignition of coal gas which had accidentally found its way
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