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6
The practical conclusions are obvious :—
1. In old, ill-kept, defective sewers—in so-called sewers
of deposit, or elongated cesspools, open grids in
narrow and crowded streets and courts or alleys
must be more or less nuisances, and ought, therefore,
to be closed, and suitable (adequate) compensatory
ventilation by shafts (or otherwise)
provided. In wider thoroughfares, however, the
nuisance from open grids must be very much less,
and the gases practically rendered harmless by
oxidation and dilution as they issue therefrom.
2. In new, properly constructed sewers, with adequate
fall, good flushing, and a fairly rapid flow, open
grids may be disregarded from the point of view
of nuisance, except, perhaps, in very rare instances,
when they may prove temporarily to be so under
certain atmospheric and other conditions, e.g.,
when inlets act as outlets ; when hot water or
steam is sent into a sewer, thereby causing increased
putrefactive changes with consequent
increased evolution of gases ; when the open grids
are too few in number and the distances between
individual ones greater than, say, 50 yards, thereby
preventing proper oxidation and dilution of the
gases ; when waste products from factories,
breweries, chemical works, &c., are run into the
sewers, and the fumes arising therefrom given off
from neighbouring open manholes, &c.
The principle of thoroughly ventilating sewers is good,
and granted a fairly wide thoroughfare, and a fairly good
sewer, I do not think I should be justified in condemning
the method by open manholes on the crowns of the sewers