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239
Surface Ventilators, that is to say' on shafts leading from the
crown of the sewer to the centre of the road, and covered at the
road-level by open gratings,' " Sir H. Roscoe observes that " the
efficiency of Surface Ventilators depends mainly on the size of
these air openings." The size of these air openings " is
altogether insufficient," largely due to the fact that they become
more or less completely filled with dirt, and hence the " general
ventilation of the sewers appears to be in an unsatisfactory condition."
He agrees with the Committee, in their afore-mentioned
report, that the mode of ventilation specified " has been
the cause of complaint owing to the imperfect manner in which
the system has been carried out, the ventilators being deficient
both in size and number; and that pipe ventilators can be
used with great advantage in addition to, and not in substitution
for surface ventilators," under proper conditions and wherever
practicable.
Sir H. Roscoe remarks that " the escape of sewer air is
practically prohibited in many quarters of the Metropolis," as
the " temperature of the air in the sewer is often ten degrees
above that of the outer air, into which the warmer sewer air
would naturally escape, were proper means provided." Sir H.
Roscoe's final observation, like so many preceding ones, is
entirely in harmony with views long ago expressed by the
officers of your Vestry and others, viz., that " the free admission
of air into the sewers is the only efficacious means of preventing
foul emanations, 1st, by the oxidation of putrescent sewage,
and 2nd, by the dilution of any offensive gases evolved." He
therefore calls special attention to this portion of the subject,
believing that" the policy of closing the air openings to the
sewers, instead of allowing as much fresh air to enter as is
possible, is based upon a wrong scientific principle, and that the
only feasible plan of rendering the sewers sweet is to carry out
thoroughly, and on a proper scale, the system of Pipe Ventilation
so strongly recommended in the excellent Report of the
Special Purposes and Sanitary Committee."