Here we have the first experience and results of free and open ventilation to sewers. As regards the number of these gulleys in proportion to the sewers we have no evidence in these reports, but judging from their being specified as large open shafts, the area for the inlet and outlet of air to the sewers would be if anything greater than that of the present day. Yet this report states that the ill odours which escaped from the gulleys, although they might not be pestilential, became more repulsively offensive, and the attention of the Commissioners of Sewers was drawn to the evil, and it was felt that some remedy or palliative ought to be devised.
The means taken to obviate this evil may be termed the first experiment in sewer ventilation.
“A gulley trap was devised and fixed in the Pavement, Finsbury, in 1834, and in 1840 nine hundred of the gulleys had been trapped with a view to remedy the evil, with the following results.
“It became apparent even before that number was fixed, that the sewers were becoming dangerous to workmen to enter, and the gases generated found vent by the house drains (then generally untrapped) into dwellings.”
[It is quite evident that the compression of gases which takes place in the sewers by the rising and falling of the liquid and sewage flowing in them was not then known, or this experiment by closing the gulleys and ventilating the sewers through the house traps would not have been attempted.
At any rate the first experiment in sewer ventilation in the City cannot be said to have been a successful one, as it left matters worse than before.]
“To obviate this, ventilating shafts connecting directly with small iron gratings in the centre of the carriageways were formed: this mode of ventilating was also first adopted in the City, and the system of trapping (with numerous modifications in manner) and ventilating the sewers in the centre of the carriageways spread through the length of the metropolis.”
Can it be said that this alteration was an improvement in sewer ventilation?
The noxious or pestilential vapours that were so repulsive in 1840 when escaping through the gulleys were not rendered less poisonous by being given off in the middle of the road or carriageways, but the constant passing of carriages over the gratings had the effect of mixing the gases of the sewer more quickly with the atmosphere of the street. Thus their noxious qualities were not so much observed, but the effect of these gases on the public health by this arrangement has not been as satisfactory as many imagine.
When the carriage traffic is suspended during the night, the effluvium from these gratings is now similar to that experienced from the shoots or gulley shafts in 1830. This opinion is formed by comparing the report with testings taken at the gratings last year.