[190] For much interesting information upon this and other subjects in connection with sewage disposal, see ‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. xlviii. p. 105 et seq. Also the report of a committee of the Local Government Board on Modes of Treating Town Sewage, 1876. ‘Sewage Disposal,’ by Henry Robinson, C.E., and other works on the subject. [191] Under the Rivers Pollution Act, no effluent is allowed to enter a stream &c., if it contains more than three parts of suspended inorganic matter, and one part organic matter for every 100,000 parts of liquid. [192] _Vide_ ‘Minutes of Proceedings of the Institution of Civil Engineers,’ vol. xlviii. p. 179. CHAPTER XXII. VENTILATION OF SEWERS. The necessity for some manner of dealing with the noxious vapours emanating from sewage other than that of letting it find its way from the sewers into the house drains and thence into dwelling houses, has induced the legislature of this country to introduce the following clause in the Public Health Act 1875, which imposes on every local authority the duty of causing their sewers to be ventilated so as not to be a nuisance or injurious to health. “Every local authority shall cause the sewers belonging to them to be constructed, covered, ventilated and kept, so as not to be a nuisance, or injurious to health, and to be properly cleansed and emptied” (38 & 39 Vic. c. 55, s. 19). The result of this compulsion upon local authorities to ventilate their sewers has been the introduction of many methods to effect the purpose, the great difficulty being to “ventilate so as not to be a nuisance or injurious to health,” the advocates of open ventilation contending that this is effected by having a sufficient number of openings in a sewer to dilute and safely disseminate the foul gas with atmospheric air so that no nuisance is caused.[193] Many other methods have been from time to time suggested, some of which have been carried into effect, and I will now proceed to give them in detail, discussing their merits and objections in each case. (1.) Open shafts are carried up from the crown of the sewer to the centre or side of the roadway, and there protected by an open iron grid or grating at the level of the street surface. This is the system which has hitherto found most favour with town surveyors, and is sometimes modified or worked in conjunction with the practice of untrapping all the gully pits and buddle holes at the sides of the roadway, which is an excellent plan if the theory of the atmospheric air dilution at which this system aims is a correct one; in fact, if this dilution by air is all that is necessary to render the foul air in a sewer innocuous and inoffensive, there cannot be too many openings into it. The objections to this system are as follows: