It has already been laid down that the air-communication between the public sewer and the house-drain shall he severed, and that the latter must not terminate in a dead end, but be amply ventilated by openings, so as to create an undiminished current of air through the drain. The necessity for this is now admitted by all sanitarians; it is sometimes urged that, if the drain be soundly constructed upon the principles laid down herein, the time taken up in the passage of the sewage through the drain will be bo short that decomposition cannot set in, but this is an erroneous notion, as, under the most favourable conditions, it is impossible altogether to avoid putrefaction. It must be remembered that the bulk of the house-sewage is discharged in eight hours, and that, for the remaining sixteen, it is often not more than a mere dribble, quite insufficient to keep the drain clean during that period, and it is then that sewage gases are largely generated.

Although here and there may be found an advocate for the maintenance of a free communication between the house-drain and the public sewer, there is a vast preponderance of opinion in favour of disconnection. It is quite sufficient for each house to have to deal with the smells created by that house itself; and it is in the power of householders to control the working of their own drains efficiently, but they are unable to control faulty construction and deposits in public sewers. Town-sewage is a much more complex mixture than ordinary sewage, rapidly changing in its constituents and temperature, thus constantly transforming the gases generated. Owing to these varying conditions, the construction and age of the sewers, and other circumstances, the generation of sewer-gases cannot be entirely avoided, and it is no part of a householders duty to ventilate the town-sewer by means of the house-drain, but, on the other hand, it is his duty to see that his house is protected against the influence of infectious matter recieved into the common sewers, and this can only be clone by severing the sewer-air from the house-drain.

There are people who allege that there is practically no motion of air in house-drains except what arises from the disturbance caused by splashing at the junction, and that all effort at ventilation is so much waste energy. Attention may be called to the experiments made in this direction by Mr. Richard Horton in January and February, 1897, extending over fifteen days, end which were recorded by him in a paper read before the Institute of Sanitary Engineers on the 4th March. The experiments were made to ascertain and compare the differences of external and internal temperature of a series of drains, and to daily measure the external wind-currents at the same time as the rate of air-currents through the drain; the relative humidity was likewise taken, as well as the height of the barometer, and the state of the weather. The first drain was a stoneware pipe 34 feet long, with a lead soil-pipe and galvanized-iron ventilating-pipe 30 feet high (rising clear of the roof); the gradient of the drain was 1 in 1<». and the depth of How of the sewage varied from 11/2 inches to nil. A ond series of experiments was made on an iron drain 5 inches in diameter and 46 feet long, having a fall of 1 in 46, the soil-pipe connected thereto being 4-inch iron rain-water pipes with lead branches for the water-closets, lead anti-syphonage-pipes and vent-pipes GO feet high; the depth of flow varied from 1m 3/8 inches to nil. A third experiment was made on a stoneware-pipe drain, 87 feet long, having a fall of 1 in 40, the soil-pipe being of lead and the ventilator 90 feet high; the depth of flow varied from 1 1/4 inches to 1/8 inch. As a result of these experiments Mr. Morton is of opinion that our preconceived ideas as to the influences of wind-pressure must be modified considerably. Other things being equal, efficiency of drain-ventilation varies directly as the relative humidity of the atmosphere. As the air Incomes drier, the velocity of the air-current in the drain increases, and as the air incomes more nearly saturated with water-vapour, the current diminishes. Where the air-current in the drain does not vary exactly with the relative humidity, the difference can be accounted for by the influence of the wind being at a greater pressure and from a drier quarter. Mr. Horton ventures to place the different influences upon drain-ventilation in the following order of value: - (1) The relative humidity of the atmosphere, (2) Materials of which the drain is composed, (3) The wind-pressure, (4) The difference between internal and external temperature, (5) Friction.

By-law 63 of the Model By-laws, issued by the Local Government Board, provides for drains being trapped from the sewer. The method usually selected as being most effective for the purpose is the water-trap, its position being as far from the building as practicable, but within the curtilage thereof, and as near as practicable to the sewer. By-law 05 prescribes two arrangements for the purpose of securing efficient ventilation of the drains, both providing an uninterrupted passage-way from end to end, with free communication to the external air. The clause is necessarily somewhat lengthy and cumbesome, owing partly to the desirability of giving two alternative ways of carrying out the principle of the by-law. By either of these ways facilities can be provided for a constant current of fresh air through the entire length of the house-drain, and each precludes the possibility of drain-air entering the house. The diagrams 330 and 331 show the two methods of ventilation prescribed by the by-law.

Fig 330   Section showing Drain ventilation with Outlet Head of Drain.

Fig 330 - Section showing Drain-ventilation with Outlet Head of Drain.

Fig. 331.   Section showing Drain  ventilationtion with Inlet at Head of Drain.

Fig. 331. - Section showing Drain -ventilationtion with Inlet at Head of Drain.