The question of drain-ventilation does not appear to be so clearly understood, nor the thought given to it that its importance demands. It is only the bond fide sanitary engineer or sanitary plumber who attaches any value to it. There are hundreds, or it may be said thousands, of houses that have the drains connected directly with the public sewer. Other houses have a trap placed in the course of the drain to disconnect it from the sewer. This trap may keep back sewage-gases, it is true, but for want of ventilation the house drains become a retort for generating these gases on the house side of the trap. The more experience the writer has the more convinced he feels that the best materials for house drains are heavy cast-iron pipes, properly protected from rusting, laid on a firm foundation, and the joints made with metallic lead. The next important item is thorough flushing with water, and they should also be ventilated, or air-flushed. There is a great diversity of opinion on this question as to the best way of arriving at success. Some sanitarians argue, and practice, that the soil pipe should also act as the drain-ventilator, and that it is necessary to have another pipe near the trap, as shown at A, Figure 195, carried up to the roof, to let fresh air into the drains, which would then pass in the direction shown by the arrows and up the soil pipe. Others maintain that the air-current should always be in the same direction as the water or sewage-current, so that any discharges from the water-closets, or other fittings, will accelerate the ventilation by driving the air forward in the drain and up the ventilation pipe A, fresh air being drawn in at the top of the soil pipe to supply what would in some cases be a partial vacuum. If the arrows were reversed it would explain what is meant. It requires very little argument to upset this theory. In the first place the discharges down the soil pipe and drain are intermittent, so that the current of air is only carried in the same direction at intervals. Between these discharges the air-current will be found to be reversed, and pass up the soil pipe.

On testing the air in a drain it will generally be found to be slightly warmer than that outside. Anyone with even a slight knowledge of aerokinetics will understand that even one degree rise of temperature is sufficient to rarefy the air in the drains, and which would then be forced upwards by the pressure of the cold air without entering at the lowest end. A very simple experiment will prove this. Take a glass tube, say 1 or 2 inches in diameter, lay it horizontally, with a slight inclination, and hold a piece of smouldering smoke-paper near the highest end, the smoke will be found NOT to enter. If hot water is poured on the tube, or a spirit-lamp held beneath, it will be found that the air inside will become rarefied and pass through with a current strong enough to carry the smoke away; but if the smoke had been applied at the lowest end, after an interval of a few seconds, it would be found to enter the tube and pass through it. By a careful study of some of these properties, which are generally considered too trivial to be noticed, results can often be achieved by natural laws, instead of resorting to mechanical or other means, which is now becoming so common.

Figure 195.

Some people maintain that if the ventilating pipe, A, was carried to a position a few feet higher than the top of the soil pipe, it would act in a way similar to the long leg of a syphon when applied to hydraulics, and in its inverted position would syphon the air down the soil pipe and through the drain. This theory no doubt looks very well on paper, but the facts are found to be quite different when applied in practice to drainage-work. An explanation may be given by pointing out that the horizontal pipe or drain is generally in what may be considered a warm situation, and discharges of hot water through it not only raise the degree of temperature of the air, but also transmit heat to the surroundings, which again acts upon the air passing in through the inlet, expanding that, and so really assisting the ventilation. The steam, also, from hot water discharged from baths, sinks, etc, will always rise to the highest point, and it is absurd to think that warm vapour of any kind would pass downward through a drain perhaps 50 or 100 feet long, so as to then ascend a lofty ventilation pipe, simply because the pipe was placed there for that purpose. Why people should persist in saying that the principles of ventilating downward are the only right ones is a mystery. If they simply stated that they had provided two stacks of ventilating pipes, one at each extremity of the drain, and did not care which was the inlet or which the outlet, so long as the drains were properly air-flushed, no person of experience would find much fault. Some people place too much dependence on the fact that carbonic acid (one of the constituents of sewage-gas) being so much heavier than air, would gravitate to the lowest point in a drain; so that it is necessary to ventilate in that direction to remove it. A man who understood his business would not advance this doctrine, knowing that, in the first place, this gas would, if generated, become diffused with the air in the drain; secondly, that it is almost always accompanied by sulphuretted hydrogen, a lighter gas, but heavier than air; and thirdly, that if the drains are properly laid, flushed, and ventilated, these gases cannot be generated, and so it is useless and a waste of time and money to provide for something which ought not to occur.

There are some few houses where this system of drain-ventilation (which is sometimes called the syphon system) is carried out, and where specially constructed cowls are fixed on the top ends of the soil and ventilation pipes to insure a current of air passing through. It is usual in these cases to use on one pipe a cowl so made that when the wind blows past it part is caught and induced to pass down and into the drain, the cowl on the top end of the other pipe being so constructed as to act as an air-extractor, and there is no doubt they answer admirably when the wind blows. But it often happens that there is little or no wind, and when this is the case the question may be asked, Are not these cowls an obstruction to the free passage of the air through them? and if so, would not open ended pipes answer just as well? If anyone had seen the number of mechanical (in distinction to fixed) cowls that the writer has, lying about on the roofs of houses, the owners in happy ignorance, and the system of drain-ventilation still going on in a satisfactory manner, they would come to the same conclusion that he has: that cowls are worthless, excepting under very peculiar circumstances.