constant quantity, being practically the same as the water supply. It is therefore an easy matter to construct sewers of the proper size to deal with the sewage. Under these circumstances the sewers need only be of a comparatively small size. All rain water from roofs, yards, etc., must be conveyed by separate pipes either into a distinct set of underground pipes or, when the gradients are sufficient, into channels at the sides of the streets. These, at convenient points, discharge into the stream which forms the natural drainage bed of the district. The sewers proper convey the sewage to the sewage farm, where it is dealt with before being discharged into the river. Under these conditions, the sewage is not only fairly constant in volume but is also uniform in composition when thus protected from dilution with storm waters, and its purification is therefore a much easier matter than when it is very concentrated one day and very dilute the next. Other advantages of the separate system are that the sewers, being smaller and with smooth walls, are more uniformly filled and more frequently flushed than the large brick sewers of the combined system; and lastly the cost of the separate system is very much less. Of course the system has some disadvantages: for instance, the fact that each house necessarily has two sets of drains obviously gives opportunity to careless builders to connect pipes with the wrong system. The flushing effect of the storm waters on the sewers is lost, and, in addition, the water that runs directly into the river is occasionally, especially after a period of drought, nearly as foul as ordinary sewage.

In the combined system it is necessary to construct sewers of a sufficient size to take a large part of the rain falling during heavy storms. Storm overflows are generally provided, consisting of channels communicating with the main sewers at a higher level than that of the ordinary sewage. Unless this precaution is adopted a sudden storm will deluge the sewage farm with a vast quantity of very dilute sewage, which it is out of the question to attempt to treat in the ordinary way. In these cases it is usual to provide large coarse filter beds of coke, breeze, or such cheap material as may be available, into which the water

may pass. From these the effluent is passed directly into the river.

In the construction of sewers the following are the chief conditions to be fulfilled:

1. The flow of sewage should have as uniform and constant a velocity as possible. To achieve this sewers up to

Fig. 74.-EGG-SHAPED SEWer.

18 inches in diameter are generally made of well-glazed earthenware socket pipes. Other materials include cement, concrete, and iron. Iron pipes have the advantage of being made in 9 or 12 feet lengths, and therefore fewer joints are necessary. For larger sewers the egg shape with the small end downwards is now universally adopted, and they should be made of well-burnt, tough, impervious bricks set in Portland cement and laid on

concrete. The advantage of this shape is that when the depth of the stream is diminished the friction is also relatively reduced, instead of being as in every other form of sewer relatively increased.

2. The velocity of the flow should be sufficiently great to keep in motion any solid particles, but not more than this, as the greater the velocity is the greater the wear and tear of the masonry. The regulation of the velocity is attained by adapting the fall or gradient to the size of the sewer. Sewers of 1 to 2 feet in diameter should have a gradient sufficient to produce a velocity of not less than 23 feet per second, and the velocity in larger sewers should never be less than 2 feet per second. If the velocity exceeds 4 feet per second it is found to wear away the brickwork very quickly, especially if the sewage contains much sand or grit. The larger the sewer the less is the gradient required to produce the same velocity, provided that the volume of sewage in each is proportional to the square of the diameter. Thus

(1) A fall of 20 feet per mile, with a sewer 1 foot in diameter,

will all produce the same velocity of flow if the volume of the sewage in (2), (3), and (4), is respectively 4 times, 25 times, and 100 times greater than in the first case (Parkes).

3. The junctions and bends must be made in such a manner as to prevent the deposition of sediment. For this reason all junctions must be made as obliquely as possible, and so arranged that the flow in the tributary sewer is in the same direction as in the main. If the sewer curves, the radius of the curve should not be less than ten times the diameter of the sewer. Properly constructed and regularly flushed sewers should never require the removal of deposits, and the fact that the sewers of all our large towns require the constant services of labourers forms a serious indictment against their method

of construction. To enable a sewer to be properly inspected manholes should be provided at intervals of not less than 100 yards, and are particularly advisable at all junctions and bends. Manholes are shafts sunk from the surface of the road to the sewer, and end in a chamber built of brickwork set in cement. The sewer and its tributary meet in this chamber, being continued along the floor in the form of half pipes set in concrete. A man entering the inspection chamber will therefore be able to tell at a glance whether there is any deposit at the junction. These manholes may also be utilised for flushing the sewers. In

TRIBUTARY

MAIN SEWER

Fig. 75.-PLAN OF FLOOR OF INSPECTION CHAMBER.

some towns it is very difficult to allow a sufficient gradient to the sewer, and consequently deposits are bound to occur unless arrangements are made for periodical flushing. The best method of preventing the formation of deposits is to place automatically discharging siphon flush tanks at various points. These must be supplied with water by a tap connected with the town supply at such a rate as to cause the sudden discharge of the contents at regular intervals of about 12 hours or less.

Ventilation of sewers. Before discussing the ventilation of sewers it is important to clearly define the distinction between sewer air and sewer gas. By sewer air is meant

the air that is present in the sewer under ordinary conditions. Recent investigations have shown that sewer air rarely contains bacteria from the sewage with which it is in contact. In fact the results of some of these experiments show that although sewer air contains more CO2 and organic matter than ordinary air, the number of bacteria may be less than in ordinary air. Sewer gas on the other hand is the gas that is given off in large quantities by stagnant sewage or putrefying deposits. This probably contains various micro-organisms which are most likely the cause of the many morbid conditions that are induced by exposure to sewer gas. The most generally recognised effects are vomiting, diarrhoea, headache, chronic bloodpoisoning, and diphtheritic sore throats.

The possibility of the sewer air being contaminated by sewer gas should cause every one to realise the importance of thoroughly ventilating sewers; and at the same time to avoid every chance of it entering houses in a comparatively undiluted condition. Not only is it of the utmost importance to have numerous openings in sewers for ventilation, but also to provide a free escape for sewer air in case of a sudden increase in its pressure, such as may be produced by the rapid increase in the volume of sewage which takes place during the early part of the day and during a heavy rainfall, or by the blowing off of steam from boilers into the sewers. Variations in the temperature of the outside air also cause movements of the sewer air, and a fall in the barometric pressure causes the gases dissolved in the sewage to be given off.

The simplest and most efficient plan of ventilating sewers is to provide openings from the surface of the road into the sewer at intervals of not more than 100 yards apart. These should be placed as nearly as possible in the centre of the road, and answer their purpose best when they form part of a manhole and inspection chamber. The opening must be covered with an iron grid, and a tray placed beneath in such a way as to catch the mud and stones that may fall through, and at the same time allow of free passage of air around it. This dirt-box should be capable of removal from the road. Street gullies should always