Metropolitan Sanitary Commission, that where 6,688 yards of foul deposit had been removed by flushing it was calculated that as the whole cost of removing it by hand labour would have been £2,387, while the cost of putting up the inside apparatus and flushing gate was £1,203, and the cost of men's time £644 12s. 7d., there was thus a saving of £539.

Pipe sewers are sometimes mechanically cleansed by dragging a chain through them and afterwards well flushing.

(138) Dead Ends of Sewers.

No sewer should terminate in a dead end, that is to say in a culde-sac, without ventilation, it is in the dead ends that the worst forms of gas and deposits of a foul kind are apt to collect. Dead ends where possible, should be done entirely away with by connecting them with the general sewer system. In the writer's district, where the dead ends of two sewers have been some 50 feet apart, the dead ends have been connected by means of a 12 inch pipe with good results, the 12 inch pipe not carrying sewage but simply allowing the air to circulate. It is a good plan to place an automatic flush tank at dead ends, so that every 24 hours or oftener the sewer will be properly flushed out, In some instances the writer has recommended a ventilating shaft run up the nearest house, the upper end well above the eaves, and this has answered. The combination of ventilation and automatic flushing ought in all cases to keep a dead end free from danger.

(139) The Size of Sewers. The Separate System.

When we come to the "separate system "-that is where the sewers only carry diluted sewage, and the storm water is so far as practicable kept out of the sewers-in this case the main sewers may be of pipes properly jointed and of very moderate size. It will give a good illustration of the large area a comparatively small pipe will drain if reference be made to an old experiment of importance in the early history of drainage. The Metropolitan Commission of Sewers initiated experiments by placing pipes within the large sewers of the metropolis. Mr. Hale laid in the sewer running through Upper George Street from the Edgware Road to Manchester Street where it falls into the Scholars' Pond sewer, a 12 inch pipe drain, 560 feet long, and by proper appliances com

pelled the sewage that would otherwise have flowed into the Upper George Street sewer, a sewer 5 feet 6 inches by 3 feet 6 inches, to flow through this pipe; the area thus drained was equal to nearly 44 acres of houses. During ninety-six hours of rain the pipe sewer was never more than half full, and it ran bore full on only one occasion for a brief period during an exceptional storm. It was also shown in the same series of experiments that the water flowing through a pipe of the kind possessed great mechanical power.

"On one occasion," said Mr. Hale, "I had the sewer in Upper George Street cleaned out immediately below the pipe, and then caused a quantity of deposit, consisting of sand, pieces of bricks, stones, mud, &c., to be put in the head of the pipe; the consequence was the whole of the matter passed clear through the pipe (560) feet long), and much of it deposited some distance from the end, on the bottom of the old sewer. When the pipe was flowing nearly half full, two pieces of brick, one weighing 12 lb. and the other 1 lb. 13 oz., were impelled by the force of the water through the whole length of pipe and struck the legs of the man at one end of the pipe with considerable force. A live rat was also washed out with great violence through the pipe and struck the legs of a man with such force as proved the rat had no control over its own motion." 1

One of the first towns that used the separate system was Slough; a system of pipe drains was laid slightly above the level of the old deep sewers. About 8 miles of pipe sewers, varying from 9 inches to 15 in diameter, were laid, 1,200 yards of the outfall sewer being cast iron, 15 inches in diameter, jointed with lead and gasket. The subsoil water is kept out of the pipes as much as possible and goes with rain water along the old channels. Besides Slough, Reading, Oxford, Halstead and a few other places in this country have adopted the separate system; in America, Memphis on the Mississippi, and Pullman near Chicago, are notable examples of separate sewerage.

(140) The Shape and Construction of Sewers.

Small sewers made of glazed pipe from 6 inches to 18 inches in diameter are circular, and there is little or no benefit in making

1 Report of the General Board of Health on the Water Supply of the Metropolis,

P. 186.

these sizes any other form; the pipes are made in short lengths and are usually jointed by passing the end or spigot into the socket or faucet of the next, the spigot end of course is laid in direction of the flow, that is down hill. In certain patent joints such as Stanford's no cement is necessary; in the ordinary pipes, a ring of gasket or tarred hemp is often forced in the space filled with cement. Larger sewers are built in the eggshaped section shown in the diagrams—

Fig. 36 is the old form, Fig. 37 is the new form; the latter has a sharper invert. The advantage of this form is that in dry weather a narrower channel is left for the small stream of sewage, and hence deposit is less likely to occur.

FIG. 38.

In Fig. 38 is shown a section of a sewer made of bricks moulded so as to suit the curved structure; the section shows two courses

of brick with an invert of glazed earthenware, the whole imbedded in a concrete setting.

Sewers are sometimes made wholly of concrete, the composition of the concrete being five to seven of sand and gravel or broken stone to one of Portland cement. A mould is first formed, the concrete run round, and when the concrete has set the mould is removed.

(141) Capacity of Sewers.

It has been already described that the capacity of the London sewers which convey the sewage to the outfall is calculated on the assumption that the sewage averages 311 gallons per head, and that they will also dispose of inch of rainfall. In sewering the town of Dantzic, in which the geological formation is chiefly sand, and the district flat, Mr. Baldwin Latham provided for a inch of rainfall every twenty-four hours, and 6 cubic feet of diluted sewage. No fixed rule can be given, because the water supply of various towns is SO different in quantity and the geological formation is different. In places having a very impervious soil, sewers will have to be larger than in the more pervious strata. The average sewage of Birmingham is 50 gallons a head, of Cardiff 66, of Croydon 76, of Plymouth 66. All these are water-closet towns. On the other hand, Preston with a small number of waterclosets only produces liquid sewage at the average rate of 24 gallons per head. It is however accepted that speaking generally a main drain-sewer should be constructed on the assumption that there will be 5 cubic feet (314 gallons) of sewage per head to be dealt with in the twenty-four hours, and a certain proportion of rain, depending on locality and area. The careful observations carried on at various meteorological stations and tabulated by Mr. Symons, F.R.S., give valuable rainfall data for every district in the British Islands, and it will be useful to remember that 1 inch of rain is equal to 3,630 cubic feet, or 22,622 gallons per acre.

The size of sewers vary so much that it is evident engineers have in practice applied no general principle; a drain-sewer will take sewage, subsoil water, and storm water: the amount of the subsoil water will vary in different localities; the sewage is mainly dependent in its volume on the water supply used or wasted, and the storm water of course varies very much; it is however generally admitted that a main drain-sewer intended to receive all the sewage of a

thickly populated square quarter of a mile, with a water supply of 20 gallons a head, if the average rainfall be equally distributed throughout the year, need have no larger area than 4 square feet, but that since storms must be considered, then double that quantity, namely 8 square feet of sectional area will be sufficient.

(142) Steep Gradients.

Special arrangements have often to be made where, from the conformation of the ground, steep gradients are a necessity; the usual way is to make a tumbling bay, the sewer delivering from above having a flap valve, the lower sewer being disconnected from the upper by means of a manhole; in this way there can be no pressure of gas, and the flap will keep any sudden rush of wind back; this is called "ramping" a sewer.

Difficulties are often met with in laying sewers from the kind of soil in which they are laid; for instance, in loose sandy soil the subsoil water often gets into the pipes before the cement is set, and the cement is in this manner washed away; the remedies proposed are either the use of cast iron pipes, or the use of a subsoil drain and pipe rest (manufactured by Messrs. Brooke and Sons, Huddersfield), this drain is in the form of the letter ; it is laid and jointed at the bottom of the trench like an ordinary pipe sewer This has the effect of draining away and therefore lowering the subsoil water, upon this subsoil drain the true sewer is laid, and it rests on a good foundation undisturbed by water or sand.

(143) Inverted Syphons.

Sewage has sometimes to be conveyed along the bed of the river; in such cases it is usually carried by an inverted syphon, constructed of iron pipes. The town of Dantzic, intersected by navigable channels and surrounded with fortification ditches over which no permanent work can be carried, necessitated the use of syphons there as an integral part of the sewer system, which was designed by Mr. Baldwin Latham; and in Mr. Latham's work on sanitary engineering these syphons and the various modifications of them are very clearly described.

It is not improbable that the sewage of the south side of the metropolis, now discharging at Crossness will have to be brought over to the north side, if so, this will be effected by a syphon carried on or beneath the Thames bed.