Until a very recent ,period, the subterranean channels of London were exclusively used as drains for the removal of surface, subsoil, and ordinary house waters; and the discharge "of any filth or soil into any common, or public drain, or sewer" was even, by the Act 57 Geo. III. c. 29, made punishable by a fine. Subsequently to that period, how ever, the universal application of the water closet system has forced the various authorities connected with the sewers to tolerate in the first place, and finally to regulate, the discharge of the excreta of the inhabitants of towns into the sewers. To such an extent does this now take place, that the modern use of those subterranean has become of infinitely more importance than the original one, and the word " sewerage," or sewage, has been iuvented for the purpose of expressing the waters employed for the removal of house refuse of the description alluded to. Unfortunately the legislature has hitherto neglected to direct its attention to the evil produced by the new system, by thus discharging, eventually, the whole of the town refuse into the natural water courses of the country; and though no doubt many of the details of the sewerage of our towns have of late years been much improved, much still remains to be done, not only for the purpose of securing the purity of the rivers and streams, but also for the useful application of the fertilising matters now so sadly wasted.
One advantage appears to have resulted from the discussions which have taken place with respect to the legislative, administra tive, and executive measures required to meet the case of the metro politan sewerage, namely, that at last many of the exclusive theories propounded by the admirers of hydraulic engineering, as it is not practised by eminent hydraulic engineers, have been set aside ; whilst all that was good in the technical modes of operation introduced by this new school has been retained. At the present day the principles most generally admitted as being applicable in designing a complete system of town sewerage may be stated to have been elicited from the discussions thus referred to, and to be as follows, in all normal eases at least. It must, however, be understood that they are based upon the supposition that the town in question is in a thriving condition, and likely to double its population in fifty years; that its relief is such as to afford tolerably favourable rates of inclination in the main sewers ; and that there is a good water supply to every house. The rain-flow to be provided for from the paved and non-absorbent parts of the town, may be taken at about inch in 24 hours if proper storm overflows can be obtained; that from open gardens, cultivated lands, &e., may vary according to the nature of the soil, from # inch to an inch per day Under-ground springs must be allowed for, if they should exist in any serious quantities, as is frequently the case in towns situated upon the outcrop of a geological stratum, or upon a bed of permeable materials surrounded by high lands able to drain into them. In some cases, the latter condition may even require that a system of intercepting, drains should be formed, so as to isolate the area of the town from the hydrogmphical basin surrounding it, and to confine the sewerage operations to that especial purpose.
These conditions premised, before settling the dimensions of the main sewers, it is necessary to divide the town into sections corresponding with the great physical divisions of the district, and to ascertain for each of the latter : 1, the area to be relieved ; 2, its actual and prospec tive population ; 3, the amount of the house sewerage it would be likely to furnish (this is usually at the rate of 5 or 7 cubic feet per individual, and it is found that at least one-fourth of that quantity is discharged betweeu the hours of eleven and one, so that it is advisable to calculate the house sewerage at nearly 1 cubic foot per head per hour); 4, the rain-flow ; 5, the length of the sewer, and the inclination it would be possible to give to its invert, because the two last named conditions will affect the dimensions of the cross section. As to the house sewers, it is useless to attempt to proportion their area and fall to the quantity of waters they may discharge, because it has been found practically that the public in general is so careless in its manner of treating those sewers, that anything less than a 6-inch pipe drain will infallibly be choked within a very short time. ,50 long as the pipe drains thus referred to run under the houses themselves, they should be jointed in cement, and under all circumstances they should be without junctions at right angles. The highest point of the sewer should be laid, if possible, at 2 feet at least below the lowest part of the basement story It is in tended to drain ; and it would be desirable to make the longitudinal fall about 1 in 65 ; the house sewers should be made perfectly impermeable, and if any land springs should happen to occur in the surface occupied ty the house they should be awned off by a special system of drains. In order to guard against the flooding of basements, by any accidental accumulation of waters in the main sewers, it is desirable that the connection between them and the house sewers should be effected at a small height above the upper line of the invert of the main sewers ; but care must be taken to prevent the formation of ail' current, or of any obstruction, able to produce a deposit neer the point of junction. Especial precautions must be taken to trap all communications with the sewers which might otherwise admit the escape of gases into the house ; all rain.water pipes should discharge their waters into the upper ends of the sewers, if possible ; and under some circumstances the rain-water pipes may be made to serve as ventilators to the sewers, when their joints' are remarkably well made, and the heads are fixed above every opening of the house. One of the favourite theories of the late General Beard of Health was to the effect, that what was then called " combined back drainage" (or the connection of a number of small house Bowen at the back of the blocks of houses, with other small sewers of an area barely sufficient to discharge the united amounts of sewerage), was the cheapest and best method of removing those waters; but so many serious inconveniences have been found practically to attach to this system, that at the present day it is generally abandoned, and the house sewers are almost always carried directly from each house into the main sewers in front of them.