In the execution of the street sewers (the sub-mains, in fact, of the sewerage of a town), care must be taken that their dimensions should be sufficient to carry off any occasional storms ; that means should be provided for the easy examination and repair of the works ; and that no obstacle should be presented of a nature to provoke a deposit of the matters brought from the houses, or from the streets themselves. The junctions of the house sewers should be made with curves of considerable radius ; the surface waters from roads and streets must be received in trapped ceespools, so constructed as to intercept all solid matters, and to collect the fine mud washed from those surfaces in au available form, because it is often of commercial value as a manure ; the ventilation of the sewers must be effected in such wise as not to inconvenience the dwellers in the neighbouring houses, and for this purpose it is preferable to insert ventilating holes in the middle of the roadway, whilst the aide gullies should be care fully trapped; and aide entrances with flushing gates, or other machinery, must bo provided according to the local peculiarities of the sewer. The principles which regulate the size and the materials of street sewers are, firstly, that the smallest possible frictional area should be presented to the flowing current, and that the water should flow without being under pressure; secondly that the actual channel for the sewerage should be impermeable, but there are some cases in which it is actually desirable that the sewer should give free passage to the land waters at levels above the line of their own maximum flow. Now of all forms of cross section, a semicircle is the one which presents the smallest frictional area ; and it may be laid down as a law that, in most cases, the impermeable cylindrical stone-ware pipes are the best materials which can be employed for street sewers, pro vided their dimensions are not required to exceed 2 feet in diameter ; beyond that diameter, however, it is difficult to obtain pipes of a sound character, and they become more expensive than brick culverta of the same area. There is another consideration which also requires to be taken into account, in cases wherein the diameter of the water course exceeds 2 feet, namely, that it frequently becomes necessary to cleanse them by hand labour. A man can crawl through an oval channel 2 feet wide by 2 feet 6 inches high, so that those dimensions may be taken as the minima ones for street sewers in districts where it would be difficult or objectionable to open the roadways; and sewers of such forms are far more conveniently made in brickwork, than in stone-ware pipes. Brickwork sewers, moreover, present the advantage of allowing lateral junctions to be made more easily than would be the case with stoneware pipes. The sizes of street sewers usually adopted, when the requisite dimensions exceed an area equal to that of a semicircle of 2 feet in diameter, are (the transverse section being now made of an egg shape) either 3 feet 3 inches deep, by 2 feet wide at the minor axis; 4 feet, by 2 feet 6 inches; 4 feet 9 inches, by 3 feet ; 5 feet 6 inches, by 3 feet 3 inches. The dimensions of such sewers as the Fleet sewer of course cannot be brought under any normal law, for it drains an area of not less than 440) acres, half of which is covered by houses, and has been known to run with a stream of not lea than 106 feet superficial area. The beet velocity of the flow in street sewers is about I to 2 miles per hour, and the rate of inclination of the bed should as far as possible bo made equal to 1 in 240; below that inclination it becomes necessary to flush the sewers from time to time, in order to keep them clear. The junctions of street sewers with one another should be made with their centre lines of invert on the same level ; the junctim a of these sewers with the main sewers should, however, take place so that the centre line of the latter should be a few inches below those of the branch sewers.' As frequent allusion has been made to the land waters it is often requisite to remove by the means of sewers, it may be as well to observe that unless draina should be laid down for the express purpose of relieving the subsoil of those waters, or unless there should exist some natural outlet for them, the main and sub-main sewers should bo made either with inlet holes or of partially permeable materials above the level of the ordinary storm flow of the sewerage. Sufficient attention is rarely paid to this condition of the soil of sandy or of gravelly districts, and it frequently happens that, in towns wherein a good system of house sewerage exists, the basements are flooded by the land waters, because the latter cannot find their way into the imper meable main sewers. When the feeding grounds, so to speak, of the springs are extensive, the best course certainly is to execute an inter cepting drain, so as to separate the inhabited area of the formation from the open soil of the country. An instance of the evil thus alluded to is to be found at Southampton, and another in the quartier Montmartre in Paris ; and in both of them the inundations of the basements, after long-continued wet weather, are often productive of serious evils. Perhaps it may be desirable to cite, as an additional reason for preferring the construction of a special system of intercepting or land drainage for the removal of the springs, that when they are removed by the agency of the sewers there is a possibility that the foul waters flowing in the latter may permeate the land around them at certain seasons. As a general principle it would unquestionably be
preferable to confine the drains and sewers to their respective functions ; but in this, as in all casea connected with practical engineering, local considerations may often render it prudent to modify absolute theoretical laws. It is worthy, however, of especial remark that the formations which are thus exposed to become charged with underground waters, are characterised by the frequency and the violence of the typhoidal fevers which prevail amongst the town populations located upon them. Aa a einitary measure, the drainage of such lands is nearly as important as the sewerage of the houses and streets can be.
The main drains of a system of town sewerage are of course to be calculated as to their dimensions upon the same principles as the sub mains ; that is to say, they must be proportionate to the area, the rain flow, nud the amount of sewerage, to be .dealt with The waters corning into the mains are usually animated by a velocity rather in excess of the one which would be requisite to maintain the rate of flow necessary to preserve a clear channel for the larger volume of water in the united stream ; and from the ordinary laws of hydro dynamics it follows that the inclinations of the main sewers may with out inconvenience be made less than those of the sub-main sewers. It is, however, desirable to keep the inclinationa of the former at least at the rate of 1 in 500, though occasionally when great facilities for examination and repair may exist, that rate may be reduced, without serious inconvenience, to 1 in 1000. Great precautions are required in the construction of main sewera to secure an efficient ventilation ; and, wherever it is possible so to do, storm overflows should be pro vided. This latter precaution of course can only be adopted when the discharge of the contents of the main sewer takes place by gravitation ; if the discharge should take place by artificial means, it will be found necessary to construct the main sewers of dimensions able to contain any occasional accumulation of storm waters.
Hitherto the final discharge of sewerage waters has been effected by merely pouring them into any natural water-course, to the great injury of the latter, and to the great disgrace of the authorities who pretend to watch over the sanitary interests of the country. It is true that some very earnest attempts have been made to apply the sewerage to the purposes of agriculture, both in a solid and in a liquid form ; and it is equally true that hitherto those attempts have not beeu successful in a commercial point of view. Nevertheless, the importance of the subject, as proved by the gradual contamination of the Thames, for instance, is ao great that no mere money considerations ought to be allowed to stand in the way of the compulsory application of some efficient measure for the deoderieation and purification of town sewerage before its discharge into any superficial water-course. The experiment tried at Leicester, of precipitating the solid matters from the sewerage, has proved at least that the operation can be effected at a moderate rate per head of the inhabitants of a town ; the results of the Rugby and Watford experiments, of pouring the sewerage in a liquid form over agricultural lands, although far from successful up to the present, have not been of a nature to discourage further attempts to dispose of that fluid by irrigation, especially in agricultural districts. In the case of Rugby the sewerage is distributed, over a mixed area of amble and of pasture lands, by a system of cast-iron pipes and move able hose, which has necessarily increased the first cost of the distri buting appal atus and of the subsequent working ; if, instead of these costly follies, the sewerage had simply been pumped up to a high level, and a surface irrigation by gravitation had been thence effected, the results would in all probability have been very different from those which were actually obtained.
Some idea of the importance of the sewerage works lately executed in England may be formed from the facts that there is now hardly a town wherein they do not exist in a more or less perfect form ; and that the average cost has latterly been at the rate of at least 1/.
per head of the population of the towns. The average sewera-rates of well-drained towns seem also to be about 21 per cent on the rental of the houses.
It may be added that in the London sewerage there appears to be present about 1 ton of solid matters to every 266 tons of water ; but in very unfavourable cases the proportions of solid to liquid may increase to 1 in 96, or even to 1 in 36; the proportion of 1 in 66 is considered to be the largest which is consistent with the safe working of pipe sewers, in which the fluid can move with a velocity of 3 feet per second.
(Consult Reports of Health of Towns Commission,' and the various Blue Books on the London Sewers ; Transactions of Civil Engineers ;' Annales des Ponta et Chaussees: &c.)