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run under the houses themselves, Uicy should be jointed in . 



and under til circumstances -they should be without junction* at right 



inrirt The highest point of the lewer should be laid, if possible, at 



2 foot at least below the lowest part of the basement story it in 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 

 by the house they should be earned off by a special system of drains. 

 In order to guard against the flooding of bascmrnU, by any accidental 

 accumulation of waters in the main sewers, it is dwirable 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 any current, or of 

 any obstruction, able to produce a deposit near 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 end* 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 cf 

 the late General Board of Health was to the effect, that what was then 

 called " combined back drainage" (or the connection of a number of 

 small bouse sewers 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 ia 

 generally abandoned, and the house sewers are almost always carried 

 directly from each house into the main sewers in front of them. 



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 obttacle 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 cesspools, so constructed as to intercept all 

 solid matters, and to collect the fine mud washed from those surfaces 

 in an available form, because it is often of commercial value as a 

 manure ; the ventilation of the sewers must be effected in such 

 wice 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 side gullies should be care- 

 fully trapped; and side entrances with flushing gates, or other 

 machinery, must be 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 hind 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 culverts 

 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 stone-ware pipes. The sizes of street sewers 

 usually adopted, when the requisite dimensions exceed on 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 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 drain* on area of not less than 440 i acres, half of 

 which is covered by houses, and has been known to run with a stream 

 of not less than 106 feet superficial area. The best velocity of the 

 flow in street sewers is about 1 J to 2 miles per hour, and the rate of 

 inclination of the bed should as far as possible be 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 junctiois 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 laud waters it is often 

 requisite to remove by the means of sewers, it may be as well to 

 observe that unless drams 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 ; 

 in,' in this, as in all cases 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. 

 As a sanitary 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 ram 

 flow, and the amount of sewerage, to be dealt with The waters 

 coming 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 inclinations 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 ore required 

 in the construction of main sewers 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 been 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 so great that no mere money considerations ought to be 

 allowed to stand ill the way of the compulsory application of some 

 efficient measure for the deodorisation and purification of t"u 

 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 result* 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 

 arable 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 appaiatus 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 hod been thence effected, the 

 results would in all probability have been very different from those 

 whii-li were actually obtained. 



Sonic 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 Veen at the rate of at least 11. 



