The first-class sewers of the modern build rarely exceed 50 inches by 30 in internal dimensions; the second class, 40 inches by 24; the third, 30 inches by 18.
Smaller class or branch sewers, from No. 4 to No. 8 inclusive, also form part of the great subterranean filth-channels of the metropolis. It is only, however, the three first-mentioned classes which can be described as in any way principal sewers; the others are in the capacity of branch sewers, the ramifications being in many places very extensive, while pipes are often used. The dimensions of these smaller sewers, when pipes are not used, are—No. 4, 20 inches by 12; No. 5, 17½ inches by 10½; No. 6, 15 inches by 9; No. 7, 12 inches by 7½; and No. 8, 9 inches by 6.
These branch sewers may, from their circumscribed dimensions, be looked upon as mere channels of connection with the larger descriptions; but they present, as I have intimated, an important part of the general system. This may be shown by the fact, that in the estimates for building sewers for the improvement of the drainage of the city of Westminster (a plan, however, not carried out), the estimated, or indeed surveyed, run of the first class was to be 8118 feet; of the second class, 4524 feet; of the third, but 2086 feet; while of the No. 5 and No. 6 description, it was, respectively, 18,709 and 53,284 feet. The branch sewers may, perhaps, be represented in many instances as public drains connecting the sewer of the street with the issue from the houses, but I give the appellation I find in the reports.
The dimensions I have cited are not to be taken as an average size of the existing sewers of the metropolis on either side of the Thames, for no average size and no uniformity of shape can be adduced, as there has been no uniformity observed. The sewers are of all sizes and shapes, and of all depths from the surface of the streets. I was informed by an engineering authority that he had often seen it asserted that the naval authorities of the kingdom could not build a war-steamer, and it might very well be said that the sanitary authorities of the metropolis could not build a sewer, as none of the present sewers could be cited as in all respects properly fulfilling all the functions required. But it must be remembered that the present engineers have to contend with great difficulties, the whole matter being so complicated by the blunderings and mismanagement of the past.
The dimensions I have cited (because they appear officially) exceed the medium size of the newer sewerage, the average height of the first class being in such sewers about 3 feet 9 inches.
Of the width of the sewers, as of the height, no precise average can be drawn. Perhaps that of the New Palace main, or first-class sewer, 3 feet 6 inches, may be nearest the average, while the smaller classes diminish in their width in the proportions I have shown. The sewers of the older constructions nearly all widen and deepen as they near the outlet, and this at no definite distance from the river, but from a quarter of a mile or somewhat less to a mile and more. Some such sewers are then 14 feet in width; some 20 feet, and no doubt of proportionate height, but I do not find that the height has been ascertained. For flushing purposes there are recesses of greater or less width, according to the capacity of the sewer, where sluice-gates, &c., can be fixed, and water accumulated.
Under the head of “Subterranean Survey of the Sewers,” will be found some account of the different dimensions of the sewers.
The form of the interior of the sewers (as shown in the illustrations I have given) is irregularly elliptical. They are arched at the summits, and more or less hollowed or curved, internally, at the bottom. The bottom of the sewer is called the “invert,” from a general resemblance in the construction to an “inverted” arch. The best form of invert is a matter which has attracted great engineering attention. It is, indeed, the important part of the sewer, as the part along which there is the flow of sewage; and the superior or inferior formation of the invert, of course, facilitates or retards the transmission of the contents.
A few years back, the building of egg-shaped, or “oviform” sewers, was strongly advocated. It was urged that the flow of the sewage and the sewer-water was accelerated by the invert (especially) being oviform, as the matter was more condensed when such was the shape adopted, while the more the matter was diffused, as in some of the inverts of the more usual form of sewers, the less rapid was its flow, and consequently the greater its deposit.
What extent of egg-shaped sewers are now, so to speak, at work, I could not ascertain. One informant thought it might be somewhere about 50 miles.