Many forms of large sewers have been adopted, but that which appears to be best adapted for the purpose, is the egg shaped, which was introduced by Mr. Roe ; the older forms having mostly a semicircular crown, with upright side-walls, and a semicircular or flat segmental invert. The main improve ment effected by the egg-shaped sewers, consisted in narrow ing the invert, thereby contracting the lower part of the channel, and increasing the depth of flow, by which means the velocity and scour was much improved ; the same quantity of water which moved but sluggishly when spread over the wide surface of a flat invert, being now concentrated in a narrow channel, was made to move with accelerated flow ; in short, this alteration had a similar effect to that produced by the employment of small pipe-sewers. Another advantage obtained by this form, was the attainment of greater height with a given area, which enabled men to pass through them with greater facility; it must be confessed though, that the narrow invert is rather inconvenient to walk upon. The shape, moreover, is economical, and based on sound prin ciples as regards construction.
Many proportions, and many methods of describing the egg-shape, have been observed at different places, some being constructed with a semicircular top, flat segmental sides, and a sharp segmental invert ; the curves of the sides and invert varying according to varying proportions of height and width, and other circumstances. Others are constructed with six centres, the upper part being described with two radii, instead of being semicircular. Of all these varieties, however, the most eligible is thus described :—Let the height of the sewer be to the width as 3 to '2 ; then having described a semicircular crown with radius 1, with radius 3, equal to the height, and with the centre on the springing line, describe a segment touching the semicircle already described ; for the invert, with a radius or of the entire height, describe a circle which will be found to touch the segments just described. This form is generally appli cable, and has the advantage of being drawn to certain pro portions.
The above are the most usual forms for sewers, but occa sionally the levels will not allow sufficient height for them beneath the roadway ; and, in such cases, it is necessary for the purpose of obtaining sufficient capacity, to construct them of a cylindrical or elliptical form ; but even then, the same end may frequently be obtained by placing two or more egg-shaped sewers side by side. Where there is a large and constant flow of water, cylindrical sewers are not ob jectionable.
The above sewers, if not more than 4 feet in height, and if in good building strata, may be constructed of half-brick work, but in other cases require to be a whole brick thick, unless the sewer be very small. They are constructed mainly of stock bricks in mortar, the invert or lower portion only being laid in cement ; the inverts are usually formed in blocks in cement before they are finally laid in the sewer ; the invert consist ing of three or more blocks, according to its dimensions. Sometimes blue vitrified bricks are used for the invert; they are very hard and durable, but do not adhere well to the cement. It has been proposed to construct sewers with radiated bricks, and they probably might be used with ad vantage where the curve is sharp : the joints of common bricks are very open in such cases. Radiated hollow bricks have also been suggested, and, if moderately cheap, they might doubtless be employed successfully. • The pipe-sewers, as at present made, are of a cylindrical form; they are constructed in lengths of 2 or 3 feet, and have a socket at one end for the adjoining pipe to fit into ; sometimes, however, they are made with half a socket on either pipe, and this is of advantage in facilitating their removal, if at any time they be required to be taken up.
With the whole sockets, it is difficult to take them up with out breaking them, and it is almost impracticable to replace them correctly after removal.
If whole-socket pipes be generally used, we should ad vise the occasional adoption of half-sockets, so that in case of stoppage, or other accident, they may admit of easy removal and replacement, without either destroying the pipes or dis turbing the adjoining ones. Cylindrical pipes are made from 3 to 2 feet in diameter. It has been suggested, that such pipes should be made of the egg-shape, rather than cylindri cal ; but it must be borne in mind, that the contraction of the invert is not of so much consequence in a small pipe, where there is likely, at all times, to be a large current compared with the capacity of the pipe ; and if the pipe be generally full, we know that the circumference of a circle contains a larger area than the same extension of boundary-line arranged in any other form ; therefore, if a cylindrical pipe be full, there is less frictional surface than in any other kind of pipe of the same capacity. As, however, in present practice, the flow of water varies to a great extent at different times, it might not be unadvisable to adopt the egg-shape, supposing that they can be manufactured with the same accuracy, and at no higher cost. If they be adopted, we should advise their being made in two pieces, the lower one forming the invert, and the upper the semicircular crown ; a flange, or socket, might be formed at the edge of the bottom-piece to receive the top, and make a correct joint ; the same practice might be adopted in cylindrical pipes of a large size. The advantages attaching to this method are, that the inverts can be laid with great nicety, a matter of considerable importance, and are open to inspection after they are laid, so as to admit of opportunity of testing their accuracy ; the objections against the whole pipes, on this head, are of considerable weight. Another advantage is, that the top-half can be taken off at any time for examination or inspection of the pipes, with out disturbing the invert, or interfering with the flow.
These pipes are manufactured of stoneware, and various kinds of clay, glazed and unglazed, some being glazed on the interior, and others on both surfaces ; the stoneware, glazed on both sides, are most frequently adopted, and are found to answer best ; but there is a process of manufacture which produces unglazed pipes equally efficient as the glazed stoneware. The peculiarity of this process consists in sub mitting the pipe, when half-dry, to an extreme pressure between two polished iron surfaces, whereby a density of substance, and truth of form, is attained, which is equivalent in practice to the best glaze, so far, at least, as the flow of water is concerned. Pipes of the above materials and manu facture have a great advantage over brick-sewers, on account of the comparative smoothness and evenness of surface, which offers very little or no resistance to the current ; the amount of frictional resistance being naturally greater over the rough surffice of the brick work. In the following table will be found the result of some experiments, for the purpose of determining the advantage gained, in this respect, by glazed pipes :—