SEWERS. Covered drains or sewers of great size, and of very solid construction, still exist under the streets of some ancient Roman cities, and especially of Rome itself. In modern times the sewers of London stand unrivalled for extent and excellent construe tion, although much yet remains to be done to render them adequate to the necessities of an immense and constantly-increasing popu lation. Full one-third of the sewers in the City of London were made in the ton years preceding 1834. Until 1847 there were seven Boards of Commissioners of Sewers in the metropolis, each having control over a par ticular district. It was often found that great inconvenience and expense resulted from the want of agreement between these boards ; and at length a Metropolitan Commission of Sewers was appointed by Act of Parliament. The Commission commenced its labours in 1847. It has control over the whole of the metropolis except the City ; but the City Comission has agreed to act as far as possible in conjunction with it.
All the sewers constructed by the Metro politan Commissions of late years are of such dimensions as to allow a man to pass through them, for the purpose of inspecting or cleansing them. The smallest sewers in the City of London division are about four feet three inches high by two feet three inches wide, the dimensions being increased according to cir cumstances, up to ten feet by eight feet, The water brought down by the Illeet Elver is conducted from Holborn Bridge by two sewers, from twelve to fourteen feet high, and six feet six inches wide, one on each side of Parringdon Street. These sewers unite, towards the mouth, into one passage about eighteen feet by twelve.
The bricks and cement in the London sewers are of the best quality; but the form of the sewers varies greatly ; some engineers preferring one form, some another. Nearly all the modern sewers however, have an : inverted arch at the bottom ; and curved forms are also generally given to the sides and the top. In the metropolitan sewers the inclina tion varies from a quarter of an inch to an inch and a quarter in ten feet. In some cases it is very difficult to obtain sufficient inclination in a sewer, and still to make it deep enough to drain the basement story of neighbouring houses ; which may be readily conceived from the fact that some parts of London are below the level of high water. Wherever it is practicable, new sewers are built at a considerable depth from the surface. • The depth of that in Watling Street, in the City of London, which is an extraordinary case, is from thirty-three to thirty-five feet.
In many cases however there is a space of not more than three feet between the surface of the roadway and the crown of the arch of the sewer. Where private drains are to be laid into a sewer for the purpose of draining houses, it is necessary that the lowest pave ment of the floor of the building be at least four feet above the level of the sewer ; because the house would otherwise be liable to be flooded with • water from the sewer, when unusually full. Drains leading from private houses are usually of a circular form, and nine inches in diameter, though some are of greater size.
The construction of galley-holes and shoots for conducting the surface drainage of the streets into the sewers varies considerably in different parts of the metropolis. It has been usual, until very recently, to make apertures, called Man-Holes, at convenient distances, to enable persons, when necessary, to enter and cleanse the sewers, ; these apertures in some instances, are built in the form of oblong shafts of brickwork, up to within about eighteen inches of the surface of the road, and covered with east-iron plates, over which the roadway is made good. A later and better plan is that of side entrances. These are passages extending from the side of the server to the foot pavement, through which they may at any time be entered by unlocking and opening a cover or trap-door consisting of pieces of flag-stone mounted in an iron frame. When a person enters the sewer by one of these openings the cover is held open by a self acting catch, and an iron grating which admits light and air rises into its place, and serves to prevent any passenger from acciden tally falling in. These entrances greatly facilitate the operation of flushing or cleansing the sewers. The necessary body of water for flushing is produced by simply accumulating the ordinary contents of the sewer, which may be done either by a cast-iron grate, fitting closely to a frame-work built into the sewer, and rising to the height that the head is required to be, or by a drop plank or gate of the same material, sliding up and down in nearly vertical grooves. In either ease the apparatus may be managed, by means of the side entrances, with the greatest facility. The ordinary run of water in the sewers has hitherto been found sufficient for the purpose ; but in case of its proving otherwise, a supply of water for flushing might be readily procured from the water companies.