The word _sewerage_ may be taken as meaning a system of sewers carrying _sewage_ which is the fluid and feculent refuse from dwellings and their yards, &c. Sewage is generally found mixed with rain water from the surface of the streets and roofs of houses, together with the liquid waste products from manufactories,[179] and sometimes, although very improperly, with subsoil water. A good system of sewerage should embrace the whole of the following requirements:-- (1.) Each sewer should be laid at such a depth as will readily drain the basements of the adjoining buildings. (2.) Its area and gradient must be so regulated as to make it self-cleansing, and at the same time carry off effectively the maximum quantity of liquid for which it is intended.[180] (3.) Each sewer should (unless quite impracticable) be laid in straight lines and with even gradients between man- or lamp-holes, and these gradients must not be excessive, or damage may be caused to the sewer. A velocity of about 6 feet per second is sufficient. (4.) Sewers must be laid at proper levels in respect of their intersection with each other, bearing in mind that they are all generally converging to one point. (5.) Manholes should be of simple construction; circular brickwork upon concrete is a convenient description. They may be made to serve the additional purposes of ventilating shafts, flushing chambers, junction shafts, storm overflows, and side entrances. (6.) Tributary sewers or drains should not join the main sewers at right angles unless the bottom of the manhole is so constructed as to give the required curve in the direction of the flow of the sewage, and they should join at a height (if of unequal section) equal to the difference of their sectional diameters, the aim of all junctions being to cause as little disturbance as possible in the proper flow of the liquids along their respective channels. (7.) Sewers should not be constructed of too large a sectional area, but none should be less than 6 inches internal diameter, as house-drains in this country are never less than 4 inches diameter, and the main sewer should of course be larger than its tributaries. It is also rather difficult to ventilate a smaller sewer than 6 inches, and very little is saved by putting in a smaller sewer than that. Stoneware pipes of greater diameter than 18 inches should never be used. Where larger sewers are constructed they should be either concrete pipes,[181] or brickwork or concrete should be employed. The position of the sewer should, if possible, be behind the houses for the following reasons:-- (1.) The waterclosets, sinks, &c., being nearly always at the back, a drain under the house (which is always objectionable) is avoided. (2.) Economy is secured to the owner of the property, as a shorter length of drain is required than if the sewer was in the front. (3.) A better fall is usually obtained.