ordinary usage, a sewer designates a drain constructed under the streets of a city to carry off the surface-water, and also liquid refuse matter from the houses. Large sewers accessible at the ends or from special entrance-places, termed man-holes, are made circular, elliptical, or egg-shaped, the latter shapes being preferred as permitting greater height in proportion to the capacity. A rate of fall of r inch in ro feet is consid ered desirable, though less suffices for a main sewer. The large sewers have been built usually of brick, the smaller ones being of glazed stone ware. Cement for the composition of sewer-pipes has been objected to as not sufficiently uniform in quality and not sufficiently durable for large constructions. To secure unyielding foundations for large sewers and cul verts under high embankments and in soft ground where great pressure may occur, inverted arches or beds of masonry are inserted within the walls to strengthen the structure. Beds of gravel, sand, or broken stone are also placed as foundations beneath culverts. The substitution of smaller earthenware pipes for the larger brick sewers has been found advantageous in increasing the rapidity of the flow of material, and thus decreasing the amount of foul gas produced by decomposition.
The catch-pools built at corners of the street receive the surface-water from gutters, and, leading it into the sewers, assist in flushing them. The ventilation of sewers has been much considered, and various methods of gratings in streets have been introduced, and experiments have been made in the use of ventilating shafts by means of tall lamp-posts and chimney flues. The deodorizing of sewers has been the subject of many experi ments, but on account of the great volume of material to be treated it is found impracticable to employ chemical substances for the purpose, and frequent flushing is the proper and efficient remedy.
Pipes and danger consequent upon the admis sion of sewer-gases into dwelling-houses has been brought so forcibly to public attention of late years that preventive methods have been the sub ject of general and scientific consideration. Ventilating pipes carried above the roofs of buildings have been largely relied upon for sanitary effects, and traps have been connected with waste-pipes to prevent the in trusion of the gas. Various devices make special claims on the score of cleanliness, effectiveness of sealing quality, and protection against siphon ing, the latter appearing to be the most serious obstacle to the effective operation of these traps in general. Security ap.-_,ainst evaporation and satu ration by the absorption of gases from beneath has been a special matter for consideration. Traps of glass with metal covers have been designed expressly for connection with washbasins, bathtubs, sinks, etc., with the
object of preventing any entrance of gas from below by the interposition of a body of water or other fluid, or by mechanical means.
.Superstructure: Object of nature of the object to be attained deeply affects the selection of material and modes of construction. The diversity of purposes is increased by differences in economic conditions and surrounding circumstances. In the United States, fluctuations in for tune have been so numerous and frequent that with many men the opera tion of constructing at one period of their lives a dwelling intended for personal use would have differed widely from an undertaking haying the same object in view commenced at another stage of their career. Differ ences in locality or latitude, and relative scarcity or abundance of particu lar kinds of material or of experts in various branches of the building trades, may also materially affect either the cost or the practicability of completing a structure within a given period. Considerations relating to such matters should receive due attention, and incite the formation of defi nite ideas, in advance of the commencement of construction.
Stability of 'Valls. —A leading purpose in the construction of walls is to insure their stability—at least, during the period of their probable use fulness. In the United States, the process of tearing down one set of buildings that they may furnish the site for improved structures has been conducted on so extensive a scale, that stability, in the sense of securing the greatest possible prolongation of usefulness, has frequently not received the degree of attention devoted to it in many other countries; on the other hand, some of the latest structures of extensive proportions represent extra ordinary efforts to insure stability. As buildings are commonly constructed, the walls are usually much the most durable portions of the structure, unless there has been exceptionally bad workmanship or inferiority and insufficiency of materials. Cases have unfortunately not been unknown in which large portions of walls have given way under pressures or adverse conditions which they should have been capable of resisting, and instances are too common in which the reaction from unnecessarily thick walls has been carried to a dangerous extreme, or in which deplorably inferior mate rial has been used. To insure the essential degree of stability, it is neces sary that an upright wall should be properly constructed upon a stifflcient foundation, and that due precautions should be taken ag-ainst the effect of pressures created by various parts of a building.