796 SEWERAGE construction. The refuse matters of the cess- pools, instead of being transported into the counfry to serve as manure, were turned into the river Thames, polluting its waters, while the sewers themselves in the lower parts of the city were incompetent to discharge the increased burdens, and the ventilating flues through the streets became avenues of the most poisonous gases. A reconstruction of many of the sewers upon a larger scale became necessary, and they were laid out upon a regu- lar system, coming down to the river from each side for a distance of 6 m. along its course. Their total length exceeded 2,000 m., and when they were completed London was regarded in 1855 as one of the best drained cities in the world. The principal sewers were of extra- ordinary dimensions, several being from 12 to 14 ft. high and 6} ft. wide. Notwithstanding the great scale of this drainage system, the most serious difficulties were experienced in its operation. The outlets of the sewers, in order to get sufficient fall for discharge, were placed but little above low-water mark, the surface of some portions of the city itself being below high-water mark. Consequently the sewers were closed by the tide except at low water, and the gaseous contents of the sewage turned back into the city and up the drains into every street and house. The dis- charge moreover tuking place only about the time of lowest water, the accumulated con- tents of the sewers were kept up the river un- til the ebb, and when at last carried down the stream they were replaced by the same amount of fresh filth. Foul banks of black mud, from which most offensive odors emanated when laid bare at low tide, collected along the banks and in the shallow parts of the river. In 1858 it was decided to adopt some active measures for the abatement of the nuisance. It was at- tempted to disinfect the sewage by discharging every day during the warm weather immense quantities of lime and chloride of lime into the river. The quantities of these thus thrown in in the summer of 1859 amounted to 110 tons of lime and 12 tons of the chloride every day, at a weekly cost of about 1,500; and in that season 20,000 more were also expended in flushing the sewers in order to aid in expelling their contents at extreme low water. Plans were also sought for from scientific and prac- tical men by which the serious difficulties encountered might be permanently overcome ; and at last one submitted by Mr. Bazalgette, chief engineer of the board of works, was adopted and executed. The main feature of this plan consists in a series of three grand parallel main sewers at different levels and dis- tances from the river, and on each side of it, which cross the old sewers and outfalls at right angles and intercept the contents of the old system, conveying them on the north of the Thames down the river to Barking, a dis- tance of 7m., and on the south side to Cross- ness in the Erith marshes, 7f m. At this point the sewage is received in enormous reservoirs, which principally discharge at favorable con- ditions of tide into the Thames ; but a portion is used in making what is called " native gua- no " by the " A, B, C " process. The work of constructing these great intercepts, pumping stations, &c., was immense; the intercepting sewers alone, 82 m. in length, cost 4,250,- 000, which was raised by a third rate levied on the metropolis, yielding 180,262 per annum, principal and interest to be paid off in 40 years. It is estimated that the total cost of these great works will not be far from 30,000,000. Not- withstanding the great improvements that have been effected in England, serious defects still exist affecting the water supply of London, from sewage outfalls above the city, but which were being remedied in 1875. In America, sewer construction has both an ancient and a modern history. The works of the mound builders prove them to have been expert in the building of reservoirs, aqueducts, and con- duits ; and various places between the north- west and Central America exhibit remains of their sewer constructions. The modern history of sewers in America is not marked by any special achievements, the sewer systems of her principal cities being the result of rap- idly increasing necessity. As a rule, there- fore, the sewers of the cities have been built piecemeal and rarely on a far-sighted plan, and generally discharge into the nearest available body of water. The construction of sewers depends, as to material, size, shape, &c., upon the uses they are to serve, and the conditions under which they must be built, embracing the consideration of area to be sewered, its geologi- cal and topographical characters, the amount of rainfall, the water supply, the present and prospective population, and the disposal of the sewage. The difficulties encountered are in part the character of the earth, this being often so soft as to demand artificial supports for the pipes and sides of trench, or so hard as to require expensive excavation, the difficulty of securing sufficient fall, and that of providing for varying degrees of foulness in street wash, and securing safe escape for sewer gases. For cleaning purposes large man-holes are now depended upon, together with facilities for flushing. The form depends upon the amount of surface water, of manufacturing waste and excrement, and of subsoil water. If the sew- age contains little solid matter, a flat or ob- tuse oval bottom will answer ; but it is found that less fluid matters will be more efficient- ly discharged by an egg-shaped oval, which secures a greater depth and velocity of flow. An important change has of late years been made by the substitution, where the drain- age is comparatively small, of small earthen- ware pipes for large brick and stone sewers, whereby leakage and earth contamination have been reduced. A difficulty attending their use is the removing of sections for repairs. The cloaca maxima of Rome and the great sewers
