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 SEWERAGE 797 of Paris are built of cut stone, as were some of the conduits of the mound builders. The great main drainage works of London are cy- lindrical sewers of brick. The system of Capt. Liernur, now so highly lauded and being adopt- ed in Europe generally, has its pipes all of riveted wrought or cast iron. Sewers are gen- erally laid along the middle of the street and at depths according to the grades. At many street corners " catch pools " are built of brick- work, which, covered by open gratings, re- ceive the wash of the street, retaining in the bottom the mud and sand and discharging the water into the sewer. In towns having dis- tricts whose surface is scarcely above high tide, the principle of interception requires at- tention from the engineer. Intercepting sewers receive the surface water, and often also the sewage, and divert it from the common sewer which passes to the lower districts. In towns where the sewage is pumped from the lower districts to higher levels on account of tide- locked sewers, the system of interception be- comes of great importance and greatly reduces the cost of drainage. This is well understood in the great drainage works in the fen districts of England, where large areas are entirely sur- rounded by intercepting drains, which lead the water off by gravitation. The great intercept- ing sewers of London, which have been men- tioned, have been constructed to convey rain- fall at the rate of a quarter of an inch in depth over the whole area drained every 24 hours, at the time the maximum flow of sewage is being discharged. When the flow in a sewer does not carry off the solid contents, it is convenient to clean it by washing it out, or "flushing" as it is technically termed. The operation is per- formed by letting in water at the higher levels, or by retaining the sewage in sections by means of gates, and, when the portion below has be- come partly empty and that above nearly full, allowing the collection by its gravity to sweep away accumulations below. The grounds oc- cupied by the annual fair at Nizhni Novgo- rod, Russia, have sewers built of cut stone ar- ranged in regular streets. They are lofty and well ventilated, and are entered by stairways through towers placed at intervals. They are flushed every day with water pumped from the Volga. The ventilation of sewers is a subject of great importance in a sanitary point of view. The effluvium from the ventilating holes in pavements has been found to taint meat in butchers' stalls in their vicinity, to avoid which connections have been made with the chimneys of manufacturing establishments. It is not improbable that the solution of the problem will be in the erection of special chim- ney stacks. The relative efficiency of deo- dorizers has received attention. In 1858 the commissioners of London sewers reporte'd that wood charcoal has a power of deodorizing currents of foul gases which is not much dimi- nished by long use. The comparative cost of disinfectants to purify a given amount of sew- age in a uniform condition, made previous to the report, is shown in the following table : Boghead charcoal (coke). 1 ton ... $8 00 Nitric acid g fl> Black oxide of manganese. . 9 20 Chloride of lime " 10 76 Peat charcoal '.'.'."..'.'.'.'.'.'.'.'.','. II 00 Animal charcoal 16 75 Bichloride of mercury ig < HI Impure chloride of zinc in damp powders 26 IK) Chloride of zinc in solution (Sir William Burnett's.). . . 87 00 Sulphate of copper 80 00 Other sulphates, as well as a number of other substances sometimes employed as disinfec- tants, proved ineffectual even in very large quantities to produce complete deodorization ; but arsenious acid, creosote, and carbolic acid were found to possess great deodorizing power. Baldwin Latham has devised a combination of man-hole, street-water, and dirt catch with overflow pipe, and a charcoal ventilator pro- tected from wet, which has been put in use in London with fair results. The gases princi- pally found in sewers are carbonic acid, sul- phuretted and carburetted hydrogen, ammonia, nitrogen, and fetid organic vapor ; but it is only in the most unfavorable localities that the permanent gases are in sufficient quantity to produce of themselves bad effects. It is the poisonous germs contained in the accompany- ing organic vapor, and which may not have any very offensive or at least powerful smell, that confer on "sewer gas" its most dele- terious properties. To prevent the entrance of sewer gas into houses is a problem which presents many practical difficulties. Various forms of traps are placed at the discharge of the house pipe into the sewer, and also in the water closets and beneath the wash bowls. In the discharge into the sewer the pipe usually dips beneath the surface of the water in a receptacle which empties into the sewer. In water closets a " sealing pan," as it is called, holds water in its cavity when its rim is horizontal, the level of which is above the lower rim of the hopper. The joints of the hopper at the upper rim being made tight, no gas can escape into the room. The waste pipe below the hopper has a syphon crook, which when filled with water prevents the escape of gas from the pipe below. Other kinds of clos- ets, which involve some additional expense, are preferred. A pump closet, used upon ships and below the water line, is modified for house purposes, and made to exhaust the hopper and force the contents out of the soil pipe. Disposal of Sewage. This is one of the vexed and unsettled problems of the day, both in an economical and a sanitary point of view, and embraces the question of utilization of sewage, which also includes many plans, none of which have been confirmed as satisfactory. It is the opinion of many that when a town is so situ- ated that its sewage may be safely run off into a harbor or a river, so as not to become a nuisance afterward, this is the best method to pursue, as by a proper system of sewers