WATER-WORKS. 361 WATER-WORKS. a river or otluT souioo to filters, after which it will have to be reimiujieil to the distributing sys- tem. V'here the water must be pumped twice, and the lift is coinjiaratively low, centrifugal pumjis are often used. Variations in the mode of supply, the extremes of which are direct pumping, with its liability to great and pcrliai)s sudden changes in working pressure, and jnimping to a reservoir, against an almost constant head, have inii)ortant l)car- ings on the choice of ])uniping maeliinery. Direct- pumping plants must be quick to respond to sudden changes in pressure, without injury to the machinery. This calls for variable speeds, automatically regulated, and for strong and simple working parts. If, however, water is pumped against a fairly constant head and at either a uniform rate of speed or under such conditions that the rate may be changed gradual- ly, economy in design and operation may receive more consideration. Questions of motive power, particularly as between steam and water, are frequently settled in advance by nature. If ample water power is at hand it is inevitably chosen, on account of its cheapness. If it is not available the j'ear around it may be supplemented with steam power, or possibly storage reservoirs may be constructed to equalize the stream flow. Since the development of the electrical trans- mission of power it is sometimes possible to utilize waterfalls remote from pumping plants. Electricalh' driven pumps may also be useil where the dynamos are driven by steam engines, but this is not economical except under special conditions, such as pumping plants remote from coal supplies, or located in the residence dis- tricts, w'here a boiler jilant for the generation of steam power would lie undesirable. Compressed air may be transmitted for tlie operations of small plants at a distance from the main ]nuii]dng station or it may be used in the air lift, for raising water from driven and artesian wells. Isolated plants in large cities, and the main pumps of small towns and villages, may be operated by oil or gas engines. Small quanti- ties of water may be lifted bj- windmills. These various forms of motive power may be applied to any one of a number of kinds of pumps, as explained under Pumps and Pumping Machin- ery. The vast majority of water-works pmnps, however, are driven tv steam, the motive power and pump being combined, except in the ease of the centrifugal jnimp, in one machine, known as a pumping engine. The more expensive the fuel the greater the reason for adopting a high-duty pumping engine, or one which, by means of better and more expensive design and workmanship, performs a relativel.y large amount of work for a small amount of coal. Uniform rates of pump- ing, as nearly as may be at the full normal ca- pacity of the pumping engine, aid in securing the maximum dutj' from any sort of pumping ma- chinery. Pumps working under high pressures are selected with special care as to strength of working parts and cylinders, and if of the re- ciprocating type they have a slow piston and valve movement. For very high pressures phmg- er pumps are used. The dividing line between high and low pressures is rather arbitrary, but 60 pounds or under might be termed low; 50 to 100 pounds, medium; while anything above the latter figure might be called high. liESEUvoiK Tanks 'AND Standpipe.s are em- |d(pyi'd to insure a reserve supply of water, to maintain the pressure in the distribution system at or above a mininuim point, ami to equalize both the rate of pumping and the head on the pumps. Tanks and stand]ii])es are forms of reservoirs, but the latter term is almost in- variably restricted to reeejitacles built in or ])artially in the earth, with either earth em- bankments, masonry walls, or a combin.ition of the two. Tanks and standpipes are built almve gi'ound, of wrought iron or steel, except that tanks are very often of wood. Tanks are gen- erally placed on wooden, metal, or masonry tow- ers, and both tanks and standjiipes are some- times inclosed with masonry and covered with some sort of roof. Standpipes rest on masonry foun- dations, the tops of which are level with or a few feet above the surface of the groun<l. Jlctal tanks and standpipes are made from nnc-fourth to one incli plates of wrought iron or steel, the latter being most commonly employed of late. The plates are riveted together to form the sides STANDPIPE WITH ROOF, DE8 MOINEB, IOWA. and bottom. In recent years the bottoms of steel tanks have been curved to a spherical or conical form and supported at the circumference of the tank, onl}', instead of resting on a floor system. This is a gain in economy and in ap- pearance. Standpipes are relatively tall for their diameters, a frequent size being from 12 to 20 by 100 feet. Ordinarily only the upper por- tion of their contents is available, the rest being too low for effective pressure, and serving chiefly as a support for the efTective portion. PisTRiBi-TiON Systems comprise the various pipes used to convey the water to the points of use. the valves and hydrants which control the flow anVi delivery of water, and the meters for recording the consumption. Sometimes the term
