IVaskington Conzbined Aqueduct and Bridge.—The ingenious artifice of utilizing- a pipe line for the double purpose of conveying water and supporting- a bridge structure is exhibited in the case of an aqueduct bridge crossing the Rock Creek near Washington, D. C., over which the structure carries the waters of the Washington conduit. It was built in 1858 by Captain Meigs of the United States Engineers. For this purpose he used two arched ribs formed of water-pipes of circular section (fi/. 54, fig. 3), through which the water flows. The span of the bridge is zoo feet, the rise is 20 feet, and the width over all 28 feet. The pipes are 4 feet in diameter and y? inches thick. The pipes are not cast to the curve of the arch, but are in straight lengths of 12 feet, with flanged joints whose faces are parallel with the corresponding- radii of the arch. At the skewbacks the ends of the pipes are faced to large conical boxes, admitting the water, and resting on the masonry. The roadway is of timber supported on span drils formed of rolled wrought-iron beams. The pipes were at one time jacketed inside with staves of resinous pine, 3 inches thick, to prevent the freezing of the water; but this prevention was found to be unnecessary, and the lining has since been removed.
Distributing-pipes.—The material universally employed for distribut ing-pipes is cast iron, as from it they are easily and cheaply manufactured, while at the same time they possess the requisite strength and admit of the easy attachment of branch-pipes. The jointing of these pipes is effected in several ways. The so-called " flange-joint " (fi/. 52,fig. 14) was for merly much employed. In this the ends of the pipe were flanged, a disc of leather or of lead was laid between the two flat surfaces, and the ends were firmly united by means of bolts and nuts. This form of joint, which has been found not sufficiently flexible, lias been superseded largely by the socket-joint. In the method now in common use one end of the pipe is cast with a bell-shape enlargement, into which the plain end of the next pipe is inserted. The space between the two, which is about equal to the thickness of the pipe-walls, is filled for about half its length with tarred hemp well rammed in, and the remaining space is filled by running lead into it. At certain intervals simple butt-joints are made with the use of a surrounding collar which serves to permit the removal of the sections when repairs are necessary. To allow for alteration in length of the line in con
sequence of changes of temperature, certain compensating sections are in troduced into the same (fig-. 15). Where a line of pipe must be carried across a river (p/. 54, jig. r), or, in other cases, where it must rest upon a curved or irregular bottom, the joints must be given sufficient flexibility to adapt themselves to the curvature. In such cases ball-and-socket (p. 53, Jig. 9) or knee joints, etc., are substituted for the usual forms of con nection.
Pipe-valves. —For emptying a system of water-mains, as well as for removing the accumulated sediment, discharge-pipes controlled by valves are placed at the lowest points of the line. At the highest points the air may collect in such volume as to prevent the flow of water, on wbich ac count it is necessary to provide suitable vents through which it may escape. Figure r6 (26/. 52) shows a form of air-valve consisting of a vertical chamber in which a float automatically closes and opens a valve in its upper portion as the level of the water rises and falls. For controlling the flow in the various branches of the system, shut-off valves are provided at the proper places. A valve of this description is shown in Figure 17.
Other Forms of Pifies. —Before the period of cast-iron pipes, wooden pipes from io to 12 feet in length were employed, and are still occasionally used; in many situations they have shown an extraordinary durability. Figure 12 exhibits a wooden water-pipe, partly in section to show one method of joining the parts with the aid of iron collars of double conical forin sharpened on each side. Water-pipes of stone-ware or terra-cotta, usually from 6 to 8 feet in length, are likewise in use. Figure 13 shows a section of this kind of pipe, whose differently-formed ends clearly explain the mode of joining section to section. The joint-packing in this case consists of hydraulic cement, and water-pipes formed of this material have been found to answer well the purpose for which they are designed. Leaden pipes, which are of small calibre, are used only for making house connections from the street-mains. Tin-lined lead pipes are somewhat in vogue in localities where lead-poisoning is feared. In a number of Amer ican cities house-service pipes of galvanized (cast) iron are used to a considerable extent, and apparently with satisfactory results.