Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/106

96 in order to supply the required pressure, and their rise and fall are regulated by means of guide-rods 2' i round the tank. For economy of space holders in which different segments “telescope ” over each other are now much employed. This form of holder consists of two or even three separate parts,—the upper having the form of the common gas- holder, and the other being open at the top as well as the bottom. They are connected by the recurved upper edge of the lower fitting into a channel which runs round the bottom of the upper, whereby the entire structure is rendered air- tight at the line of junction. Holders of great capacity are now erected in connexion with large works. The Imperial Company in London possesses two, at Bromley and Hackney, telescopic in form,——the outer segment measuring 200 feet in diameter by 35 feet deep, and the inner 197 feet by 35. These holders are each capable of storing 2 million cubic feet of gas, which at sp. gr. 480 would ;weigh 73 tons. A still larger holder is at the Fnlham station of the Gas Light Company, it being 223 feet in diameter and rising 66 feet, with a capacity equal to 3 million cubic feet. The Governor.——An eflicient control of the pressure of the gas, along its Whole course from the gas-holder to the point of consumption, is an object of great importance for the avoiding of leakage, for equal distribution, and for supply- ing the burners at that pressure which yields the largest illuminating efl'ect. Ilncontrolled pressure may supply certain levels in a proper manner, but will leave low- lying districts insufliciently supplied, while the’ pressure in high districts will be excessive. The variations from simple difference of level may be very great. Thus, with a pressure of 1'7 inch at the Leith works, the gas would be delivered in some parts of Edinburgh at a pressure of 4'5 inches. The varying consumption from dusk onwards also greatly affects unregulated pressure. To control and correct these and other irregularities and disturbances governors are now used,—at the works or station for delivering the gas to the mains, in districts to correct variations owing to level, and beyond the consumers’ meters for controlling house supply 3 while in certain forms of burners a regulat- ing apparatus is also inserted. The principle on which all governors are based consists in causing the gas by its own pressure to act on seine form of sensitive surface which opens or closes a valve or aperture in proportion to the variations of pressure exerted on it. Fig. 14 is a dia- grammatic section of the common form of station governor. The course of the gas is indicated by arrows, d being the inlet and e the outlet pipe; c is a valve of conical form fitted to the seat 1' and raised or depressed by the weight f working by a cord over a pulley; bb is the hell or liolder,—a cylindrical vessel of sheet iron which rises and falls in the exterior vessel act, in which water is contained to the level represented. The gas, entering at a’, passes through the valve, fills the upper part of the inverted vessel bb, which it thus partially raises, and escapes by e. If the pressure from the holder be unduly increased or di- minished, the buoyancy of bb will be in- creased or diminished in like proportion, and the valve being by this means more or less closed, the quantity of gas escap- ing at e will be unaltered. And not only will the governor accommodate itself to the varying pressure of the holder, but also to the varying quantities of gas required to escape at e for the supply of the burners. Thus, if it were necessary that less gas‘should pass through c, in consequence of the extinction of a portion of the lights, the increased pressure thus pi-o(1u¢e,1at the iold_ei' would raise the governor, and partially shut the valve, leaving Just sufficient aperture for the requisite supply of gas. Numerous improvements have been made on the ordinary FIG. 14.—Section of Governor. GAS station governor. In the form invented and manufactured by D. Bruce Peebles, the bell or holder is enclosed in a gas-tight case or chamber, and a small portion of the inlet gas flows in and out of this chamber above the holder. The pressure of this small quantity of gas is regulated by passing it through a small separate governor; and, acting on the outer surface of the holder, this, in a very delicate and sensitive manner, performs the duty of weights in the older forms of governor. An arrangement similar in principle is applied to the district governor by Bruce Pecblcs, the minimum day pressure being secured by means of a stopcock or screw-valve on the apparatus, and the inaxiniuni night pressure is controlled by a small subsidiary governor. The principle of the small governor, which thus plays an im- portant part in regulating large flows of gas, will be ex- plained under consumers’ governors, the apparatus being shown in section in fig. 18 below. Supply Pipes.—The street main and service pipes are tubes of malleable or of cast iron, the gauge of which must be arranged according to the quantity of gas to be supplied, the length it has to travel, and the pressure under which it is carried forward. Practical gas—engineers possess elaborated tables of data for the regulation of the size of their various supply pipes. Notwithstanding the utmost care and accuracy in the laying and fitting of street mains, leakage at joints is a constant source of annoyance. Under the most favourable conditions there is a discrepancy of from 7 to 8 per cent. between the gas made and the amount accounted for by coiisumpticn, and the greater part of that loss is due to leakage in street pipes. To convey the gas from the main pipes and distribute it in houses, pipes of lead or of block tin are generally used. Conszmzers’]|[eters.—-Of these there are twofornis in actual use, the “ wet ” and the “ dry.” The former, the invention of Mr Clegg, is represented in the two sections (figs. 15 and 16), where cc represents the outside case, having the form 6 Fig. 15. Fig. 16. of a flat cyclinder; a is the inlet tube and b the outlet pipe 3 g, _r/ are two pivots, and It a toothed wheel fixed upon. the pivots and connected with a train of wheel-work to register its revolutions. The pivots are fixed to and support a cylindrical drum-shaped vessel dcld, having openings 9, e, e, e, internal partitions Pf, Pf, ef, eff, and a centre piece fr_‘r‘f. The machine is filled with water, which is poured in at Ia up to the level of 13 ; and, on gas being admitted under a small pressure at a, it enters into the upper part of the centre piece, and forces its way through such of the opcmngsf as are from time to time above the surface of the water. By its action upon the partition which curves over the opening ct, a rotatory motion is communicated to the cylinder,——the gas from the opposite chamber being at the same time ex- pelled by one of the openings e, and afterwards escaping at b, as already mentioned. Wet meters work easily, and, when well set and properly supplied with water, measure

the gas with much accuracy. But excess or deficiency