GAS-LIGHTING 



101 



thus tend* to descend and press the gas (contained 



between tin- water and the crown of the holder) 

 out into the mains, and ;il-<> back through the 

 Mai i"u meter ; hut they HO nearly pome the floating 

 holilei ili;ii i lie Hinall pressure at which tlie gas is 

 ili-!i\ I-I---I through the station-meter is sufficient to 

 lift tin- holder, and thus to enable gas to accumu- 

 IM it when there is uo outflow through the 

 iiuiiii ; and when there is such an outflow, the gas- 

 holder oscillates up and down according to the 

 proportion between the gas taken oft' from the 

 mains and that supplied from the retorts. When 

 the diameter of a gasholder is proportionately great, 

 it does not need counterbalancing. It is com- 

 p.uatively not a heavy structure, and it contains a 

 gas which is lighter than air, so that the pressure 

 ujion the base, so far as due to the sheet-iron 

 holder and its contents, readily comes to be but 

 little more than that which would have been due 

 to an equivalent quantity of air. Mechanical 

 ingenuity has been spent upon framing the holder 

 ly means of ribs, and internal bars, so as to give, 

 tli" maximum strength (freedom from buckling) 

 with the least weight ; and upon the construction 

 of telescopic holders, in which the holder is con- 

 structed in two, three, or four lifts or cylinders, 

 of which only the inner one has a crown. In each 

 pair of cylinders the inner one has its lower free 

 edge turned up, so that when it rises it hooks into 

 tin- down-turned upper free edge of the outer cylin- 

 der, and, as the gasholder goes on filling, lifts the 

 outer cylinder from the tank, and so, if there be 

 more than two lifts, for each succeeding cylinder ; 

 the gas being prevented from escaping between 

 any two of these mobile cylinders oy the water 

 which the inner one lifts from the tank in its 

 upturned edge. Recently the construction of the 

 g i-'>meter has been managed in such a way as to 

 dispense with the columnar guides. Necessarily 

 1 1 IM space within the gasholder above the tank water 

 is, by means of pipes, placed in communication both 

 with the station-meter and the mains. The function 

 of the gasholders is a most important one ; they 

 act as a reservoir, and usually are of a capacity suf- 

 ficient to contain a twenty-four hours' maximum 

 supply (the quantity used on a midwinter day); 

 and they also equalise the pressure. The gas- 

 holilor of the South Metropolitan Co. at East 

 Greenwich has six lifts, a diameter of 300 feet, a 

 bright when inflated of 180 feet, and a capacity of 

 12,000,000 cubic feet. The gasholder ensures a 

 regular supply at all hours both of day and night; 

 and by its means a comparatively small plant, kept 

 continuously working, is enabled to meet demands 

 for which, ir the gas were supplied direct from the 

 retorts, it would be quite inadequate. 



Before reaching the mains the pressure of the 

 gas is regulated by the station-gwernor ; an 

 excessive pressure in the mains would result in 

 \i-.-ssive leakage. There are various devices for 

 securing the automatic adjustment of resistance, 

 whose amount is made to increase or diminish with 

 the pressure ; either by the gas lifting to a greater 

 or less degree the floating bell of a small gas- 

 holder, and thereby altering the position of a 

 conical or parabolic plug suspended within the 

 entrance to the main, or (Hunt's) by working a 

 throttle-valve. 



The gas is conveyed from the works by main- 

 pipes or mains, generally of cast-iron, carefully 

 jointed ; the jointing is effected either by turn- 

 ing and boring so as to make the pipes fit 

 ea>ily with a little white and red lead, or by 

 using pipes which do not exactly fit, and making 

 them do so by means of caulking, melted lead, 

 india-rubber, or rust cement; in some cases the 

 pipes are connected by ball-and-socket joints ; in 

 others, special provision is made for expansion. 



At each lowest point provision U made for taking 

 oil' water, as by a trapped drip- well, the liquid in 

 which can ! pumped out into a cart and taken to 

 the gas-works, when mains supply a district the 

 altitudes in which vary considerably, the tendency 

 is for the local pressures to vary correHjMindingly ; 

 a difference of 100 feet in level makes a difference 

 of 1 '5 inch of water in a pressure-gauge ; and 

 therefore it is necessary to use district-governors 

 which control the pressure in particular district*. 

 To the mains are connected branch or service pipe*, 

 usually of wrought-iron or lead, in whicn the 

 deposition of moisture is provided for, either by 

 making the whole service-pipe drain into the 

 main, or by fitting up a drip-well at each lowest 

 point. 



The gas supplied is measured by meters, of 

 which there are two main varieties, the wet 

 ami the dry. The wet meter is a device for 

 measuring out successive units of volume of gas ; 

 the reading will be the same whether the gas be 

 delivered at low or at high pressures ; and there- 

 fore the lower the pressure the less the absolute 

 quantity of material in gas measured through a 

 wet meter, and vice versd. In a wet meter there is 

 a cylinder mounted on an axis ; this cylinder is 

 hollow, the hollow being divided into four parts or 

 chambers by partitions, the longitudinal boundaries 

 of which present the form of an Archimedean screw 

 or the rifling of a gun ; the gas enters one of these 

 spiral chambers at one end ; as the gas is pressed 

 in, it displaces water and makes the hollow space 

 lighter than water; it thus makes the hollow tend 

 to rise, and in that way works the cylinder partly 

 round. No gas can pass through the chamber until 

 it is completely full. When one chamber has been 

 completely filled, two things happen : the entering 

 stream of gas now finds an inlet into the succeed- 

 ing chamber; and, secondly, the gas in the first 

 chamber finds a possible outlet at its opposite end, 

 through a slit which now begins to emerge above 

 water-level. As the cylinder goes on rotating, the 

 first chamber comes to sink under water; water 

 enters the chamber and gas leaves it ; and so for 

 each of the four chambers in succession. The axle, 

 thus made to rotate in proportion to the amount of 

 gas delivered, works a train of wheelwork which 

 by means of pointers shows the numl>er of 10,000's, 

 the number of 1000's, and the number of 100's of 

 cubic feet of gas which have passed through the 

 cylinder. The water must be kept at a constant 

 level ; it may freeze, for which reason the meter 

 should be kept in a sufficiently warm place (not 

 too warm, else the gas will expand and the meter 

 give too high a reading), or else a non-freezing 

 Rquid should be used ; and the water damps the gas. 

 There are contrivances for maintaining the water- 

 level constant; the meter sometimes shuts off thegas 

 when the water is too low. Thus there may l>e an 

 automatic addition of water from a subsidiary reser- 

 voir, or an automatic maintenance of level by a 

 hinged float which sinks into the water when liquid 

 fails to support it in its uppermost position (as in 

 the constant-level inkstands); or, there may be 

 (Warner and Cowan) a contrivance for transferring 

 the excess of gas delivered at each revolution, when 

 the water is too low, back again for measurement. 

 When the meter is driven too fast the record is too 

 low ; but backwash in the meter then causes flicker- 

 ing at the jet; and the general use of meters too 

 small for the work which they have to do is con- 

 ducive to leakage in the district within which they 

 abound, on account of the high pressure necessary 

 to force gas through them. 



Dry meters are, in principle, a variety of piston- 

 meter ; the fluid is measured by displacing a piston 

 or diaphragm, and thereby filling a measured 

 cavity. They consist of two or three separate 



