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TANKS AND GAS-HOLDERS.

The gas, after having been measured by the station-meter, passes on to the gas-holderwhere it is stored for use.

Gas-holders consist of two distinct parts—a tank for containing water, and the invertedcover or gas-holder itself, within which the gas is contained.

The construction of tanks requires considerable skill and judgment on the part of theengineer, as they vary in their structure according to the nature of the soil, the value of theland, the materials at command, and their price. The tanks may be either entirely sunk in theground, or raised above it. They may be excavated from the solid rock, or built in brick-work or masonry, rendered water-tight by puddled clay or cement; or they may be made ofcast or wrought iron; or they may be constructed by a combination of both plans.

When the solid rock reaches near the surface, the construction of the tank occasions littledifficulty. At Chester, Mr. Clegg found so firm and impervious a bed of sandstone that nobrickwork or puddling was necessary; the tank being simply fashioned with the pick, anda few land-springs stopped.

When the excavation takes place in less compact strata, a mound of earth may be left inthe centre of the excavation with a sufficient slope to guard against slips, and it may, insome cases, require to be wattled. These slopes will vary according to the nature of theground. Good gravel will stand at the slope shown in Plate XXIV., which represents thesection of an excavated tank with the supports of the gas-holder surrounding it. If thestratum be of clay and sand mixed, the inner part had better be entirely removed. Clayalone will stand at a slope of about lj to 1. In either case, the surface of the mound mustbe puddled two feet thick. In tanks whose diameter does not exceed fifty-five or sixty feet,the earth, if requiring puddle or other finishing, may be got out entirely, as it will be foundcheaper to do so. In rock, the centre portion may be left in for much smaller diameters.

It is necessary to make provision, in all tanks, for the introduction into the gas-holder ofthe pipe that admits the gas from the purifiers, and a separate pipe is generally introducedto conduct the gas to the mains for distribution. In brick tanks a small well is sunk forthe purpose, as shown in the section in Plate XXIV. This well may be about seven feetdiameter, built of brickwork in cement, and well puddled, and so much lower than the tankof the gas-holder as to allow the top of the receiver to be below the bottom of the tank.

The construction of a “ dry well ” for the introduction of the inlet and outlet pipes is nowdiscarded by some engineers as unnecessary; the pipes being carried down the face of thetank, or in a recess made for that purpose. There are certain conditions under which that

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