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The first thing to be done is to sink a well or sump, 3 feet to 4 feet in diameter, at a convenient distance from the circumference of the proposed excavation. This should be lined with open, unmortared brickwork (technically called steining), to allow of the free percolation of the water into the well or sump through the joints of the lining.

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Into this, when the sinker has reached the water-bearing strata, he conveys the suction-pipe of the pump, puts the latter in operation, and clears out the inflowing water, to enable him to proceed with his work. This well is carried down to a depth of 3 feet to 5 feet (depending on the volume of water present) below the bottom of the intended excavation, and is then paved with bricks set in cement. If the strata are of uniformly open character, consisting, say, of a mixture of gravel and sand, one sump will be sufficient to clear the ground of water; otherwise, if it is not uniform, but barred by intervening clayey deposit (not an unusual thing), and even by solid-bedded sand, for this sometimes is almost as impervious as clay, it may be necessary to drain the water to the sump, or even to put down two or more sumps outside the ground operated on. Duplicate pumps should be provided where the flow of water is very great. With these arrangements completed, the work of excavating and building can be proceeded with, unhindered by the presence of any undue amount of water."

When employing puddle in the construction of a tank, after the necessary excavation has been made, which is to be well shored up and strutted, a layer of puddle, 2 feetthick, prepared from good stiff clay by well soaking it with water, cutting it up, and turning it over until it becomes a soft, homogeneous mass, is first put all over the bottom and under the brickwork footings, and a portion of the circular wall of the tank is raised, say, 3 feet high. Puddle is then thrown in between the excavation and the

brickwork, and this is continued until the necessary height is obtained. In order to save expense, both in labour and water, it is usual to leave a portion of the centre of the tank untouched. This unexcavated portion is known as the "dumpling," and it is also necessary to puddle up the sides, and over the top of this dumpling, so as to make the tank water-tight. The dumpling serves to support the king post of a trussed gas-holder; or, if the holder is of the untrussed type, the timber framing which supports the roof when the holder is down.

During the progress of the work it is necessary to see that the circle required is adhered to, this is insured by the use of the contrivance known as a "trammel;" and, also, that in the building of the wall, the latter shall rise to the same height all round the tank, as if some portions are higher than others, then there would be a risk of unequal settlement, which would probably result in a fracture.

On the bottom of the floor of the tank "rest stones are bedded, for the purpose of "resting" the bottom curb of the holder when the latter is empty. At certain points in the tank wall stones are inserted, for the purpose of receiving the bolts which fasten the tank guide rails, and at specified intervals piers are formed which must be tied in with the rest of the work; these piers take the holdingdown bolts, etc., of the gas-holder columns, and are capped at the top with a stone, which serve as the base for the columns. The holding-down bolts are fixed and built in during the construction of the tank. They are secured at their lower end by cast-iron holding-down plates, which are built into the piers.

According to the size of the tank so does the thickness of the tank wall vary, the range being, say, from four and a half to two and a half bricks at the bottom, to from two and a half to one and a half bricks at the top,

the reduction in the thickness of the wall being made by "set offs" at predetermined heights, and on the outside of the wall. The top of the tank is usually finished off with a brick or stone coping.

As previously remarked, it is necessary to sink a sump, and to keep pumps continuously at work, in order to keep down the water, until the completion of the tank, when the permanent water required in the tank is gradually filled up to the necessary level, and its weight on the puddle over the bottom and dumpling prevents the bottom from blowing, and the nuisance and expense of a leaky tank.

Concrete is now greatly used in the construction of gasholder tanks, and in places where proper ballast is easily and cheaply obtainable, it is a most economical material if properly mixed and built up. The concrete is made of Portland cement, the proportion being about one of cement to seven of well-washed ballast, with a small proportion of clean, fine sand. The excavation for the concrete is an annular ring, rather wider than the width of the concrete sides, carried down to the necessary depth, and polled and strutted from the sides. The bottom part should be made wider than the actual wall, and left rough, so that when the interior of the tank is taken out a rugged part may present itself to combine with the new concrete forming the bottom of the tank. When the whole is completed, it is rendered over with a coating of neat Portland cement of from three-quarters to 1 inch in thickness. Of course the water has to be kept out of the work during its progress, and must be particularly watched in the construction of tanks of this description, or it will wash away the fine particles of cement from the ballast before it has time to set.

During the construction of gas-holder tanks, the inlet and outlet pipes for the circulation of the gas through the holders must be arranged for. The pipes, which vary in diameter according to the size of the holders, are usually

spigot and socket. They are carried down the outside of the tank and below the bottom, and return up the interior of the vessel; the end, or open mouths, of the pipes. being some 8 or 9 inches above the level of the water. The bottom of the interior pipes fit into cast-iron bends, and are carried, with a proper amount of fall, into what is called a "bell syphon," which is arranged to take the pipes rising outside the tank, and also to form a syphon to catch any leakage of water, should there be one, or any condensed water that may be made during the travel of the gas through the pipes. These syphons require regular supervision and pumping.

Cast-iron tanks are constructed of flanged plates cast to the radial curve of the tank, the plates being usually made, 4 feet 6 inches long and 3 feet 6 inches deep, provided with flanges 2 or 3 inches wide. The flanges are cast with holes about 6 inches apart, for the purpose of receiving the bolts which join the plates together. The thickness of the plates vary according to the size of the tank. In large tanks, say from 80 to 150 feet in diameter, the bottom plates range from an inch to an inch and a half in thickness, while the side plates vary from an inch to an inch and an eighth in thickness for the bottom tiers, reducing from five-eighths to three-quarters of an inch for the top tiers. The plates for smaller tanks are, of course, made thinner in proportion, but in all cases it is necessary to place iron bands round the outside of the tank, secured together by means of cotters, otherwise it would not be safe to trust to the tank being sound. The flanges of the bottom plates are always placed on the inside, while the side flanges are generally on the outside.

Iron tanks should be bedded on a concrete foundation, the latter being perfectly level.

The columns, or standards, for guiding the holder are generally attached to brick piers by means of holding-down

bolts built into the latter, the piers being kept separate from the holder; but in the case of tanks of a very small size the columns are carried on brackets cast on the tank, the columns being connected at the top by T-iron girders in the usual manner.

Iron tanks are made of different diameters up to 150 fcet, but are rarely more than 25 feet deep.

The Gas-Holder.-The gas-holder is the vessel in which the gas made is stored, and the storage capacity of a gas-works should be equal to the maximum quantity of gas consumed in twenty-four hours.

The holder is a cylindrical vessel closed at the top, which is known as the roof, and open at the bottom; holders are made of sheet-iron of varying thickness, according to the size of the vessel. The holder is rather less in diameter, but of the same depth as the tank in which it floats, and is guided by rollers, which are attached to its upper and lower edges at intervals along its circumference, the rollers working against suitable guides fixed in the tank, and on a series of columns, or standards, placed at equal distances around the top edge of the tank, so as to allow the holder to rise and fall with freedom.

The action of the gas-holder, by which it rises and falls, is as follows:-Supposing the holder to be empty of gas, and the tank filled with water, then, on allowing gas to enter the inlet pipe, as there is a space between the surface of the water and the roof of the holder, and the top of the inlet pipe is above the level of the water in the tank, the gas fills this space, and as it possesses a certain amount of force or pressure, this pressure is exerted on the surface of the water and the underside of the roof, thus causing the holder to rise. The water in the tank may be said to serve three purposes: it offers the necessary resistance to the gas, which enables the holder to rise, and when the holder is falling it is the medium by which the gas is ex

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