TANKS AND GAS-HOLDERS.

231

nesses shown in Fig. 3, the whole of the brickwork was solid in every part, and groutingwas nowhere allowed. The interior brick and a half was carried up in three separate anddistinct rings, each of one-half brick thick, the external ring being built scaffold-high beforethe middle ring was built, and the same being done with the middle and inner ring; thehorizontal joints were broken in the manner shown in Fig. 3. The space between the brick-work and the sides of the excavation were filled up with concrete composed of ground bluelias lime, mixed with clean sharp sand and small shingle not larger than pigeons’ eggs, insuch proportions as to bed the pebbles. It was further specified that the bottom of theexcavation should be carefully levelled, and any interstices or irregularities in the chalkfilled up with the same concrete, and that on the bottom two courses of bricks should belaid flat, bedded in cement, the horizontal joints between the courses being three-quartersof an inch thick.

A similar tank to the above, also excavated out of chalk but of smaller dimensions, wasdesigned for the Basingstoke Gas Company by Mr. Barlow. The interior diameter of thetank is 31 feet 6 inches, and its depth from the upper surface to the seven stones placed atthe bottom for the gas-holder to rest upon is 10 feet. The sides were built of bricks laidin Portland cement, it being specified that the bricks should be hard, well-burnt squarestocks, the hardest being selected for the exterior faces, and the whole to be dipped in waterbefore being laid. There is one main pier fi feet wide, and five secondary piers, two ofwhich are 3 feet wide, and the remaining three 2 feet wide.

The tank for a gas-holder 25 feet deep and S7 feet 6 inches diameter, is shown in PlateXXIV. The dimensions marked in the engraving showing the slope of the internal moundof earth are given on the assumption that the nature of the ground is favourable for exca-vation. In such circumstances the tank may be built with a counterfort every ten feet,projecting eighteen inches. The outside of the tank to the thickness of two feet must bewell puddled. The estimated cost of such a tank is £749. 12s. 6 d*

The brick tanks at the Great Central Company’s Works at Bow Common are 105 feet9 inches diameter and 26 feet 6 inches deep. The specification for the construction was tothe following effect:—

That all the earthwork should be excavated to a depth of twenty-five feet from the surface linewhich included the depth of the brickwork, puddle concrete and the like—less five feet—which thetop of the tank might be above the ground line when finished ; an additional five feet of excavationto be made if the tank should be required to be sunk level with the ground. The substratum of theentire tank to be of puddled clay properly tempered to a depth of three feet. The brickwork to becommenced on the top of the puddle, the walls to be three aud a half bricks thick for six feet high,exclusive of three double crossings of footings, together nine inches broad on the back of the wall;and one inverted arch to be constructed on the front portion of the wall fourteen inches deep. Thewhole of the footings, the inverted arch, and the wall to the height of the top of the arch, to be builtin Roman cement, and good, clean, hard, sharp sand, in the proportions of one-half of each. The

* The structure aud dimensions of the tank figured in Plate XXIV. are retained from the previouseditions of this work, as they present a good illustration of the general form of excavated tanks, but theconstruction is not of a kind to be now recommended.