Three pairs of the above mentioned L levers are represented in fig. 2, in the length of the gas holder to support it in different parts and to prevent it altering its figure. The weight that must be used is according to the magnitude of the machine. The pairs of levers F, G, K, K, fig. 3, are placed side by side on the same centre pins, and cross each other. K, K, are counterpoises at the ends of the arms i, i, they are long pieces of iron extending from one lever K, to the next lever. The tank is furnished at the bottom with a recess, as seen in fig. 3, and 4, to allow the arms i, i, and counterpoises K, K, to descend beneath the edges of the gas holder. In the course of the movement of the machine, the sides of the gas holder are shorter at the top or ridge joints than at the bottom edges, as seen in fig. 2, in order that the under edges of the folding ends can move in an horizontal plane. Each of the folding ends is made of two triangular plates, connected together by an air tight joint, and each plate is again jointed to its respective side plate, and they are made tight by introducing a piece of leather or oil-cloth, or any other flexible substance impervious to air in the angle at the joint.
Fig. 5, represents the end plates of the gas holder when nearly extended, but when it is closed up, the two parts N, O, of the end assume the position as shewn by dotted lines. L, M, fig. 5, shews how the ends of the two side plates are turned outwards at b, to render them stiff and firm. As all the flexible joints are made strong by metallic joints or hinges, the leather has no stress to bear but only to prevent the escape of the gas; R, R, fig. 2, are the collars of leather to prevent the escape of the gas at the openings in the top or ridge joint where the guide rods c, c, pass through.
The tank must be filled with water to such a level that the under edges of the sides and ends of the gas holder will be a few inches immersed in the water. The counterpoises K, K, fig. 3, tend to close the sides of the machine together, and expel the gas from the gas holder through the pipe B, fig. 2. The counterpoises are so adjusted in weight as to force out the gas with the requisite pressure.
If more gas be introduced by the pipe A, it distends the sides of the machine and moves them outwards upon the ridge joint. A man hole, as seen at S, fig. 2, is made in each side of the gas holder, to give entrance when any repairs are necessary, or to oil or examine the joining leathers. It is scarcely necessary to add that the form and dimensions of this gas holder, and the materials of which it can be made may be varied without any deviation from its essential properties as they have been now described. For instance, the ends of the gas holder may be formed of more than two folding plates, united together, if it is judged necessary, and the levers F, G, may be varied in number, form, or proportion, provided they balance the weight of the sides and cause the lower edges of the gas holder to move nearly in an horizontal plane. Or the balance levers may be laid aside entirely, and the gas holder may be suspended from the upper part of the guide rods C, C, without moving up and down thereon. But in this case it will require more water in the tank to keep the open end of the gas holder always immersed in the water; the weight of the sides of the gas holder will then tend more to bring them together and to expel the gas.
In proportion as the quantity of water sufficient for the tank of the collapsing gas holder is less than that required for the tanks of other gas holders, it is attended with this further advantage, that the water can be let off or removed without any expence when repairs are necessary. If the repairs indeed are trivial, they can be made without letting off the water at all, the depth being no more than one foot. In the case, on the contrary, of the gas holder, with or without specific gravity apparatus, the quantity of water is so considerable, that the means provided for carrying it off must always be attended with great difficulty and expence; and yet it is a provision which is in all cases indispensable, no matter however difficult or expensive, for no material repair to the interior of the apparatus can be otherwise effected.
With regard to the best size of a gas holder adapted to a certain number of retorts, it may be stated, that this machine should be of a sufficient capacity to hold the whole quantity of gas that is required for the supply of the lights during one night, exclusive of what may be furnished by the retorts during that time.
Rule for finding the capacity of a Collapsing Gas Holder of given dimensions.
The bulk of gas which a collapsing gas holder of given dimensions will contain, may be found, by multiplying the area of the triangle contained between the side plates when at their greatest extent, and the surface of the water, by the mean length of the side plate. For example, suppose the base of the triangular end plate be 30 feet long, and 30 feet high, and that the length of the side plate at the top be 40 feet, and at the bottom 60 feet,
30 × 15 = 450 area of end plate,
450 × 50 = mean length of end plate,
= 22,000 cubic feet capacity.[44]
[44] A collapsing gas holder of 22,000 cubic feet capacity, costs about £. 800, it weighs eight tons; a collapsing gas holder containing 15,000 cubic feet, which weighs seven tons, costs about £. 700, and a ditto containing 30,000 cubic feet costs about £. 1000. The depth of the cistern for either is one foot.