The holder is a cylindrical vessel closed at the top, which is termed the roof, and open at the bottom, made of sheet iron, varying in thickness according to the dimensions of the apparatus, the smaller sizes being constructed of thin material in order to avoid an excess of pressure, whilst those of very large dimensions are made of stout plates for the purpose of obtaining sufficient pressure to expel the gas to the burners. The holder is somewhat less in diameter, but of the same depth as the tank in which it is placed, sometimes being partially suspended by chains which pass over grooved pulleys and counter-balance weights, but more frequently only guided by rollers attached around its lower and upper edges, which work against suitable guides in the tank and on the columns in such a manner as to permit the holder to ascend and descend in the tank with the greatest freedom. The action of the gasholder is very simple. The tank being filled with water, and the holder immersed therein ready for use, there is a space between the surface of the water and the roof of the holder; the gas enters by the inlet pipe into this space, and with the force it acquires in being expelled from the coal, pressing on the surface of the water and underneath the roof, and over the whole area of both, causes the holder to rise. Thus, by its own force or pressure, the gas provides room for itself, and in proportion to the quantity entering so does the holder rise out of the water. For instance, a holder having 100 feet area, or about 11 feet 4 inches diameter, in rising 10 feet will receive 1,000 cubic feet of gas, and in descending, the same quantity would be expelled. Gasholders, though often suspended, are never entirely counter-balanced, having always sufficient weight to give the necessary pressure for forcing the gas through the mains and smaller pipes to the burners, all through the neighbourhood which is supplied from it. The gasholder should be so constructed that, when it is full or at its greatest height, its lower edge will be so far under water as to prevent the gas from escaping. The water in the tank serves three purposes; it is the means of resistance for the gas to lift the holder, it prevents the gas escaping or mixing with the atmosphere; and it is the means of expelling or forcing out the gas as the holder descends. THE IRONFOUNDER. [Illustration: FOUNDRY.] Having already described the various operations of the trades employed in building and fitting a house, we will say something of the manufacture of those cast iron columns, girders, gratings, balconies, pipes, gutters, air traps, coal plates, stoves, and other articles which are so necessary to the Builder before his work can be completed. All these, as well as a great variety of other goods made in black or bronze iron, such as gates, bridges, pieces of furniture (like umbrella stands), iron taps, and even pots and frying pans, are made at the _Iron Foundry_.