Scale of an inch to a foot FIG. 10. Scale % of an inch to a foot FIG. 11. of the flue should be from one-sixth to one-fourth that of the fireplace. Defective draught very often arises from the bad construction of the flues, for when the draught holes from several furnaces open into a common conduit the currents, being continued beyond their orifices, modify each other. Figs. 7 to 12 show in plan and elevation a row of 6 large sized furnaces with their flues opening into a common stack. Furnaces for melting metals, heated with gas instead of solid fuel, have been adopted in some works. Fig. 13 shows an arrangement formerly used in the Berlin Porcelain Manufactory, and in the mint. A is a chamber of fire-brick having an ordinary flat grate, composed of iron bars, and another grate in which the bars are placed in steps b. On a grate of this kind small free-burning coal may be used, which would in a great measure drop through an ordinary grate. The surface of the grate, by this step-like arrangement of bars, is considerably increased, and ample space allowed for admission of air. The two openings d, e are closed by iron doors. A blast of air is introduced under the grates by the pipe f. opening g, which is a sliding damper h. The fuel is charged through the closed with an iron cover, and by The gases generated, chiefly carbonic oxide, pass through the pipe k, provided with a regulating valve l, and escape at m. Immediately in front of the opening m, and extending across the furnace, is an iron pipe n, from which air issues. The carbonic oxide here burns, and the heat is communicated to the furnace in which the crucible is placed. In order to represent both the gasproducer and the melting furnaces in the same vertical section, it has been necessary to deviate from the actual position of these furnaces at the mint, where the melting furnace is at the side of the gas furnace. The air is forced by a small fan through the pipes ƒ and n, which are provided with regulating valves.1 Great improvements in the construction of melting 1 Percy's Metallurgy, p. 201. furnaces heated by gaseous fuel have been made during the last thirty years, notably through the labours of Messrs. Siemens. By the use of gas-producers fuel may be used which is too inferior for ordinary furnaces, and a higher temperature attained by the combustion of the gas formed, than by using solid fuel directly. A gas-producer is generally a somewhat rectangular chamber, in which fuel is burnt for the formation of carbonic oxide as already explained. Siemens's producer, Fig. 14, is lined with fire-brick; the side A is formed of iron plates lined with fire-bricks, and has a step-grate B, with wrought-iron bars C. The fuel is charged through the hopper D. The gas passes up the pipe E, which is cased with iron, and issues into a horizontal wrought-iron pipe, which conveys it to the regenerator, where it is highly heated before entering the furnace in which it is burnt, thus producing a much higher temperature. The waste-heat of the furnace is not lost, but utilised in heating the regenerators. The arrangement of a Siemens's crucible furnace is shown in Fig. 15, where the pots are heated by the combustion of hot gas and hot air. The regenerators are chambers of open refractory brickwork, built in pairs, two pairs being required for each furnace; each pair being used alternately for absorbing the heat of the gaseous products from the furnace, and heating the gas and air required for combustion. By means of a reversing valve, the waste gases pass to the right or left pair at will. When the waste gases are passing down through the right pair, the cold air and producer gas are passing up through the left pair, the direction being reversed when sufficient heat has been absorbed. It will be observed that the combustible gas and air enter the furnace alternately at the right and left, according to which pair of regenerators is being used for heating them. By using gas for melting metals a neutral or nonoxidising flame can be obtained, which prevents loss of metal by oxidation, a manifest advantage in cases where it is necessary to preserve the composition of an alloy intact, and should be of great advantage in the manufacture of such alloys as brass. Reverberatory furnaces are only used for the manufacture of brass where large quantities require to be cast, as in the case of ingots of yellow metal used for ships' sheathing, etc. The furnace commonly employed, Figs. 16, 17, is capable of holding about a ton of metal. It is somewhat rectangular in shape, with flat sloping bed, which inclines from the bridges and back towards the front, or working door. The bed is formed of fire-brick carefully set edgeways, or a well rammed sand bottom is used. It is advisable that the copper should be melted first in |