the fire and reducing its temperature. The amount of water admitted must be carefully regulated.

Immediately in front of the grate is the cleaning opening, closed tightly by a self-sealing door which is opened only when the fire requires cleaning.

The charging opening, through which the furnace is filled with fuel, is in front at the level of the charging floor for the retorts, the brickwork of the front wall of the furnace being stepped out gradually until it projects enough to permit the opening to be entirely in front of the front wall of the setting, and in such a position that the coke from the middle retorts of a bench of 9s, or the top retorts of a bench of 6s can be drawn directly into the furnace. This charging opening is closed by a loose cast-iron lid fitting into a groove in a castiron frame so as to make an air-tight joint with the aid of a little coke dust which is allowed to collect in the groove.

The primary air is drawn in through two flues, A, A, one on each side of the furnace. The fronts of these flues are closed by air ports with slides by which the area of the opening and therefore the amount of primary air can be closely regulated. From these flues the air passes to the recuperators if the setting is regenerative, or direct to the furnace if the setting is not regenerative. In either case it enters the furnace through a number of openings, B, B, B, B, in the side walls directly under the grate bars, the object being to have it distributed as much as possible over the full depth of the grate.

The secondary air is admitted through air ports, similar to those for the primary air, into two flues, C, C, one on each side of the furnace, and passes through the recuperators, when these are provided, or simply through flues running to the back of the setting and forward again. In either case it is taken off from the last longitudinal flue in the series, by short transverse flues, D, D, D, D, each of which opens directly in front of one of the nostrils through which the producer gas passes from the generator to the setting. Each stream of gas is thus supplied with a stream of secondary air for its combustion. There are usually five of these openings on each side. of the combustion chamber, as that part of the setting in

which is begun the combustion of the gas is called, spaced equal distances apart from the front to the back of the setting.

To clean the fire, the water is shut off from the step grate, the cleaning door opened, secondary bars driven in above the top plate of the step grate until their points are supported by the sloping back wall of the furnace, the regular bars removed and the ashes allowed to drop into the ash pan, the grate bars replaced, the clinker cut away from the walls and removed, the secondary bars pulled out, dropping the fire on to the grate, the water turned on the step grate, the ashes removed from the ash pan, the primary air openings cleaned out, and the cleaning door closed to stay closed until the fire has to be cleaned again. If the coal contains only a normal amount of ash and the proper amount of water is supplied to the furnace, the cleaning need not be done oftener than once in 48 hours.

All generator furnaces are not built exactly as shown in the cuts, but every such furnace should have a closed top, and the arrangements of primary and secondary air flues and openings and of filling and cleaning doors will be substantially as is here illustrated and described. (Trustees.)

3. Give a description, illustrated by a sketch, showing a vertical section of the apparatus, of the construction of a double superheater Lowe carburetted water gas apparatus, and give a brief description of its operation for the manufacture of carburetted water gas.

Ans. As is shown on the cut, the double superheater Lowe carburetted water gas apparatus is composed of three cylindrical vessels connected together in sequence. The shells of these vessels are made of wrought iron and they are lined with fire clay blocks. An annular space from 1 in. to 11⁄2 in. wide is left between the lining and the iron shell and is filled with asbestos fibre, or other fire-proof non-conducting material, to diminish the radiation of heat from the shell.

The first vessel, A, is called the generator; the second, B, the carburetter or first superheater, and the third, C, the superheater, main superheater or fixing chamber.

The generator contains the fuel for heating up the apparatus and producing the water gas, and is provided with a grate and cleaning and clinkering doors, at the grate level, and a charging

door or doors on the top. It is usually connected from both the top and bottom to the top of the carburetter.

The carburetter is filled with a checker work of fire brick on edge, set with from 11⁄2 in. to 2 in. clear space between the

rows and so that the rows break joint with each other. It is connected at the bottom to the superheater, or fixing chamber, which is also filled with checker brick. On the top of the superheater is a take-off casting, the top of which opens to the atmosphere and the side to a pipe leading to a small cylindrical vessel, called the wash-box, which contains water in which the open end of the take-off pipe is sealed. The

opening to the atmosphere can be closed by lowering a heavy iron lid called the stack valve, which by its own weight forms a gas-tight joint with the face of the casting. When this lid is lowered, the only outlet from the superheater is through the take-off pipe and the wash-box.

The generator has suitable air blast and steam connections, the carburetter, air blast and oil connections, and the superheater, air blast connections as shown on the cut. The opera

tion of the apparatus is as follows:

A fire is started in the generator, A, and this vessel is filled with coke or anthracite coal which is brought to a high temperature by the air blast. Owing to the depth of the fuel bed and the comparatively limited amount of air admitted to the generator, the combustion of the fuel is not perfect, and the products of combustion passing off through the carburetter to the superheater contain carbon monoxide. By blowing air into the carburetter and superheater this carbon monoxide is consumed in these vessels and the checker work, with which they are filled, is highly heated. The products of the final perfect combustion so obtained pass off through the open stack valve to the stack and the atmosphere. This blowing up is continued until all parts of the apparatus have been brought to the proper temperature. The blast is then shut off and the stack valve closed. Steam is admitted through the steam pipe to either the top or the bottom of the generator, as the case may be, and passing through the fuel bed is decomposed by the hot carbon, its oxygen uniting with the carbon to form carbon monoxide, while the hydrogen is set free. The mixture of hydrogen and carbon monoxide, called water gas, passes over into the carburetter, through the top of which oil is admitted. The oil being vaporized by the heat, the vapors are picked up by the water gas and pass forward with it over the hot checker brick in the carburetter and superheater. The long continued exposure to heat converts the oil vapors into permanent oil gas, this oil gas being mixed during its formation with the water gas, and the result is an illuminating gas, which, properly speaking, should be called carburetted water gas, but is commonly known as water gas. The stack valve being closed, this gas passes out of the superheater by way of

the take-off pipe and wash-box, from which it goes to the scrubber and the rest of the apparatus.

The decomposition of the steam absorbs heat from the fuel in the generator and the distillation of the oil takes up heat from the checker brick in the carburetter and superheater, so that the apparatus gradually cools down during the periods of gas making. When it has been cooled to a temperature at which gas cannot be economically produced, the oil and steam are shut off, the stack valve opened, the blast turned on and the heating up process repeated. The operation thus consists of a series of alternating periods of heating up called "blows," and periods of making gas called "runs." (Trustees.)

4. Describe two forms of positive rotary exhausters, commonly employed to draw gas from the retorts, or relief holder, and force it through the rest of the apparatus.

Ans. The two forms of positive rotary exhausters commonly employed are the Mackenzie exhauster and the Root exhauster.

iron

An elevation and vertical cross-section of the Mackenzie exhauster are shown on the accompanying cut. A is a cast case, the general shape of which is cylindrical, with inlet and outlet openings as shown. The moving parts consist of the hollow drum D, open at both ends, the blades B, B, B, and the rolls R, R, R. The length of the drum is such that there is just clearance at each end between it and the inside of the projections of the case (shown on the elevation), the inside diameter of these projections being only a small fraction of an inch larger than that of the drum. This is set with its centre below that of the case, and its circumference tangent at the bottom to the rib E, running along the bottom of the case. It is rotated about its centre by means of a hollow shaft projecting from one end, which shaft is turned by a belt pulley or direct from an engine on the same bed plate as the exhauster. Between the pulley and the drum the shaft is supported by a bearing cast on the case, and a similar shaft projects from the other end of the drum to support it by means of another bearing on that end of the case.

The blades B, B, B, are free to rotate around a fixed shaft, the axis of which coincides with the axis of the main cylinder