reveal at the front, to enable the front wall of setting to cover instead of forming a straight joint. The top of the bench above the arches should be haunched up five courses with fire-bricks laid solid, and finished with one course on edge to form a coping, making the total height of the bench about 10 feet 6 inches from the floor line; these particulars of course referring to a ground-work retort house.

In the crown of the arch two flue-holes are formed, one for each furnace. These are usually 12 inches square, and situated at the back of the setting. On the top of the bench the main flue communicating with the chimney is constructed; this is divided in the middle, so as to form two flues, and is constructed of fire-bricks set in fire-clay; it is provided with dampers, which cover the two flue-holes in the furnace arch.

It was formerly the practice to have only one tall chimney-stack, which served for all the benches in the retort house, but it is now the common practice to erect dwarf chimneys, built on the top of the arches, and carried only a few feet above the ridge of the roof, one such chimney serving for a bench of six or eight settings.

"It is very important that the main flue and chimney should be of ample area, in order to ensure good working, and to ensure this the area should be about 40 superficial inches for each ton of coal carbonized by the bench of retorts for which it is constructed. Where one chimney serves a large bench of retorts, say of twelve settings, this figure may be reduced to 30 superficial inches per ton of coals carbonized” (Herring).

The whole of the brickwork forming the retort bench must be braced together by means of buckstays, tie-rods, etc., both longitudinally and cross-wise, in order to prevent any undue expansion that may be caused by the heat of the setting.

A suitable arrangement of buckstays is shown in fig. 2.

The arrangement consists of two flat wrought-iron bars, connected together at intervals by cast-iron distance pieces, and trussed as shown. One of these buckstays is placed at each end of the division wall of the setting, and is carried a few feet above the top of the retort bench, the tops of the buckstays being connected together by a wrought-iron joist, which not only acts as a tie, dispensing with the customary transverse tie-rods, but

also serves to support the hydraulic main. The lower ends of the buckstays should be securely fixed in concrete to a depth of at least a foot. The end buckstays should be stronger than the front ones, on account of the greater thrust on the buttress walls, and generally consist of H-iron girders. There should be four buckstays fixed at each end buttress of the bench, not less than 12 inches deep. These buckstays are attached to longitudinal tierods, which run the full length of the bench; the tie-rods are of wrought-iron, 2 inches in diameter, divided into two or more lengths. The tie-rods are attached to the

H-iron buckstays, either by a flat iron strap, which fits over the top of the H-iron, and is welded to the tie-rod, or they are screwed for a distance of about 12 inches at their ends, and pass through a hole tapped to the same thread at the top of the buckstay, the projecting end being secured by a nut. Stretching screws and unions are employed for making the joints between the lengths of the tie-rods.

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Fig. 4 is a longitudinal section of a bench of "through" retorts, showing the furnace,' the arch," and the connection with the main flue.

The methods of setting and heating retorts employed by different engineers necessarily vary, but the following brief description of one method, with the aid of fig. 3, should make the subject tolerably clear.

As will be seen, the retorts are placed in an arch horizontally, within a few inches of each other. Situated in the centre, below the level of the bottom retorts, is a furnace, which may be described as a rectangular firebrick chamber with an arched top, and having an iron door for the purpose of charging with fuel. The furnace is open at the bottom, which is provided with fire-bars, on which rests the fuel, the air necessary for the combustion of the latter, passing through the spaces between the bars.

The furnace arch is made with openings at certain intervals so as to allow the heating gases to pass into the setting, such openings being known as "nostrils."

The retorts are supported on fire-brick walls, which are formed by carrying up the brickwork corresponding to the ́solid portion of the furnace arch, so that there is alternately a solid wall and a hollow space corresponding to the nostril holes in the furnace arch; the walls are also provided with openings which communicate with the hollow spaces at intervals, so that the heating gases circulate in every direction round the retorts, the arrangement described answering the purpose of a series of flues. After

traversing the top retorts the heating gases pass over a solid wall, known as the bridge wall, and descending, enter, by means of an opening at the bottom, a hollow chamber or upright flue, which is in communication with the main flue. The main flue is provided with suitable dampers for

the purpose of regulating the amount of draught, which should be sufficient, and no more, to admit the necessary amount of air required for the perfect combustion of the fuel. Fig. 4 shows another method of setting retorts. Here it will be seen that the furnace is placed between the retorts, and the heating gases, after traversing the top retorts, descend, and then enter a flue underneath the

bottom retort; this flue is divided by a mid-feather, so that the heating gases travel first from back to front, and then back again, where it enters the upright flue in communication with the main flue, as in the first example.

Fig. 5 shows a slightly different arrangement. In this case there are four partitions, which are continued along the whole length of the retorts, so that the heating gases are blocked from passing in any direction except over the

two top retorts, over which they descend, and passing to the front of the setting enter a flue beneath the bottom retorts, and from thence to the main flue. It will be noticed that in this method of setting retorts the transverse walls are dispensed with.

In retort settings of the description given, the heating gases are produced by a furnace placed at each end of a through setting. The furnace is perhaps the most important part of a setting of retorts, and it is very essential that the student should have a clear idea of what goes on