From each end of the retort a tube projects horizontally, which serves to support and guide the scraper rods, which slide through them.

The form of the scraper b is shown by the separate figure 2, and the form of c by Fig. 3; in each of them are two square notches, which, sliding upon square bars of iron, placed longitudinally on the upper side of the interior of the retort3, are thereby suspended to it, and kept uniformly in their proper positions. The process of working the retort is as follows: - previous to charging it, the scraper c is drawn outwards from the body of the retort close up to its end, and the scraper b is pushed inwards so as to come in contact with" e; both scrapers being then beyond the opening d, the charge of coals is admitted through the latter by opening the cover e; the scraper b is then drawn back from its position beyond d, and thus spreads the coals in an even layer over the bottom of that part of the retort exposed to the fire. About a foot from each scraper, the rods are connected by a solid and a hollow screw, so that when the rods are drawn out, they may be renewed by unscrewing them at these parts. As the distillation of the coal proceeds, the gas escapes by the tube f, and from thence passes through the condenser to the purifying vessels.

When all the gas has been separated from one charge of coals, the scraper c is thrust forward, pushing before it, and clearing out all the coke, which falls into the coke box g, from which it may be discharged by the mouth piece h into a barrow, and wheeled away. As it is necessary that the apertures at d and h should be closed air-tight during the extrication of the gas, the lids or doors are lined with lead, which, being screened from the heat of the furnace, serves to close the joints as effectually as the troublesome process of luting them at every charge. Fig. 4 gives an external view of that end of the retort where the coke is discharged, and Fig. 5 a similar view of that end at which the coals are put in; the letters have reference to the like parts in each figure. Fig. 6 gives a vertical section of a series of three vessels, ij k, which constitute the purifying vessels; the lower parts of them are occupied with a stratum of lime as high as the dotted line f f; the impure gas passes from the retort into the tube m, and by its pressure descending along the bent arm, it passes out at the bell-mouthed end, and mixes with the lime contained in the vessel i, wherein it deposits much of its impurity by condensation; from this vessel the gas again rises and enters the tube n, thence filtering through the second lime vessel j, it reascends and enters the tube o into the third filtering vessel k; thence it proceeds by the tube r, to the gasholder, for subsequent distribution.

The covered tubes p p p above the filtering vessels are for the purpose of charging them with lime; the dotted circles are perforations in the sides of the vessels (furnished with plugs) for the purpose of ascertaining the depth of the stratum of lime, or other purifying material; and the three tubes q q q at the bottom of the vessels are for discharging the lime when saturated with the various matters that have been condensed during the purifying process. To aid the complete discharge or clearing of these vessels of their contents, three bars of iron are employed as scrapers, one in each vessel, and a rod, as a handle, is screwed into each, and passes through a stuffing box in the side of each vessel. We are not aware whether the preceding apparatus has been adopted in practice. Retorts of a semicircular form in their transverse section have been tried, but although the coal was very rapidly and effectually decomposed, they were so speedily destroyed that we believe retorts of this figure are no longer employed.

Upon the subject of retorts generally, it may be observed, that they, together with the hydraulic mains, and other necessary connexions, form one of the principal items of charge in the erection of gas works on the ordinary plan; and that in conducting works on the same principle, an enormous expense is annually incurred by the oxidizing or burning away of the retorts. Indeed the oxidation of these vessels is so rapid, that however the time of their duration may vary from a difference of their form, the quality of the iron, or the mode of setting them in the furnace, they cannot withstand, on an average, more than eight or nine months' wear. To avoid these sources of expense, and with a view to other advantages, Mr. S. Broadmeadow, of Abergavenny, devised the following process, which he secured by patent. The plan was adopted at the Abergavenny Gas Works, but has not come into use elsewhere; and although it may not realize all the advantages contemplated by the inventor, is deserving of notice in this place.

The plan consists, first, in substituting brick ovens for iron retorts; secondly, in exhausting the ovens of gas, as fast as it is generated, by means of an exhausting cylinder, or any equivalent apparatus; and thirdly, in purifying the gas so generated, either wholly or partially, by admitting into the gasometer a certain portion of atmospheric air. The engraving on page 593, with the following description, will sufficiently explain the process; a is the oven in which the gas is generated; b the oven door; d door of the fire grate; e a pipe through which the gas is conveyed from the oven to the condenser f, into which a small hand-pump g is inserted to draw off the coal tar; h a pipe through which the gas passes from the condenser into the top of the exhausting cylinder i: the piston of this exhausting cylinder receives its motion from a small steam engine, which is supplied with steam from a boiler fixed in the flue, and heated-by the waste fire of the furnace, k k two pipes, one leading from the top and the other from the bottom of the exhausting cylinder to the purifier l; m the outlet pipe to convey the gas from the purifier into the gasometer; and n is a pipe branching from the pipe h to convey the gas, at the alternate vibration of the beam, into the lower part of the same cylinder.