Proceeding on erroneous data, many have persuaded themselves of having noticed that parallelopipedal and semi-cylindrical retorts last longer fit for use than those of a cylindrical shape, an assertion of which subsequent trials, conducted in the manner just stated, has clearly shown the fallacy. Enough has been done at the different gas works in the capital to settle this point, and there is now but one opinion amongst those who are best qualified to judge of the subject. Every body who has made the trial on a large scale, is convinced as already stated, that the best form of the retort for manufacturing coal gas where the process is conducted on the plan of decomposing coal in masses or layers of from four to eight inches in thickness, is a cylinder six and a half feet long, and one foot in diameter, and accordingly retorts of this shape and dimensions are now used in all the best regulated gas establishments in the metropolis. A cylindrical retort of the description before named, weighs about nine and a half to ten hundred weight. These and all other shaped retorts are furnished with a moveable lid or cover having a conical edge to fit the mouth-piece; the cover is rendered air-tight, not as formerly by grinding, a mode which was costly, but by the interposition of a thin coat of loom, between the lid and the mouth of the retort. The mouth-piece forms a separate part of the retort. It is bolted and screwed to a flanch which terminates the mouth of the retort, so that when the retort is worn out, the mouth-piece may be detached and applied to new retorts. There are now in action 620 cylindrical retorts, at the two chartered Gas Works[12] in the metropolis; and the total number of retorts at all the London gas establishments amounts to 960. [12] { Westminster Station 250 Retorts. Westminster Gas Works, { Brick Lane ditto 190 ditto { Norton Falgate ditto 50 ditto City of London Gas Works, Dorset Street, 130 ditto --- 620 _Application of heat.--Flue Plan originally adopted._ It must be obvious that the durability of the distillatory apparatus, greatly depends on the manner in which the heat is applied, to effect the decomposition of the coal contained within the retort. If the heat be very intense the whole vessel is rapidly destroyed. If it be too languid, the distillatory process is protracted, and much fuel, time, and labour wasted to no purpose; and the retort is speedily deteriorated, if the heat acts upon one part of it more than upon another.