Furnace, an utensil, or apparatus, in which a strong lire, either of coals or of wood, may be raised and maintained.

There is a great diversity of furnaces, according to the different purposes to which they are applied; but, as it would exceed our limits to specify them, we shall only state the chief points to be attended to in their construction, and next mention the various patents that have been granted to specula-tive individuals. - The chief objects in building, and arranging" a furnace, are:

1. To confine the heat as much as possible to the matter which is to be operated upon. Hence the fire is usually limited to a cavity formed with that intention, and which is provided with a door for supplying it with fuel, as likewise with a grate for supporting it, and permitting the air to pass through, as well as the ashes to drop down into what is called the ash-pit.— Thus, the heat is restrained so as to exhaust its force on the subject inclosed.

2. To prevent such heat from being dissipated; which design is effected by simply shutting the door of the furnace, and placing the matter to be acted upon, in such a direction as to receive the whole force of the lire, in its pas-sage up the chimney.

3. To produce an intense heat with the smallest possible quantity of fuel. Hence the throat, or funnel of the chimney, is occasionally contracted by a sliding plate; which, when shut closely, prevents the passage of any smoke or air; and, on drawing it out in a greater or less degree, leaves a vent propor-tionally large or small. Thus, a large quantity of fuel may be put in the furnace at one time, that will be slowly consumed, and consequently require less attention, than those furnaces which are destitute of this improvement. Where no great degree of heat is required, the sliding plate may be of cast-iron; in some cases, however, fire-clay will be more serviceable ; but contrivance is inapplicable to such furnaces as consume large quantities of fuel, and especially where metals are to be melted.

4. To arrange the whole, so that the degree of heat maybe regulated at pleasure ; which intention is ef-fected by admitting only a certain portion of air to pass through the fuel. For this purpose, the late Dr. Black recommended to fill the upper part of the furnace frequently with small portions of soot, so that by closing the door of the ash-hole, and perforating it with a certain number of holes corresponding to each other other, a suf-ficient controul may be obtained over the fire. When the heat is to be increased, all the passages should be opened, and the height of the vent extended ; by which means the column of rarefied air will be enlarged, at the same time its passage through the fuel promoted, and consequently also the heat of the furnace rendered more intense.

In June, 1783, a patent was granted to Mr. James Watt, of Birmingham,for his newly-improved method of constructing furnaces or fire-places for heating, boiling, or evaporating water, or other liquids; and also for heating and melting metals, or smelting ores; by which greater effects are produced from t e fuel, and the smoke is in a great measure prevented or consumed. The patentee effects these different objects, by closing every passage to the chimney or flues, excepting those left in the interstices of the fuel; by placing fresh fuel above or nearer to the external air, than that which is already converted into coke or charcoal; and by constructing the fire-places so that the flame must pass down-wards, or laterally, or horizontally, through the burning fuel, and also from the lower part or internal side of the fire-place, to the flues or chimney. - In some cases, Mr. Watt causes the flame to pass through a very hot funnel, or Hue, previously to its arriving at the bottom of the boiler, or at that part of the furnace, where it is intended to melt metals; by which contri-vance the smoke is still more ef- . fectually consumed. In other cases, he directs the course of the flame from the fire-place immediately into the space beneath a boiler, or into the bed of a melting or other furnace. - A minute account of this machinery is inserted in the 4th vol. of the "Repertory of Arts and Manufactures," where it is describ-ed and illustrated by engravings.

In 1794, Mr. Henry Browne, of Derby, invented an ingenious furnace, calculated to -facilitate evaporation; for which the Society for the " Encouragement of Arts," etc. rewarded him with a gold me-dal. - By this arrangement, the heat is first carried under the vessel, then reverted back on the sides, and, at length, conveyed over the surface: thus the air in contact with the liquor is heated and rarefied to such a degree, that the fluid is raised into vapour or steam, much sooner, and with less fuel, than in the cold atmosphere; and, as the air necessary to keep the fuel in combustion passes over the surface of the liquor, every pernicious vapour is carried with it into the fire, where it is decomposed, or at least rendered innoccuous. Mr. Browne's furnace is likewise so constructed, that as much fuel may be laid on the fire at one time as will be required for twelve, or even twenty-four hours; and thus one man is enabled to perform the labour of three, with much greater facility than by the usual method. Beside-this advantage, the evaporation is more speedily effected; less fuel is consumed than in the common boilers now in use; and, neither the operator nor the neighbourhood will be annoyed with the most per-nicious vapour. - Those who wish to be informed of the various parts of this useful contrivance, we must refer to the 12th volume of the "Transactions' of the Patriotic Society before mentioned, where it is minutely described, and illustrated by an elegant engraving.

A patent was granted in Decern-ber, 1798, to Mr. William Rally, of Newbald, in the East Riding of Yorkshire, chemist; for his invention of a philosophical furnace and boiler, with an actuating wheel appended to them; and which are applicable to the drawing of foul and inflammable air from pits, mines, etc. to several branches of pharmacy, and various mechanical purposes. As, however, this machinery is scarcely suitable to domestic economy, we shall only add, that the specification of it is contained in the 10th volume of the "Repertory of the Arts and Ma-nufactures."

The last patent which claims our notice, is that granted in November, 1799, to Mr. James Burns, of Glasgow, builder; for his invention of certain improvements applicable to furnaces, fire-grates, stoves, and chimnies, by which a greater supply of heat may be obtained from a given quantity of fuel; and rooms of every description may be heated more speedily andeffectually than by the methods now in use; while they are calculated in a great measure to prevent accidents from ' women's and children's clothes taking fire, and also to give a degree of cleanliness which cannot be attained where grates and stoves of the common construction are employed. The design of this contrivance is to prevent the heat, generated and thrown out into any room or apartment by combustion, from being unnecessarily wasted by the air of such room being made to maintain the combustion of the fuel in the grate. To effect this purpose, the air supporting the fire in the grates or stoves made with the patentee's improvements, or in other grates to which they may be applied, ought to be conveyed through a tube (which he calls an air-tube) from the outside of the house : or it may be made to pass from the outside of the house between the joists, so as to be brought to the bottom bars of the grate, without communicating with the interior air of the room ; while the grates, and other parts connected with them, should be so construct-ed, that the passage may be closed in a greater or less degree by means of a valve, small door, cock, or any similar contrivance, wherever it is not requisite to supply the fire with coid air from the outside of the house: or, the same object may be attained by directing the tube to a cellar, larder, etc. which will thus be thoroughly ventilated, and prevented from acquiring unhealthy or disagreeable smells. - As our limits will not permit us to specify the constituent parts of Mr. Burns's design, we refer the reader to the 12th vol. of the Repertory, etc above quoted, where it is minutely described, and farther illus-trated by two plates. But we can-not conclude the subject, without stating, that his improvements are affirmed to be an effectual cure for smoky chimnies ; and when a fire is lighted in grates of the patentees construction, it burns up, and be-comes lively in a few minutes, without the aid of bellows, and that watchful care which common stoves or grates require. - See also Boilers, and Pipe-place.

Furnace. - The construction of furnaces, so arranged as to consume the whole volume of smoke, is an object which has long engaged the ingenuity of artists. - Hence various contrivances have been proposed ; but few, we believe, for efficacy and simplicity, can come in competition with that invented by Messrs. Robertsons, of Glasgow. The opening of their furnaces, instead of being closed by a door, consists of a quadrangular hopper or funnel, which is contantly supplied with coals ; so that, in proportion as the fuel is exhausted, a fresh stock continually descends through the hopper. Thus, the first combustion, which disengages the greatest part of the smoke and flame, takes place near the mouth of the fire-place, and a considerable quantity of the smoke will, without any other contrivance, be consumed by passing over the red-hot fuel in the farther part of the furnace. Bat, as a perfect combustion of smoke cannot be obtained without the aid of atmospheric air, a cast-iron plate, about three quarters of an inch above the top of the hopper, is introduced, so that a slit is formed of this depth, and of an equal breadth to the front of the furnace; through which a current of air constantly enters, and is then combined with the smoke. This aperture may be enlarged, or diminished, as occasion may require, by raising or lowering the iron plate, by means of an iron pin : thus, the supply of air may be proportioned to the quantity of smoke produced, and tie whole of the latter will be advantageously used, before it can escape through the chimney.