A vessel or apparatus, wherein fuel is burnt in chemical, manufacturing, and culinary operations. Furnaces are as various, and even more so, than the particular objects for which they are designed; to accomplish these so that they shall perform their offices in the most economical and convenient manner, is the proper study of those who have to construct or employ them. The proper choice of materials, adapted to the degree of heat and other circumstances, is also of the greatest importance; indeed, the resulting products of furnaces greatly, and often wholly, depend upon the combined application of chemical knowledge, manufacturing experience, and inventive skill. The following appear to be the essential qualifications of a good furnace: first, to be able to concentrate the heat, and direct it as much as possible to the substances to be acted upon; second, to prevent the dissipation of the heat after it is produced; third, to obtain the greatest quantity of heat from a given quantity of fuel; fourth, to be able to regulate at pleasure the necessary degree of heat, and have it wholly at the operator's management Under the articles Boiler, Iron, Air, Foundry, and various others that occur in this work, numerous practical examples are given of the construction of furnaces; it will therefore be our business, under the present head, to supply the deficiencies that are left on the subject, which we shall premise by some observations on the nature and proper construction of furnaces in general.

In the construction of furnaces for boilers every thing should be combined that has a tendency to add to the effect of the fuel, and to avoid that which is calculated to diminish its effect; but without a knowledge of the nature of burning, we should be like seamen traversing the ocean without a compass. When a portion of fuel is ignited in a close fire-place it must be supplied with air to enable it to burn: and the fuel itself in the process of burning is partly converted into gaseous matter, which escapes up the chimney with a portion of the air supplied to the fire; but the greater part of the air so supplied ought (as Mr. Tredgold observes to be changed in the process, by its oxygen uniting with the carbon, and other combustible parts of the fuel, forming carbonic acid gas, vapour, etc. Now, in order that perfect combustion, or burning of the fuel, may take place, the air should have free access to every part of the fuel, which is heated sufficiently to burn; as fuel must be heated in a certain degree, otherwise its elements will not combine with the oxygen of the air.

And we see clearly the advantages of a regular supply of fuel: this advantage is greater in proportion to the quantity of hydrogen contained in the same; for if a large body of such fuel be at once put upon the fire, much of the hydrogen will escape in a gaseous state unconsumed, carrying off with it a very considerable portion of heat; whereas if the fuel be thinly scattered over the surface of the fore part of the fire the hydrogen would most likely be consumed in passing over the red hot embers in the after part of the fire, and the product go off in steam; and that the latent heat of such steam may not be lost, it will be desirable to have a horizontal flue of metal for the smoke to pass along after it has left the boiler, when the steam can be condensed, and the heat applied to warm water for the boiler or other useful purposes. But to succeed in consuming the combustible gases, it is necessary that they should mix with the air that has become hot bypassing (as expressed by Mr. Watt, in the specification of his patent in 1785,) "through, over, or among, fuel that has ceased to smoke," or by being drawn through small flues or channels in the brickwork round the fire, in such a manner as to be heated before it mixes with the gas to be consumed; but unless hydrogen, or some of its combinations, are constantly passing off, the introduction of a stream of air into the fire-place will only take away the heat from the boiler; and therefore in a slow fire, consisting chiefly of carbon, it will do more harm than good; while in a quick fire of cherry coal, or cannel coal, in which hydrogen is abundant, it must be a great advantage, and particularly when the fire is regularly supplied with fuel.

The quality of the air to supply the fire, Mr. Tredgold remarks, is worthy of being considered, although any dirty wet hole is usually esteemed good enough for the fire place. Now the air ought to be dry, for air charged with moisture is improper, and only takes away heat; but where there is a very low chimney, and consequently an imperfect draft, some water in the ash-pit will increase the draft, by being converted into steam by the heat of the ashes, the mixture of the steam rendering the smoke much lighter than common air. The air should be cool when it enters the ash-pit, that it may pass with greater velocity through the fire, and the fire-place shed should be dry, in order that the apparatus may be durable, and be kept in order with little attention. The opening to admit air to the fire should be sufficiently large for producing the greatest quantity of steam that can be required, but not larger, and it should be constructed so as to increase in size as it approaches the fire. The area of the spaces between the bars should clearly be much greater than the area of the place that admits air to the fire.

The fire should be made immediately under the boiler or other vessel to be heated, that its full effect may be exerted upon the bottom; and after quitting the fire, the mixture of flame and smoke should pass through a wide and shallow aperture, called the throat; - wide, that it may spread under the greatest surface of the boiler; and shallow, that it may pass through with considerable velocity, and consequently be impelled against the bottom of the boiler. In making the flue circulate, according to the usual mode, round the sides of a long boiler, the heat never extends far enough to render it effectual throughout its length, and the action being oblique the advantage gained is very trifling; for the same reasons, the making of a flue to return through the boiler offers no advantage that compensates for its complexity of construction, since the heat may as well be confined to act upon the bottom, and have less depth of water. The depth of fuel to be on fire at the same time should be sufficient to ignite the fresh fuel, without impairing its action on the boiler in a sensible degree. From the observations and experiments made by Mr. Tredgold to determine this point, it appears that the depth of burning fuel should be about three or four times the depth of what is added at a time in feeding the furnace; that is, four times when you feed frequently and three times when you feed seldom; and according to the nature of the fuel, there will be greater or less space wanted between the bars and the boilers. In the construction of furnaces, the slowest conductors of heat should be used; some metal work is absolutely necessary - that is, simply the bars and a frame at the mouth, where the fuel is put in, with or without a door; in the latter case, the space is to be filled with fuel. The rest of the brickwork should be built with hard well burnt bricks; and in order to confine the heat to the boiler, it will be proper to leave cavities in the brickwork; and, with the exception of the necessary ties, to form a double wall, with a hollow space between, keeping the maxim of Morveau always in view - to insulate the fire-place from all bodies that are rapid conductors of heat.

Between the fire door and the bars there should be a dead space; on this the fresh coals are laid, previous to their being pushed forward on the grate, which should not be done until they have given out their gas over the brightly ignited fuel on the bars. This dead space is usually covered with an iron plate, called a dead plate, but it is preferable to floor it with fire tiles, as the latter are less liable to affect the fittings of the door and frame. The bars are of course proportioned to the size of the furnace; they are usually from 11/2 to 3 inches in depth, and the thickness varying from 3/4 to 11/4 inch; the length seldom exceeds three feet; and where a more extended grate is required, they are generally laid in separate lengths upon transverse i earing bars, which receive both ends. The spaces between the bars are from 3/8 to 1/2 an inch. The whole area of the grating should be about one-fourth the area of the bottom surface of the vessel to be heated; each foot of such grating is adapted, according to Mr. Tredgold's calculations, to burn one-eighth of a bushel of coal per hour.

The same area will answer for either a slow or a quick fire, but in a slow fire a greater depth of fuel is necessary; and also for equal bulks of any other kind of fuel, the same area will apply as for coals; but it will be obvious from this rule that the areas to produce equal quantities of steam will be inversely as the power of the fuel. The damper is best situated at the opening into the flue; it should be supported in its slide by a counterbalance weight, and its action be rendered easy and certain. The door should be made to shut as close as possible; but there is a difficulty in keeping it so, when exposed much to the action of the fire; they are best defended by making them double, with a hollow space for air between them. Mr. Atkinson's mode of constructing them is good; he rivets on the inside a hollow cast-iron box, which just fits the doorway; the depth of the sides of the box so strengthens the door as to prevent its warping, while the hollow space of confined air prevents the escape of heat.

A sliding door, balanced by a weight, in the manner of a sash window, has many advantages; it is more easily opened and shut; is out of the way when open, and shuts close: when any thing is to be done at the fire, a smaller opening suffices.