Oven is a general term applied to variously formed apparatus employed for baking or drying different substances, many of which have been described in the course of this work; we shall therefore confine ourselves, in this place, to an account of that particular class of ovens which are used for the baking of bread. The common baker's oven, " upon the old principle" (as it is now distinguished), is usually a vaulted chamber of brickwork, of an oval shape, and having an iron door and frame in front; and there is mostly added in the upper part an enclosed closet with an iron grating, for the "tins" to stand on, called the proving oven. To heat these ovens, faggots are usually employed; these are put inside, and burnt to ashes, which are afterwards removed, and the bottom cleared out. During this process a great deal of the heat escapes; and as a still farther length of time is required to charge the oven with the bread, the oven must necessarily be made much hotter at first than is required for the baking temperature; and, consequently, a great waste of fuel is the consequence.

It this heat be not greatly in excess at first, the oven gets chilled before all the bread is put in, and causes the latter to sink and become "heavy." To remedy this inconvenience, ovens of more recent construction are built upon a solid base of brickwork, with a door of iron in front; and on one side of this is another iron door, opening into a small furnace, provided with a grating, on which the fuel (coal) is laid, and an ash-hole underneath. The fire-chamber is separated from the oven by means of a partition, but is open at the end: over this is usually erected a copper with a cock to it, for heating and supplying water to the bakehouse; and on one side of the copper is situated the proving oven, also inclosed in the brickwork. In heating the oven, the draught from the fireplace causes the flames and heated gases to sweep and reverberate around the circular walls and dome; and the soot that may be at first deposited upon the brickwork is subsequently burnt off, as the fire burns clear, or is brushed away before the bread is put in by a swab, such as is used for cleansing the decks of ships at sea.

The farthest extremity of the fire-chamber is usually provided with a sliding door, by the opening or closing of which the heat may be regulated; and thus, as well as from the proximity of the fire and chamber, the heat of the oven may be kept up during the time the bread is being put in.

An oven, invented by Count Romford, and termed the perpetual oven, has been much extolled; and though we have never seen it in use, it deserves, from its originality, ingenuity, and convenience, to be better known. For the baking of small bread, pastry, and the like, its utility is manifest. The following is the description given of it by the Count, with the manner of using it: - "In the centre of a circular, or rather a cylindrical mass of brickwork, about eight feet in diameter, which occupies the middle of a large room on the ground floor, I constructed a small circular closed fireplace for burning either wood, coals, turf, or peat; the diameter of the fireplace is about 11 inches, the grate being placed about 10 inches above the floor, and the top of the fireplace contracted to about 4 inches; immediately above this narrow throat, six separate canals (each furnished with a damper, by means of which its opening can be contracted more or less, or entirely closed,) go off horizontally, by which the flame is conducted into six separate sets of flues, under six large plates of cast-iron, which formed the bottom of six ovens on the same level, and joining each other by their sides, which are concealed in the cylindrical mass of brickwork.

Each of these plates of cast-iron being in the form of an equilateral triangle, they unite in the centre of the cylindrical mass of brickwork; consequently, the two sides of each unite in a point at the bottom of it, forming an angle of sixty degrees. The flame, after circulating under the bottoms of these ovens, rises up in two canals, concealed in the front wall of each oven, and situated on the right and left of its mouth; and, after circulating again in similar flues on the upper flat surface of another triangular plate of cast-iron, which forms the top of the oven, goes off upwards, by a canal furnished with a damper into a hollow place, situated on the top of the cylindrical mass of brickwork, from which it passes off in an horizontal iron tube, about 7 inches in diameter, suspended near the ceiling of the room, into a chimney, situated on one side of the room. These six ovens, which are contiguous to each other in this mass of brickwork, are united by their sides by walls made of tiles, about an inch and a half thick, and ten inches square, placed edgeways, having its separate canals furnished with a register communicating with the fire-place. Any one, or more of them, may be heated at the same time without heating the others; or the heat may be turned off from one of them to another, in continual succession; and, by managing matters properly, the process of baking may be uninterrupted.

As soon as the meat-pies, or puddings, are drawn out of one oven, the fire may be immediately turned under it to heat it again, while that from under which the fire is taken is filled with other dishes, and closed up." We have heard of several ovens having been erected, of which this plan of Count Rumford forms the groundwork.