Annexed is the common reverberatory furnace. At d a is the ash pit and fire-place; b b the body of the furnace; c c the dome or reverberating roof; d the chimney; e door of the ash pit; f door of the fireplace; g handles of the body; h aperture to receive the head of the retort; t handles of the dome; k receiver; f stand of the receiver; m retort, represented in the body in dotted lines.
Aikin's Portable Blast Furnace is represented in the figure on the following page, it is composed of three parts, all made out of the common thin black lead melting pots, sold in London for the use of the goldsmith. The lower piece c, Fig. 1, is the bottom of one of these pots, cut off so low as only to leave a cavity of about an inch deep, and ground smooth above and below. The outside diameter, over the top, is five inches and a half. The middle-piece, or fire-place a, is a larger portion of a similar pot, with a cavity about six inches deep, and measuring seven inches and a half over the top outside diameter, and perforated with six blast holes at the bottom. These two pots are all that are essentially necessary to the furnace for most operations; but when it is wished to heap up fuel above the top of a crucible contained, and especially to protect the eyes from the intolerable glare of the fire when in full height, an upper pot b is added, of the same dimensions as the middle one, and with a large opening in the side, cut to allow the exit of the smoke and flame.
It has also an iron stem with a wooden handle (an old chisel answers the purpose very well,) for removing it occasionally.
The bellows, which are double (d), are firmly fixed by a little contrivance, which will take off and on, to a heavy stool, as represented; the nozle is received into a hole in the pot c, which conducts the blast into its cavity. No luting is necessary in using this furnace, so that it may be set up and taken down immediately. Coke, or common cinders, answer well for fuel; and the heat which this little furnace affords is so intense, that its power was first discovered by the fusion of a thick piece of cast-iron.
Chevenix's Wind Furnace is represented in the following engraving. It is, in some respects, Dr. Ure observes, to be preferred to the usual form. The sides, instead of being perpendicular, are inclined, so that the hollow space is pyramidical. At the bottom the opening is thirteen inches square, and at the top eight. The perpendicular height is seventeen inches; the form appears to unite the following advantages: 1st, A great surface is exposed to the air, which, having an easy entrance, rushes through the fuel with great rapidity; 2d, The inclined sides act in some measure as reverberating surfaces; and 3d, The fuel falls of itself, and is always in close contact with the crucible placed near the grate. The late Dr. Kennedy, of Edinburgh, whose opinion on this subject claims the greatest weight, found that the strongest heat in our common wind furnaces was within two or three inches of the grate: this, therefore, is the most advantageous position for the crucible, and still more so when we can keep it surrounded with fuel.
It is inconvenient and dangerous for the crucible to stir the fire often to make the fuel fall; and the pyramidal form renders this unnecessary.
It is also more easy to avoid a sudden bend in the chimney, by the upper part of the furnace advancing in this construction.