The fourth part, which constitutes about two thirds of the height of the fur nace from the base of the hearth up to the throat, presents the figure of a sur face of revolution, generated by a curve whose concavity is turned towards the axis of the furnace, and whose last tan gent towards the bottom is almost verti cal. This surface is sloped off with that of the boshes, so that no sharp angle may exist at the belly. In some furnaces of considerable dimensions, as in that with three to arcs, this portion of the furnace is cylindrical for a certain height.
The conical orifice called the tugre, in which the tapered pipes are placed, for imparting the blast, is seen near the bot tom of the furnace, fug. 59, at A. Nose tubes of various sizes, from 2 to 4 inches in diameter, are applied to the extremity of the main blast-pipe. Under A is the bottom of the hearth, which, in large fur- . naces, may be two feet square. a is the top of the hearth, about two feet six in ches square. A n the height of the hearth, about six feet six inches. a shows the round bottom of the conical or funnel part, called in this country the bashes, standing upon the square area of the hearth. c is the top of the boshes, which may be about 12 feet in diameter, rind feet in perpendicular height. n is the furnace top or mouth, (gueulard in French,) at which the materials are charged. It may be 41 feet in diameter. The line between c, D, is the height of the internal cavity of the furnace, from the top of the bosh as upwards, supposed to be 30 feet. A, D, is the total height ot' the interior of the furnace, reckoned at 441 feet. E E is the lining, which is built in the nicest manner with the best fire bricks, from 12 to 14 inches long, 8 inches thick, and curved to suit the circle of the cone. A vacancy of 3 inches wide is left all round the outside of the first lining by the builder ; which is sometimes filled with coke dust, but more generally with sand firmly returned. This void space in the brick-work is for the purpose of allowing for any expansion which might occur, either by an increase in the bulk of the building, or by the pressure and weight of the materials when descending to the bAtom of the furnace. Exterior to E E is a second lining of fire-bricks simi lar to the first. At a, on either side, is a cast-iron lintel, 81 feet long, by 10 inches square, upon which the bottom of the arches is supported. a, 43, is the rise of the tuyere arch, which may be 14 feet high upon the outside, and 13 feet wide. The extreme size of the bottom or sole of the hearth, upon each side of A, may be 10 feet square. This part and the bosh
ing stones are preferably made from a coarse sandstone grit, containing large rounded grains of quartz, united by a siliceo-argillaceous cement.
The blowing machines employed in Staffordshire are generally cast-iron cyl inders, in which a metallic piston is ex actly fitted as for a steam engine, and made in the same way. Towards the top and bottom of the blowing cylinders, ori fices are left covered with valves, which open inside when the vacuum is made. with the cylinders, and afterwards shut by their own weight. Adjutages conduct into the iron globe or chest, the air ex pelled by the piston, both in its ascent and descent ; because these blowing machines have always a double stroke. As soon as the blast furnace gets into a regular heat, which happens about 15 days or three weeks after fires have been put in it, the working consists simply in. charging it, at the opening in the throat, whenever there is a sufficient empty space; the only rule being to keep the furnace always full. The coke is measur ed in a basket, thirteen of which go to the ton. The ore and the flux (limestone) are brought forwards in wheel-barrows of sheet iron. In 24 hours, there are, thrown into a furnace the following: 144 tons of coke, 16 tons of roasted ore, and 61 tons of limestone; from which about 7 tons of pig iron are procured. This is run off every 12 hours • in some works the blast is suspended during the discharge. The metal intended to be converted into bar iron, or to be cast again into moulds, is run into small pigs 3 feet long, and 4 inches diameter • weigh ing each about two hundred weight and a half.
The disorders to which blast furnaces are liable have a tendency always to pro duce white cast-iron. the color of the slag or scoria; is the surest test of these derangements, as it indicates the quality of the products. If the furnace is yield ing au iron proper for casting into moulds, the slag has a uniform vitrification, and is slightly translucid. When the dose of ore is increased in order to obtain a gray pig iron, fit for fabrication into bars, the slag is opaque, dull, and of a greenish yellow tint, with blue enamelled zones. Lastly, when the furnace isproducing a white metal, the slags are black, glassy, full of bubbles, and emit an odor of sul phureted hydrogen. The scoriae from a coke are much more loaded with lime than those from a charcoal blast furnace. This excess of lime appears adapted to absorb and carry off the sulphur, which would otherwise injure the quality of the iron. The slags, when breathed on, emit an argillaceons odor.