Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/314

 298 IKON in Sweden, Norway, and Lapland furnaces of 30 to 40 feet in height and 1000 to 2000 cubic feet capacity are em ployed. Of late years, however, large furnaces have been built in Sweden of 50 to 60 feet in height, especially for Bessemer pig smelting. The internal shape of the blast furnaces in general use is some what variable. Those of the older construction may be described as being made up of two truncated cones placed base to base, the greatest diameter (the boshes) being about one-third way up ; those of more recent construction exhibit much less angle at the boshes, and are internally shaped more like a barrel, or like an inverted soda water bottle with most of the neck and the conical bottom cut off. Fig. 8 illustrates the alterations in size and shape that have taken place in the blast furnaces of the Cleveland district during the last thirty years or so (abridged from a paper by J. Gjers, Journal Iron and Steel Institute, 1871, ii. 202). Similar alterations in dimensions and shape of furnaces have taken place in other localities. The smaller furnaces of 30 to 35 or 40 feet in height have mostly been replaced (when worn out) by larger and higher ones, the angles of the internal cavities of the older shapes being rounded off. Fig. 9 illustrates the section and ground plan of one of the jalder form of open-mouthed furnaces used at Dowlais (Truran), consisting of a heavy mass of masonry, square at base, strongly braced together with iron tie-rods, rising in the shape of a truncated pyramid to the height of the boshes, and then surmounted with a conical top surrounded at the throat by a gallery for the introduction FIG. 8. Different Forms of Blast Furnaces. of the charging materials. In the square base were four arched re cesses or tuyere houses, one on each side, F, F, for the introduction of the blast through blowing holes by means of tuyeres, the front recess G also serving for the removal of cinder and the tapping of the fur nace for the running of the pig. The lowest portion of the hearth or crucible, A, was constructed of refractory sandstone, grit, or con glomerate, or of difficultly fusible firebrick, the inner portion of the upper part of the furnace being also built of firebrick set in fireclay with an air course between the double lining thus constructed ; ex teriorly the furnace was built of less expensive and refractory materials, usually of stone, strongly bound round with iron hoops. Above the charging gallery D a slighter brickwork continuation of the internal cavity arose, E, termed the tunnel head, in which door-holes, closed with movable iron doors, were perforated for the introduction of the charge. At the level of the boshes BB or thereabouts, the pyramidal base was finished off exteriorly with a cap or coping, tho conical shaft C rising upwards therefrom. Fig. 10 represents a somewhat later form, chiefly differing from the former in that the base is circular instead of square, the whole forming a truncated cone into which the tuyere houses A, A, A, B are sunk, the cinder notch and tapping hole for the outflow respectively of the cinder and pig iron being in the tuyere house B, towards which the walls of the hearth and crucible are cut away ; the outer portion of this form of furnace consisted of a shell of boiler plate ri vetted together, anil the masonry was considerably less massive than that of the older form. The more modern furnaces (such as fig. 11) are con structed like this, but the masonry at the base is still less massive, so that, instead of there being four distinct tuyere houses, the separat ing walls of the houses are wholly cut away and replaced by a number of stout iron pillars on which rests the greater portion of