IRON ORES. 



253 



The annexed figure 2, exhibits the interior laid open. The main 

 •tincture is called the stack. Of the interior cavity, the lower part, 

 2 H, h, is the hearth, H is four- 



sided ; B B, the boshes* having 

 nearly the shape of a funnel, ex- 

 cept that it is square below ; above 

 b, is the proper furnace, usually 

 about 30 feet high ; below the cru- 

 cible, lies the hearth, commonly of 

 refractory grit rock. The furnac 

 is circular, and is lined with fir 

 brick (I) ; next to this, is a layer ol 

 dry sand (r,) and then one of brick 

 (r 7 ,) constituting the inner part of 

 the stack. The layer of sand al- 

 lows the interior to expand by 

 heat, without cracking the exte- 

 rior ; and moreover, the whole, I, 

 r, r', may be removed for repairs 

 without injuring the exterior work. 

 At t, is one of the twiers, (or tuyeres,) the tubes by which the blast of 

 air is driven into the furnace. At m, is a partial partition of fire brick, 

 called the tymp, separating the back and front of the hearth, but not 

 extending to the bottom or hearth-stone. The hearthstone is made of 

 a refractory grit rock. 



In each side of the four-sided stack, at bottom, there is a door-like or 

 arched opening, (A, figs. 1 , 2,) which extends in to the stonework that en- 

 closes the hearth. Three of these opening are called the twier- arches, 

 and the other is the front or working arch ; the twiers enter by the 

 twier-arches to the interior, and at t, (fig. 1,) is shown the place of en- 

 trance of one. The view in figure 1, gives a front view of a twier arch ; 



and 



in figure 2, at A, there is a side 

 3 



view, with the twier in place. 

 To prevent the melted metal, 

 which often rises above the 

 twiers, from flowing into the 

 blast pipe, in case of the blast 

 being accidentally checked, 

 there is at V (fig. 2) a valve, 

 which is raised by the blast and 

 closes when it stops ; and at k, 

 a place for inserting a rod to 

 remove any slag that may cling 

 to the twier. 



Figure 3, is a horizontal sec- 

 tion, at bottom ; A, A, A, are 

 the twier arches, separated by 

 the masonry of the stack ; H, 

 h, the position of the hearth or 

 crucible ; m is the tymp be- 

 tween H and h; f, t, t, are the 



* This word is from the German word boschvng, a slope. H. 



