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and permanency. When bricks are ufed, it is found of 

 utility to make the whole part of the building folid, back as 

 far as the external fquare of the hearth, fo that it the bofhes 

 fail in part as to difplace one layer of bricks, another furface, 

 equally fredi and entire as the former, prefcnts itfelf to the 

 aftion of the lire. 



Fig. 2. Ground plan of the top of the ho.fhings o( fg. I. 

 A A and B correfpond to the fame letters in the elevation. 

 The dotted fquare C defcribes the form and dimenfions of 

 that part of the hearth immediately above the tuyere, as feen 

 in the elevation C C. The large dotted fquare D D is the 

 external lizc of the hearth, as feen alio in the elevation 

 DD. 



Flg.^. Ground plan of the fquare and bofhings of 



J'S- 4- 



While we profccute the detail and hillory of theconllnic- 



tion of the blaft furnace, the fame diverfity of plans formeriy 

 noticed comes under revievir, in every department of the erec- 

 tion. The importance of the hearth is admitted by every 

 clafs of reafoners upon this fiibjeft ; and to deviie a form 

 better calculated for fmclting than another, has been an ob- 

 jeft of general concern with the nianufaiturcr. Much as may 

 be deemed to depend upon its form and conllrutlion, infinitely 

 more benefit is derived from u proper quality of Hone, to refilt 

 for a given length of time the powerful effetts of a continued 

 and unremitting blaft. To both of thefe important dcfiderata 

 much of the manufafturers attention has been from time 

 to time diredtcd. . 



The firft fmgularity that ftrikes us forcibly in the figure 

 of the hearth, is, that in place of being circular, hke the 

 upper parts of the furnace, it is conftrufted of a fquare fun- 

 nel-form, with angles as acute as reprefented in Plate IX. 

 Jig. I. This narrowing form is continued on three fides 

 of the fquare to the bottom of the hearth, where it generally 

 meafures from 22 inches to 24 inches. The top of the 

 hearth, at B or A, fg. I. and 4. or as it is commonly called 

 the fquare, is never lefs than 30 inches, nor more than 33. 

 The height of the hearth from E to B, Plate IX. fg. i, 

 7 feet, and none are made higher. From C to B, Jig. 3, 65 

 feet, wliich is now reckoned the moft advantageous height ; 

 and from C to A, fg. 4, the hearth meafures 6 feet, under 

 which height there are no hearths ever attempted. 



The ftrufture of a hearth, properly fpeaking, confifts of 

 three folid fides only, the fourth, or front, is filled up by the 

 tymp, or key-ftone. Plate IX. fig. l. The block E is ge- 

 nerally in one piece, and from four to five feet long, accord- 

 ing to the height of the hearth. It defccnds towards 

 the bottom till within two feet or two feet four inches, 

 and then leaves an opening of fim.lar dimenfions as to 

 height, into the centre of the he. rt':i or funnel, as at 

 letter F. 



As the fquare form in which the hearth is finifhcd cannot 

 laft a day after the blaft is introduced, and is even frequently 

 deftroyed in the aft of annealing, or heating, it cannot be 

 effentially necefiary to the making of iron. The hearths of 

 all furnaces whtn blown out, are entirely round, or if wafted 

 more upon the tuyere fides, oval. The general ufage of the 

 fquare muft haoc been derived from long acquired habit, or 

 perhaps from the conveniency of working and finilhing thofe 

 immenfe blocks of tlone which are ftill deemed ncceffary to 

 the perfeftion of a hearth. The interior of charcoal of wood 

 furnaces was at one time entirely fquare from top to bottom, 

 fo that in the progrtfs of the trade, from finelting with wood 

 to the ufe of pit-coal, although the general (hape of the fur- 

 nace has been altered, the fquare figure of the hearth has 

 been retained. 



Whatever may have been the utility of this general pre- 



7 



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dlleAion in favour of eftabl'died forms, the advantages 

 hitherto fuppofed to be derived from this fource are now by 

 many doubted, and all thofe nice fpeculations relative to the 

 precife dimenfions and figure of bodies and fquares, threatened 

 with total annihilation. This innovation is not confined to 

 figure alone, but. extends to dimenfions, and to the nature 

 and bulk of the material necefiary for the conftruftion of 

 hearths. 



Fig. I. Plate X. is the feftion of a hearth and bolhings, 

 conftructed upon an enlarged principle as to fize. 

 Diameter of the bodies at A - - - 10 feet 



Diameter of the hearth at B - - - 4 



Diameter of the hearth at C - - - 3 



Thefe enlarged dimenfions, in place of being fquare as 

 formerlv conftrufted, are now entirely round, excepting where 

 the tymp (tone forms the key to the front of the hearth, as 

 may be feen in Plate X. fg. 2. where the external circle A A 

 reprefents the diameter of the bodies, B, the termination of 

 them, or the top of the hearth, and the form at C, a plan 

 of the infitle figure of the hearth acrofs the bottom of the 

 hearth at C,fg. i . fam.e plate. 



The difficulty of always obtaining a fand-ftone well calcu- 

 lated to ftand the violent effefts of the blaft, the frequent 

 great exptnce incurred, the immenfe lofs of time fuftained in 

 cutting out old and putting in new hearths, and afterwards 

 annealing them, has induced more matters to fpeculatc upon 

 the ule of bricks of ftiapes larger than the common forms, 

 made from good fire clay. No permanent advantage has 

 hitherto been derived from this fcheme, although it is 

 abundantly obvious, that a fuccefsful experiment of this kind 

 would leffen the expence of a hearth greatly, and fave at 

 leaft half the time now required to replace an old one. 



Neither have any uncommon advantages refulted from the 

 hearths laid down in Plate IX. fg.6. and in Plate X. 

 fg. I, 2, and 3. While fome approve, more are ready to con- 

 demn a meafure, which has for its objeft the enlargement of 

 a fpace before blowing, which too fpeedily becomes fo 

 afterwards. There cannot, however, be any objeftion to the 

 circular, in place of the Iquare form, unlefs a little additional 

 workmandiip is fuftained as fuch. The matter refts with 

 experience, accompanied by accurate obfervation, to prove 

 the fanguine hopes of the projeftors, or falfify the prophetic 

 forebodings of thofe who now condemn the meafure. The 

 amount of our progrefs hitherto, in the making of pig iron, 

 is afcertained with certainty ; to affign hmits to its ultimate 

 bounds would be prefumption. Of one faft, however, we 

 may reft alfured, that the perfeftion of the fteam engine, 

 and the confequent command of blaft, has alone done more 

 for the manufafture of this article, than all thofe nice diades 

 of diftinftion as to furnace taken colkftivcly, which relieve 

 each other in a fucceflive train of minute gradation from one 

 extreme to the other ; to all, or to moft of which, the moft 

 wonderful effefts have been from time to time afcribed. 



One fubjeft of confidtrable importance ftill remains to be 

 difcudtd, relative to the conftruftion of the blaft furnace ; 

 namely, the abfolnte and relative heights of the tuyeres, the 

 dam-ftone, and tymp. 



On the fubjedt of tuyeres, the general opinion is, that 

 the nearer the cinder the blaft is introduced, the greater 

 is the effeft as to the abfolute quantity of reduftion. 

 But this may be produftive of conftquences more than 

 fufficicnt to counterbalance the doubtful advantage of acce- 

 lerated reduftion, either by blowing the cinder from off the 

 furface of the iron, and de-carbonating it, or by the cinder 

 ruftiing back through the blovv.pipe at any ftop of the 

 blowing machine, and deftroying the leather bag which con- 

 nefts the blowr-^ipe with the main laying pipes. This never 



Lappena 



