B L A 



B L A 



moment it enters into complete fiifion, continues without 

 diminution or augmentation throughout the whole operation 

 of the furnace ; or, in other words, caft iron neither receives 

 nor lofes carbon whilft it preferves its fluidity. The firft 

 fact explains the realon v/hy the iron is preferved from com- 

 buftion, when it defcends oppofitc to the current of blaft. 

 The fecond is a proof that the carbonaceous matter is con- 

 veyed to the iron in the furnace by a fpecies of cementation 

 previous to fufion ; and that nftcr this point, call iron will 

 not take up any addition of carbon. 



To prelerve and eftablifli the relation of cementation and 

 fufion in the furnace cnfures uniform produfts. Combullion 

 in this prcfeuts us v;ith a gradation of temperature, diminilh- 

 ing from the tuyere upwards through thirty or forty feet of 

 ignited matter. The inferior temperature towards the top 

 ol the furnace heats the materials to rednefs ; an affinity is 

 here commenced betv^ixt the carbonaceous matter and the 

 oxygen of the ore ; the latter is gradually removed, and a 

 fecond affinity is iiillituted betwixt the de-oxygenated par- 

 ticles of metal and the carbon : this, as the ore defcends to 

 higher temperatures, is rapidly mcreafed, and by and by the 

 faturation of the coally pnnciple is complete. As the fatu- 

 ration of carbon always increafes the fuiibility of iron, the 

 metal of the furnace enters into fufion at a comparatively low 

 temperature, and fpeedily precipitate?, through the high tem- 

 peratures in the neighbourhood of the blaft, to the general 

 refervoir below. 



It is not therefore neceflaiy to fuppofe, that the great 

 volume of air thrown iiito the furnace, :iiid the great tempe- 

 rature of courfe excited, are neceflary to ttie manufafture of 

 the iron, fo far as it regards quality ; this, it is more than 

 probable, may be injured by it, and even the economy of 

 the manufafture itfelf. Quantitv, however, is in general 

 fecured ; but this is more the effeft of mechanical reduftion, 

 than of any neceflary operation of the blaft upon the ore and 

 materials above. 



The quicker the body of cokes can be reduced, which 

 occupy that part of the furnace between the point of fepa- 

 ratioii and the tuyere, the greater will be the reduftion of 

 the whole, and the greater the quantity of manufactured 

 metal. To this point the whole force of the blaft is direfted ; 

 here the chief part of the decompofition of the atmofpheric 

 air takes place ; and here the dettrutlion of the intervening 

 cokes is effefttd, and that always in proportion to the quan- 

 tity of air poured upon their highly ignited furfaces. 



If we affume, with a blaft of a certain denfity, any two 

 points in the furnace, the one as the point of decompofition, 

 and the other of ftparation and fluidity of the metal, fup- 

 pofe the former at the tuyere, and the other at the lower end 

 of the boihes at A, [S^e dffcriplion of Blast furnace,) then 

 it muft be allowed probable, that a change taking place in the 

 denfity, or even in the quantity of the blaft, t'nat change will 

 affect not only the points themfelves,but alio their relative dif- 

 tances. The point of leparation may be brought nearer (and 

 perhaps injurioufly fo) to the level of the blaft, the elevation 

 of which is luppofed to remain the fame. The contrary may 

 with equal truth be inferred ; that if the point of leparation 

 is carried to a more elevated fituation by a change or incrtafe 

 of temperature, the ore may enter into fufion bef -re it lias 

 remained fufficitntly long in contaft with tiie ignited fuel, 

 and thereby both the quality and quantity may be injured. 



2d. Since pit-coal coke became the ftaple fuel at the blaft 

 furnace, the denfity and quantity of air deemed neceflary to 

 enfure combuftion and quantity, have been yearly incrcafing. 

 The various qualities as to hardnefs or foftnefs, punty and 

 elfeft, have given rife to a multitude of opinions, which are 

 the moll appropiiate quantity and denfity of air for rcfpec- 



tive qualities of cokes. The blaft of the furnace, in confe- 

 quence, has at different places varied from il to 41b. of ex- 

 panfive force upon the fquare inch of the air velTel. Mod 

 of the Englifti works are blown with air not exceeding' 2 lb. 

 upon the fquare inch, as being the mofl proper medium of 

 denfity, and beyond which the materials would be over- 

 blown. Enghfh coal, in general, is foft in it» fo.Tile Hate, 

 but rich in carbon, and free from mixture. In Scotland, 

 where the coal is found in denfe ftrata, and forms heavy coke, 

 the blaft is ufed from 2 lb. to 4 lb. per inch. Thofe who 

 have adopted denfe blalls declare, that quantitv of iron is 

 incompatible with a column of air inferior to the meafure of 

 their ftandard. Either the prejudice is vei'y general, or 

 there really muft exiil a dircft analogy between the nature 

 of the blail and the denfity of the coal. 



The operations ef the charcoal pig manufaftory vrere 

 condufted with blafts of a trifling denfity, ftldom exceeding 

 li lb. upon the inch, and often under this. Denfe blaft, 

 it was believed, over-ran the furnace, moll probably by 

 exciting too great a temperature, and frequently had a ten- 

 dency to difcharge the m.atcrials from the furnace top. 

 There never yet have been any direft experiments made to 

 afcertain upon what this variety of pit-coal depends ; whe- 

 ther exclufively from its denfity, or from containing the car- 

 bonaceous matter in more purity cj- greater difengagement. 

 Perhaps both are neceflary to be taken into account, before 

 any fatisfaftory explanation can be given of the facls now 

 , ftated and generally admitted. 



3d. The denfity of a column of air depends upon the 

 power of the blowing machine, and the proportion of the 

 area of the fleam cylinder to that of the blowing cylinder. 

 Tables of the powers of fteam engines, and the diameters of 

 cylinders requifite to condenfe air from i| lb. to 41b. upon 

 the circular inch, will be found under the article Blowing 

 Machine. It will appear evident from thefe tables, that 

 fteam cylinders of the fame diameter, and working at the 

 fame power, when employed to raife air of various denfities, 

 do not difcharge the fame quantity of atmofpheric air in any 

 given time. The larger the area of the blowing cvlinder, 

 the number of ftrokes being the fame, the greater will 

 be the quantity of air dilcharged into the furnace. The 

 reverfe is the cafe with blafts progreflively more denfe ; fo 

 thai any part of an engine's power may be employed, not 

 in raifing the true principle of combuftion — air, but in coii- 

 denfing a comparatively fmall body of air, fo as to give it 

 additional velocity. 



To fix the point, or maximum, of the moft profitable 

 denfity, has hitherto been unattainable. The circumllances, 

 deemed intimately connefttd with coal, render it neceflarv 

 to accommodate the blaft to the combuftibility of the fuel : 

 were this not the cafe, it would be difficult to overturn the 

 following reafoning, and to exhibit an inllance where it 

 might not be found generally applicable. Combuftion in 

 the furnace will be excited m proportion to the quantity of 

 air introduced. A blowing machine, that with the fame 

 power of fteam cylinder threw into the furnace double the 

 quantity of air, though of an inferior denfitv, would reduce 

 a greater quantity of combuftible matter than one oppo- 

 fitely conftrucled ; or, in other words, cooo feet of air per 

 minute entering a furnace would produce greater cfTedls than 

 3000 feet, although the latter were comprefltd into nearly- 

 half the bulk of the former. 



The moft planfible theory of blaft is to fix upon the low- 

 eft denfity at which the air can be forced into the furnace, 

 and then proportion the diam.eter of the air-pump to the 

 power of the tleam end. Suppofe that this could he eflefted 

 at half the denfity ufually employed, then that part of the 



epgine'» 



