B L A 



B L A 



happens but a confiderable portion of lime is facrificed, befides 

 the expenci; of the bag. 



In common, the furface of the tuyere plate is laid eight 

 inches above the cinder, or, which is the fame tiling, above 

 the level of the dam-floi;e. Some blow at a dillance of 

 four inches, others at fix and eight, and fomc again 

 as high as twelve and fourteen inches. However, under 

 fome circiimflances, the height of the tuyere is determined 

 by the nature of the materials. In thefe cafes, if the 

 tuyere is only raifcd one inch above its proper height, the 

 bottom of the furnace lumps up immediately, and will invari- 

 ably rife in the fame progreffive manner in which the tuyere 

 is heightened. 



Plate X. Jig. I. reprefents the relative proportions of 

 height betwixt the dam, tuyere, and tymp, in ordinary 

 cafes. 



G, the dam, or notch of the dam plate, 17 inches above 

 the level of the bottom at H. 



I, the centre of the tuyere 26I inches from the furface of 

 the bottom, and 9+ inches above the level of the dam. 



K, the bottom of the tymp plate, 23 inches from the 

 bottom of the furnace, and 6 inches above the level of the 

 dam. 



At iron-works where different opinions cxift as to the 

 proper or working height of the dam, very different relative 

 heights enfue, regarding the tymp and tuyere. The former 

 fhould always regulate the other two. The height of it is 

 feldom ufed lefs than 16 inches, nor more than 28 above the 

 bottom. 



Confiderable advantages refult from placing the tuyere, as 

 to its horizontal pofition, at a judicious dillance from the 

 front or back wall. This is, as in the cafe of height, often 

 regulated by the nature of the materials. If the furnace, 

 owing to this circumftance, tends to work cold and languid, 

 behind, with a propenCty to lump at the back wall, the 

 blow pipe ought to be direfted as near to the extremity of 

 the hearth backwards as it is poffible to get in the tuyere 

 iron; Fig. 1^. P/att: X. letters: but where the operations 

 of the furnaces proceed with cafe and facility, the centre of 

 the tuyere fhould more generally approach the centre of the 

 hearth, as at b. 



Of late years a new mode of blowing has been introduced, 

 which, from its great prevalency and good cfFefts, feems to 

 bid fair to come into general ufe. Furnaces till lately were 

 only erefted with one arch, or tuyere fide, and the blaft or 

 column of air introduced by means of one blow pipe ; now 

 nioft of the new furnaces are built with double tuyeres, with 

 two fets of main condudting pipes, p d the blaft. introduced 

 by means of two pipes in place of one. 



The general cfteft-; and fuppofed properties of this mode 

 of blov.-ing are attended to under the article blalh 



In the mean time, the proper height and diftance of the 

 tuyeres, and their relative pofition to each other, have been 

 fubjecl to endiefs difputation. Fig.^^. Plate 'K.. a and b 

 fhew how, in common cafes, the tuyeres are placed to each 

 other in their horizontal range : a is placed with its centre 

 three inches from the extremity or back .wall of the furnace, 

 and b at the dillance of nine inches from its centre. That 

 there (hould be a difference of diilance'in their horizontal 

 pofition none are inclined to difpute ; but that this (hould 

 take place in their vertical fituation, is by fome contended ; 

 while others infill that the difference ought riever to be lefs 

 than four inches. Fig. 6. Plats IX. ee. 



Some lefs faflidious aifert, and witli many evidences of 

 found rcafoning on their fide, that if the blail is introduced 

 into the furnace, and at a proper diftance, to keep the back 

 wall clear, thofe nice diflinftions aS to inches go for nothing, 



in a region where an inllantaneous increafe of volume mull 

 dcftroy all repulfion or mechanical contact. This philofo- 

 phical reafon is flatly denied, and the contrary minutely arid 

 gravely afierted, that were two pipes placed ever)- way im- 

 mediately oppofite to each other, the action of the oppofite 

 columns would retard the velocity of the air, and diroinifh 

 the real elevated quantity in the furnace, by locking up in 

 mutual oppofition a portion of their refpeclive quantities in 

 the laying pipes. There might be fome foundation for this 

 conjecture, were the refpeftive nozlcs or blow pipes brought 

 into aftual contaft, or iiiferted into each other ; but to thofe 

 who confider, that in moil furnaces there is never lefs than 

 four feet of diftance between nozle and oozle, and the mo ft of 

 the intervening fpace filled with a column of femifufed 

 materials, ignited to the higheft pitch of vvhitenefs, this 

 fuppofition will appear to reft upon very unfatisfadory 

 grounds. 



A lefs fcrupulousclafs of obferversandrcafoners upon this 

 fubjcft even go the length to aifert, that the tuyeres ought 

 to be put in direft oppofition, and that this, fo far from being 

 detrimental, would be found to poffefs unqualified advan- 

 tages. This it is faid would refult from a certain degree of 

 coolnefs which the extremity of each column of air confers 

 upon its oppofite tuyere iron, and prcveiit the fame from 

 heating and burning. To whatever caufe it is attributable, 

 the fad ilands in many inftances unqueftioned, that not 

 half the tuyeres are lofi: or burnt out, with the double blaft, 

 that was formerly deftroyed, where the fingle blall was in 

 ufe. 



Fig. 7. Plate X. reprefents a tuyere iron, 16 inches wide, 

 and iz inches high at the wide end, 18 inches long and 

 narrowing at the other end to 4 inches wide, and 4^ inches 

 in height. Fig. 6. is a plan of the under furface of the 

 tuyere iron. Fig. 5. reprefents the fize and dimenfions of 

 the tuyere plate, which when bedded receives upon its furface 

 the tuyere iron, fg. 7. This plate is firft laid upon a bed of 

 fire clay, with its narrow end towards the hearth, and in- 

 clined to rife a little. The tuyere is then introduced upon its 

 furface, height and diftance lieing attended to in the difpo- 

 fition of the plate, and the fpace betwixt its furface, and the 

 fand-ftone of the furnace, rammed very perfeftly with balls 

 of good fire-clay mixed with fmall fragments of fire bricks. 

 When about to blow, the nofe or inner end of the tuyere is 

 covered with a very plaftic clay, to prevent it from heating 

 and burning away. This is always carefully attended to, and 

 the blaft put off at any time to replace it. Should it be 

 neglefted at any time, the iron would inflame with fuch ra- 

 pidity, that an opening would be inftantly made, by which 

 the cokes and igi.ited m.atter of the furnace would be recoiled 

 with the greatell violence imaginable. 



Fig.z. PhleXlU. Thedara-ftone. This is adually the 

 dam, or barrier, which prevents the fluid contents of the 

 furnace from advancing, and making their efcape into the 

 fand of the cafting houfe. It is generally made from the fame 

 ftone as the hearth, but is found ftill more difficult to ftand for 

 any length of time the aftion of the fluid iron, than the 

 hearth to refift the ravages of the blall. 



Fig. J . dam plate. This is laid againft the dam ftone with 

 a bed of fire clay interpofing, and clofes the front of the 

 furnace. Its form is double, fo that bv turning it fcrves the 

 purpofe twice. It often fails, owing to the conftant current 

 of lava paffing over the curvature a, and deepening it, till the 

 iron flows over along with the cinder. 



Fig. 4. the tymp plate. This embraces the under end of 

 the tymp ftone, and the fides of the hearth for three feet up. 

 The ihicknefs at bottom, called the heel, or cod, ispreferved 

 from the adion of tlie fire by a llrong Hopping of clay. 



This 



