jiir iifed in the "Blaji-Fuvnace. 6l 



irtoll minute and thorough inveftigatlon. In order to take a 

 comprchenfive view of this fubjeft, the following divifion will 

 be requifite : — 



ift, The intimate connexion which the quantity of blaft 

 bears to the area of the internal cavity of the furnace, and to 

 the natune of the pit-coal. 



ad, The various modes by which air is procured, and 

 how thefe refpedtivelv affeft the quality of the air, 



3d, The various changes to which air is fubjeftcd by a 

 change of temperature in the atmofphere, with the confequent 

 efFeiSts. 



4th, How far increafed or diminifhed velocity and com- 

 preffion alter the rcfults of the furnace. 



5th, The form and diameter of the difcharging-pipe. 



I ft, Then, in the conftruftion of a blali-furnace and 

 blowing-machine, the quantity of air to be ufed ought to 

 depend upon the internal dimenfions of the former; which, 

 again, ought to be formed according to the quality of the 

 pit-coal. Upon the foftnefs or hardnefs of the coal, ought 

 more immediately to depend the height of the blaft-furnacek 

 Ihis necelfary precaution has given rife to a vaft variety of 

 furnaces, of different capacities, from 30 to 50 feet in height, 

 and from 9 to 16 feet diameter at the bofhes. Furnaces 

 from 30 to 36 feet are ufed for the fofter qualities of coal, 

 fucb as a mixture of free-coal and fplint. Furnaces from 

 36 to 45 are appropriated to the burning of fpllnt-coal cokes ; 

 and in Wales, fuch is the fuperior ftrcngth and quality of the 

 pit-coal, that the furnaces admit of being reared to the height 

 of 50 feet. ' 



Thefe various qualities, of coal, it has been formerly fliewn,' 

 have appropriate weights of iron-ftone, and, to ufe the lan- 

 guage of the manufactory, are capable " of fupporting a 

 greater or leffer burden of mine." The former qitalitics 

 admit not of having the air difchargcd in great quantity, un- 

 lefs it is impelled under an uncommon degree of comprelhon 

 and toiifcqucnt velocity incompatible with the operations of 

 a fteam-cngine. The reafon is obvious : when air, loofely 

 comprefl'cd, or comparatively fo, is thrown into a body of 

 jjjnited fuel, the mechanical Itruclurc and continuity of whofc 



paiticlts 



