Iron, 12& 
3d, The various changes to which air is subjected by a 
change of temperature in the atmosphere, with the conse- 
quent effects* 
4th, How far increased or diminished velocity and com* 
pression alter the results of the furnace* 
5th, The form and diameter of the discharging-pipe, 
1st, Then, in the construction of a blast-furnace and 
blowing-machine, the quantity of air to be used ought to 
depend upon the internal dimensions of the former ; which 5 
again, ought to be formed according to the quality of the 
pit-coah Upon the softness or hardness of the coal, ought 
more immediately to depend the height of the blast-fur- 
nace. This necessary precaution has given rise to a vast 
variety of furnaces, of different capacities, from 30 to 50 
feet in height, and from 9 to 16 feet diameter at the bosh- 
es. Furnaces from 30 to 36 feet are used for the softer 
qualities of coal, such as a mixture of free- coal and splint. 
Furnaces from 36 to 45 are appropriated to the burning 
of splint-coal coaks; and in Wales, such is the superior 
strength and quality of the pit-coal, that the furnaces ad- 
mit of being reared to the height of 50 feet. 
These various qualities of coal, it has been formerly 
shewn, have appropriate weights of iron-stone, and, to use 
the language of the manufactory, are capable “ of support- 
ing a greater or lesser burden of mined ? The former qua- 
lities admit not of having the air discharged in great quan- 
tity, unless it is impelled under an uncommon degree of 
compression and consequent velocity incompatible with 
the operations of a steam-engine. The reason is obvious : 
when air, loosely compressed, or comparatively so, i§ 
thrown into a body of ignited fuel, the mechanical struc- 
ture and continuity of whose particles are soft, the air is 
much more easily decomposed ; the ignition, of course, 
is more rapid : the descent of the materials is promoted 
beyond their proper ratio, and long before the carbons^ 
R 
