682

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Blast Furnace generally resembles externally a round tower, or it is square at the bottom,and circular at the upper part; occasionally it is entirely square, but in either case itsinternal construction is the same. The exterior of the roof is flat, and from its centre risesa cylindrical structure, which serves as a chimney, from 20 to 25 feet high ; it has anopening at the side called the filling place, into which the coke , ironstone, and limestone , arethrown. In many furnaces the external wall round the top is raised 10 or 15 feet higherthan the floor, to protect the people employed from the cutting blasts of the wind. InStaffordshire , when the ground is level, the blast furnaces rise like the towers of an ancientcastle, and the ascent to the top is by an inclined plane; the materials and the fillers aresometimes hoisted by the steam-engine, working ropes over a pulley at the top.

In Shropshire advantage is often taken of the difference of level of the ground, and thefurnaces are built on the lowest part, so that the top may be even with the surface above ; bysuch an arrangementthe materials can bewheeled and throwninto the furnace withoutany difficulty.

In these furnaces Arepresents the regu-lating cylinder, 8 feetin diameter and height;

B, the floating pistonloaded with weights,proportionate to thepower of the machine;

C, the valve, 26 incheslong and 11 inches wide,by which the air ispassed from the pump-ing cylinder into theregulator: D, the aper-ture by which the blastis forced into the fur-nace, its pipe being 18inches in diameter ; thewider this can be madethe less is the friction,and the more powerfulthe blast; E is theblowing or pumpingcylinder, 9 feet high,and 6 feet in diameter,the piston within ithaving a stroke of from5 to 7 feet; F, theblowing piston with its valve or valves, of which there are sometimes several distributed overthe surface of the piston, the area of each being proportioned to the number; G is a pier ofstone or masonry supporting the regulating cylinder, to which is attached the flanch andblowing cylinder ; II is the safety-valve or cock, by the simple turning of which the blastmay be admitted to or shut off from the furnace, passing to a collateral tube on the oppositeside. I, the tuyere, by which the blast enters the furnace ; the end of the taper pipe whichapproaches the tuyere receives small pipes of various diameters, from 2 to 3 inches, callednose pipes ; these are applied at pleasure, as the strength and velocity of the blast mayrequire. K, the bottom of the hearth, 2 feet square. L, the top of the hearth 2 feet 6 inchessquare. K, L, the height of the hearth, 6 feet 6 inches ; L is also the bottom of the boshes,and where they terminate of the same size as the top of the hearth, only the former is roundand the latter square. JVI, the top of the boshes, 12 feet diameter, and 8 feet perpendicularheight. N, the top of the furnace at which the materials arc charged, commonly 3 feetdiameter ; M N, the internal cavity of the furnace, from the top of the boshes upwards,30 feet high; NK, total height of the internal parts of the furnace, 44£ feet. OO, thelining ; this is done in the nicest manner with fire-bricks made on purpose, 13 inches longand 3 inches thick. PP, a vacancy round the outside of the first lining, 3 inches broad,and filled with coal dust; this space is allowed for the expansion which might take placein consequence of the swelling of the materials by heat when descending to the bottom ofthe furnace. QQ, the second lining, similar to the first. R, cast-iron lintel, on which thebottom of the arch is supported. US, the rise of the arch ; ST, the height of the arch onthe outside, 14 feet and 18 feet wide. V V, the extremes of the hearth, 10 feet square; this

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Fig. 5RG,

SECTION OP IRON FURNACB.