The external figure of a blast furnace is that of a truncated pyramid, while its interior form has been very aptly compared to that of a decanter, supported upon a funnel. In the subordinate details of their construction, there is much variation by different iron masters, which, however, probably does not essentially affect the results. In the annexed sectional representation of one of these great laboratories, we have not selected one of the latest mould, but one that was described and found to be very efficient by that great master in metallurgic operations, Mr. David Mushet; since which, we believe there have been no improvements of essential importance, except in the blowing machinery, which we shall duly notice, a is the regulating cylinder, 8 feet in diameter, and 8 feet high; b the floating piston, loaded with weights proportionate to the power of the machine; c the valve by which the air is passed from the pumping cylinder into the regulator; d the aperture by which the blast is forced into the furnace; diameter of the pipes, 18 inches; e the blowing or pumping cylinder, six feet diameter, 9 feet high, 7 feet stroke; f its piston, with a view of one of its several valves; g solid masonry, on which the regulating cylinder rests, and to which the flanch and tilts of the blowing cylinder are attached, h the safety-valve or cock, by the simple turning of which, the blast may be admitted to, or shut off from the furnace, and passed off to a collateral tube in the opposite side, i the tuyere, by which the blast enters the furnace.

The end of the tapered pipe, which approaches the tuyere, receives small pipes of various diameters, from 2 to 3 inches, called nose pipes, which are applied at pleasure, according to the strength of the blast required, k is the bottom of the hearth, 2 feet square; l the top of the hearth, 2 feet 6 inches square; k l the height of the hearth, 6 feet 6 inches; f is also the bottom of the boshes, which here terminate, of the same size as the top of the hearth, only the former are round and the latter are square, m the top of the boshes, 12 feet in diameter, and 8 feet high; « the top of the furnace, 3 feet diameter, at which the materials are charged; m n the internal cavity of the furnace of the top of the boshes upwards, 30 feet high, n k, total height of the internal parts of the furnace, 441/2 feet; o o the lining, which is built with fire bricks,

Fig. 1.

13 inches long by 3 thick, in the nicest manner; p p a vacancy left all round the outside of the first lining, 3 inches wide, which is rammed full of coke dust, this space being left for the expansion of the materials by heat; q q a second lining, similar to the first; r a cast iron lintel, in which the bottom of the arch r a is supported - this arch is 14 feet high on the outside, and 18 feet wide; v v the extremes of the hearth, 10 feet square. Fig. 2 represents a plan of the foundation of the furnace; a a are the bottom stones of the hearth; b stratum of bedding sand; c c passages for the escape of vapour; d d pillars of brick; these parts, in the dotted elevation, are indicated by similar letters of reference. The subjoined, Fig. 3, affords an horizontal section of the diameter of the boshes, the linings and vacancy being indicated by similar letters of reference to those in the elevation. Fig. 4 exhibits a vertical side section of the hearth and boshes, showing the tymp and dam-stones, and the tymp and dam-plates, a the tymp-stone; b the tymp-plate, wedged firmly to the stone, to hold it in case of splitting by the heat; c, dam-stone, which occupies the whole breadth of the bottom of the hearth, excepting about six inches, which, when the furnace is at work, is filled every cast with strong sand.

This stone is surmounted by an iron plate of great thickness, called the dam-plate, the top of which is about 3 inches below the tuyere hole. The space between the bottom of the tymp and the dotted line is rammed full of strong sand or fine clay, called the tymp stopping, which prevents any part of the blast being wasted. The square of the base of this blast furnace is 38 feet; the height from the false bottom to the top of the crater, 55 feet.