each of the longer sides of the hearth, thus causing a better distribution of the air.

The interior profile of a few different types of the blastfurnace is shown in Fig 19 A—F.

Collection of waste gases.-The waste gases of blast-furnaces were formerly allowed to escape, the furnace tops being always left open, but this system has been almost universally abandoned, except in a few localities where fuel is cheap and cold-blast is used. Various contrivances have, from time to time, been devised for closing the throat, so as to prevent the escape of the waste gases, which are conducted into suitable apparatus, where they are burnt to generate heat for heating the blast, etc., as before mentioned.

In the charcoal furnaces of Sweden, and some old coke furnaces, a portion of the gases is collected without closing the top, by introducing a number of iron pipes through the brickwork, at a depth of 10 or 12 feet below the top. In the Upper Hartz, and other places, an annular space is formed in the upper part of the masonry of the furnace, communicating with the interior, beneath the surface of the charge, by a number of openings inclining upwards, in order to prevent the lodgment of the charge in them. A large pipe opens into the annular space, and is connected with a high stack, so as to obtain the necessary draught for drawing off the gases. With some small straight furnaces a cast-iron cylinder is inserted in the throat, leaving an annular space which forms a collecting flue for the gases. The mode of charging by means of a central cylinder causes the more compact and finer particles to go to the centre, and the larger pieces to the sides, which latter then become more permeable by the gases. In order to correct this defect a fixed cone Fig. 20, supported by radial arms, is added, so as to distribute the charge more uniformly.

Escalle uses a fixed cone without a cylinder, Fig. 21. The top being closed, the gas is collected by the cone, the radial

arms which support it being hollow, so that the gases pass through them to the off-take. Moveable cones are now

more generally used, both for collecting the gases and distributing the charge. The most simple and general form consists of a fixed cup (a), Fig. 22, and a moveable cone (b) suspended by a chain so that it may be raised or lowered at will. Various contrivances are in use for controlling the movement of the cone, a frequent form being a pinion moved by a hand-wheel, and gearing into a rack attached to a counterpoised weight. The gases pass through a lateral flue into the main pipe, which conducts them to the boilers, heating stove, etc.

At Ulverstone in Cumberland a central iron tube Fig. 23, lined with brick, has been adopted for collecting the gases. The tube is about 5 feet in diameter, and extends about 5 feet into the throat of the furnace. It rests on six ribs of brickwork. In Langen's apparatus the gas collecting

tube is placed externally to the furnace, which may therefore be kept filled with the charge. It consists of a bellshaped tube, which rests on an inverted conical charging cup. The bell is suspended from a lever, and the charge is placed in the conical cup, so that when the bell is raised the materials glide into the furnace.

Twyers. The twyers, used for introducing the air into the blast-furnace, are subjected to intense heat, and require to be protected by a continuous circulation of water. They are therefore made with a double casing of wrought-iron, cast-iron, or bronze. Fig. 24 shows the arrangement of a Staffordshire twyer. Another form, known as Lloyd's spray-twyer, is open at the back, and the cooling water, in

the form of spray, is driven from the end of a perforated pipe against the port of the twyer. Another form, termed the Scotch twyer, Fig. 25, consists of a spiral wrought iron tube enclosed in a cast-iron casing; a current of water circulates through the coil to keep it cool.

The number of twyers employed varies in different furnaces, according to the nature of the fuel and class of ore. An excess of blast causes the consumption of too much fuel, with the production of white iron; it also tends to cool the slags and impede the working. With an insufficient blast, the temperature of the furnace is lowered, and the production of iron lessened. With small charcoal furnaces two or three twyers are used. In large coke furnaces six or seven are arranged symmetrically round the hearth, with their axes pointing a little away from the centre. When producing forge-pig, the twyers are arranged horizontally, or slightly inclined

downwards, thus causing a partial decarburisation. For foundry-pig, the twyers are often slightly inclined upwards.

Hot-blast. The use of hot air was first introduced by Neilson at the Clyde Iron Works in 1828, and was soon attended with great economy of fuel, and an increased make of iron. The arrangements for heating the blast are of two kinds:-cast-iron pipes through which air passes, heated externally; and chambers of refractory brickwork, constructed on the principle of Siemens' regenerators, which are now in more general use.

Cast-iron stoves.-Fig. 26 represents an old form of stove, which consists of a series of arched pipes of cast-iron, arranged in an oblong fire-brick chamber; along each of the long sides of this chamber are two circular mains, fitted with sockets, into which the legs of the vertical pipes are received, while between the mains, and running the full length of the stove, is a rectangular fireplace. The pipes are very unequally expanded by the heat, and this, when the extremities are fixed to the mains, leads to frequent breakages. Fig. 26. The defects of this arrangement have caused many modifications to be introduced. Thus the horse-shoe pipes have been replaced by an inverted V shape, which gives a smaller radius of curvature to the arch; and greater uniformity of heating is obtained by introducing stops at intervals in the entry main, so that the air passes alternately backwards and forwards. across the arch. In some works, the vertical pipes instead of being arched are united by short horizontal limbs at right angles, and are placed close together.

In many German works a number of horizontal pipes of elliptical section are arranged in a fire-chamber, in a similar manner to the tubes of a locomotive,

and united into a continuous serpentine coil by arched bends, which are external to the fire, thus lessening the tendency of these bends to break by irregular expansion.

Of the regenerative type, two principal forms are employed, invented respectively by Cowper and Whitwell. The Cowper stove Fig. 27, is a circular wrought

Fig. 27.

iron tower, closed with a dome-shaped roof, and lined internally with firebrick. It contains a circular fire-brick flame flue A, into which the waste gases from the blast-furnace pass by the valve B. The body of the stove is occupied by a checker work of fire-brick for absorbing the heat. The gas entering the flue A, is there burnt, the air necessary for combustion entering by the valve C. The hot products passing down through the checker work make it red-hot, and finally pass into the chimney-flue D.

The stove having been thus heated, the valves B,

C, and D are closed, and the cold-blast valve at the bottom of the stove opened, together with the hot-blast valve F. The cold air enters at the bottom or cooler end, and ascends through the brickwork, getting gradually hotter, and then escapes by the valve F, which communicates with the blast-furnace, at a temperature of 800° C. Two stoves are worked in conjunction, one being heated by the combustion of the waste gases, while the other is