For iron plates, bar-iron of suitable quality is made into piles of various lengths and thicknesses, then reheated, and rolled to the desired thickness.

For large sheets, the piles are 20 inches long, 7 inches wide, and 4 inches deep, weighing about 70 lbs. The white-hot pile is made into a square bloom, by passing it through the grooves of the blooming-rolls, then through plain roughing-rolls, and finally through the polished finishing-rolls.

Black-plates, as the thin plates intended for tinning are called, are doubled over upon themselves after every re-heating, so that several sheets are rolled together. The plates are then cut to proper size and separated.

Nail-rods. Two different methods are adopted for making nail rods-by rolling a bar to the desired thinness; and by slitting a strip of iron into parallel rods, which is by far the more common. Two or three pieces of iron are made into a pile, and raised to a welding-heat in a reheating furnace. It is then taken to the rolls and extended into a bar 10 to 12 feet long and 4 inches wide; then passed between smooth rollers to obtain the desired thickness. It is then transferred

to the slitting rolls Fig. 50. These consist of spindles carrying steel discs fixed at suitable distances. When revolving, the discs on the upper spindle project into the spaces of the lower series, thus forming a rotatory shearing-machine. On inserting one end of

Fig. 50.

the iron strip between the guides, it is drawn forward by the shearing-discs and cut into rods, which are afterwards cut to length.

Hoop-iron is made from small piles, which are heated and rolled between grooved rolls, and finished between a pair of cylindrical rolls, where it is pressed to the desired thickness. The great length of the bars, and their tend

ency to cool quickly, make it necessary to drive the grooved rolls at great velocity, but the finishing-rolls work at an ordinary speed of 100 revolutions per minute.

Small sizes of flat, square, and round iron are rolled with trains having three rollers in height, so as to hasten the work while the iron is hot, the mill being driven from the middle roll. The bar is passed between the bottom pair and returned between the upper pair, so that the operation is performed in both directions. A speed of 230 revolutions per minute is common. In rolling bars of small section, guides are used to keep them straight. It is usual to attach parallel guides, with friction-rollers, to the tables in front of the rolls, which are then termed "guide-mills" or "trains." In the short finishing-rolls for round iron, each has a semi-circular groove, and the two together form a complete circle.

Plate-mills are usually made with two sets of rolls, each being a plain cylinder of uniform diameter. The first or roughing-pair has the top roll balanced by counterweights, whilst the top finishing-roll runs freely. These latter rolls are cast in chills and highly polished. The size varies from 5 to 9 feet long and from 20 to 36 inches in diameter. The speed is 25 to 40 revolutions per minute, the higher speeds being used for light plates.

Guillotine-shears.-These shears are used when a cut of considerable length is required, as in the shearing of heavy plates, sheets, etc. The form Fig. 51 is now also used in some works, in place of the crocodile, shears, for puddled-bars, blooms, etc. It contains two diagonal-edged knife blades (aa), the upper one moving vertically between guides, and worked by a crank (b).

Rails and thick bars are finished by cutting off, whilst hot, the rough or crop ends with circular saws, 3 to 4 feet in diameter, and driven at great velocity.

The rolling mill employed for finishing puddled iron, so as to form merchant bar, consists of two sets-the roughing and the finishing rolls. The rolls are arranged

in housings with adjustments similar to those described for the forge-train, except that the finishing-rolls are provided with lateral tightening screws for keeping them always in exact position. In order to prevent lamination, the pile is passed through the roughingrolls with the joints alternately flat and edgeways, which renders the welding uniform.

Universal rolling-mill.--The contrivance consists of a combination of vertical and horizontal rolls,

so arranged as to compress the pile equally in both directions at once. Fig. 52 shows the position of the rolls. The vertical rolls work upon slides, and can be adjusted to different distances by means of

Fig. 51.

screws. The horizontal pair are driven in the usual way from the bottom roll, and the vertical pair are connected with them by an

intermediate shaft, which receives its motion from the driving-pinion.

In the forging of heavy piles, steel ingots, etc., great advantage is gained by the use of a hydraulicsqueezer or press. It consists of a vertical

Fig. 52.

cylinder with its ram acting downwards against a table or anvil; or a pair of dies are used for special shapes. The ram is connected with the piston of a smaller press, by which it is raised.

Re-heating furnaces.-The shingled masses, and bars of iron obtained from the forge, contain slag and impurities, which have not been eliminated in the previous operations, and the object of re-heating is to remove these substances as much as possible, and raise the billets or slabs of iron to a welding temperature. The enclosed slag partially liquates out as a fluid basic silicate, leaving a basic residue mixed with black oxide of iron Fe3O4, which is largely removed in rolling. The basic slag also acts on the manganese, silicon, phosphorus, etc., retained by the iron, oxidising them, the oxides passing into the fluid slag. The sulphur and carbon are also partly oxidised in the same way, and probably removed as gases.

The re-heating furnace Fig. 53 is somewhat similar in shape to the puddling-furnace; the bed is flat and

Fig. 53.

slightly inclines down to the flue, and is lined with sand. In some works basic material is used. It is essential that the fire-place should be large enough for the furnace to be rapidly raised to a white heat; the ratio of the section of the fire-place to the bed being about 2: 1. The arch of the roof should be as low as is compatible with the size of the piles, or other mass of iron, to be heated. The slag runs off the bed constantly to the bottom of the flue, and is termed "flue cinder."

Of late years gas-furnaces have been used for re-heating, such as Siemens' furnace. A modification of the regenerative furnace has been applied to re-heating by

with hot air, which is heated by a "recuperator" placed under the bed of the furnace. The recuperator is a brick chamber, partly solid and partly hollow, containing a number of vertical passages, the adjacent ones being separated, and the alternate ones connected by horizontal passages in the perforated bricks. The flame passes downwards, while the cold air, admitted at the bottom, passes upwards by separate channels, the action being

continuous.

The Boëtius furnace differs from the preceding one, in the manner of heating the air required for the combustion of the gas used. Fig. 55 shows the general arrangement in vertical section. The gas-pro

ducer is constructed with passages in its walls, through which the air supplied to the furnace is compelled to pass, thus being heated. It meets and mingles with the gas at the furnace-bridge, where combustion