ally urge a stream of air into the furnace. The volume of air thus supporting the combustion, may be estimated by the contents of these cylinders, which are six feet diameter, with a stroke of eight feet long, repeated thirteen times per minute, and doubled by a reciprocity motion, causing a consumption or decomposition of 6240 cubic feet of air per minute. In the foundery attached to these works, were cast the ironwork of Vauxhall Bridge; the columns of the King's Theatre in the Haymarket; many of the works in the dock-yard at Sheerness; the famous roof of the Rum Quay at the West India docks; and nearly all the pipes of the West Middlesex water-works. Sugarmills for expressing the juice from the cane in the Colonies, are also manufactured here; and the proprietors have exported upwards of 150 steam-engines to the same quarter within the last seven years. Here was made the steam-engine which supplies Calcutta with water: the nabob or king of Oude, has had one to work a pleasure steam-yacht, and another on a small scale to work punkas or large fans, to ventilate and cool his apartments.

At the Cognor Park works, on the borders of Derby and Notts. the Butterley Company manufacture bar iron in all its forms of convenience and utility. Their vastness is thus described: " Conceive a space as large as Lincoln's Inn Fields, covered with extended fires and smoke, with the rumbling of blasting engines, the thumping of welding-hammers, and scores of men carrying about masses of iron at a white heat: imagine furnaces of melted iron, with their narrow doors, through which light flows with sensible momentum, and blinds those who dare to look upon the liquid lakes within: behold sets of revolving wheels, one of them twentyfour feet in diameter, weighing twenty tons, yet whirling seventy-two times in a minute; and see the connexion of this balance and regulator: view twenty kinds of apparatus, alive, as it were, and with Cyclops

moving among them, and you have before you these vast Derbyshire iron-works. To comprehend, in a sentence, the works carrying on by the Butterley Company only, I may observe, that in its iron and coalworks it employs twenty-five steam-engines with the power of 700 horses, and at this time gives employment to 1500 men, as miners, colliers, furnace-men, moulders, steam-engine fitters, smiths, labourers, &c."*

Why is Swedish superior to British iron?

Because the Swedes smelt with wood instead of coke. It is imported into England in great quantities, and is chiefly used for carbonization in steel.

Why is iron deprived of its malleability by long-continued hammering?

Because it loses a portion of its latent caloric; Dr. Black being of opinion that metals are malleable in proportion to the matter of heat which they contain in a latent state.

Why is cast-iron puddled and rolled?

Because a principal part of the foreign substances are thus burned away or squeezed out, and malleability is conferred upon the metal by rendering it more pure.

By this curious process of puddling, cast-iron, after it has been to a certain extent refined, by refusion in a forge, is, in this country, converted into wrought iron. The cast iron is put into a reverberatory furnace, and when in fusion, is stirred, so that every part may be exposed to the air and flames. After a time, the mass heaves, emits a blue flame, gradually grows tough, and becomes less fusible, and at length pulverulent; the fire is then urged, so that the particles again agglutinate at a welding heat, and are gradually wrought up into masses. In that state of intense heat, the masses are passed successively between rollers, and the bars made malleable. They are cut into pieces, placed in parcels in a very hot reverberatory, and again ham

* Abridged from Sir R. Phillips's Personal Tour, Part ii, 1830,

mered or rolled out into bars. They are thus rendered more tough, flexible, and malleable, but much less fusible, and may be considered as nearly pure iron. Why is iron better cast perpendicularly than horizontally?

Because of the pressure of the upright column, which renders the iron much less liable to air-bubbles and imperfections of that kind, which defeat the skill and calculations of the machinist. If this upright pressure be increased by a weight of extraneous metal, the casting is still more likely to be sound.

Why does a rod of wrought iron, if plunged into cast iron in fusion, become steel?

Because the iron absorbs part of the carbon. What is called case-hardening, is a conversion of the surface of iron into steel.

Why is the process by which iron is converted into steel, called cementation?

Because it consists in heating bars of the purest iron in contact with charcoal; it absorbs carbon, and increases in weight, at the same time acquiring a blistered surface. This, when drawn down into smaller bars, and beaten, forms tilted steel; and this broken up, heated, welded, and again drawn out into bars, forms shear-steel.

In this process it has been commonly considered that the carbon combines mechanically with the iron; our chemists have, however, long been of opinion, that it is a chemical combination that takes place, by the gradual absorption of carbon in the gaseous state, by the iron. This fact has been proved by Mr. Charles Mackintosh, of Crossbasket, Lanark, who has taken out a patent for preparing steel, by subjecting the iron to a stream of carburetted hydrogen gas, evolved from coal under distillation. This iron is enclosed in a pot or crucible in the furnace, and when arrived at the proper heat, a stream of gas is directed by a pipe into the

ARTS AND MANUFACTURES.

2.39

crucible, which has another aperture to allow that part of the gas to escape, which has not been taken up by the metal. Steel, in ingots, is porous; but, to confer solidity, it is hammered, tilted, and rolled. At Attercliffe, near Sheffield, are extensive works for these purposes. Here, by the power of a water-wheel, fifteen feet in diameter, hammers are worked, weighing from 3 to 4 cwt. and strike, at ten or twelve inches fall, from 100 to 220 times in a minute. The ingots, at a strong red heat, are exposed to the action of these hammers, and the metals condensed into bars, which are next submitted, at the same degree of heat, to the tilting hammer, which gives 300 strokes per minute: lastly, they are rolled or flattened into sheets, and drawn into lengths. Six tons a week are hammered down by one hammer; about three tons are tilted; and twenty-four tons can be rolled, working night and day, by relays of hands.

The making of steel is a British manufacture scarcely sixty years old. Previously it came from Austria and Syria, and was dear and little used. It is, however, now heated, welded, cut, and moulded in this country, with nearly the same facility as deal wood by an ordinary carpenter.

Why does a drop of nitric acid, let fall upon steel, occasion a black spot?

Because the iron is dissolved, and the carbon thereby exposed to view.-Parkes.

Why is steel tempered?

Because, when steel is heated to a cherry-red colour, and then plunged into cold water, it becomes so extremely hard and brittle, as to be unfit for almost any practical purpose; and tempering reduces it from this extreme hardness, by heating it to a certain point or temperature.

The polishing of steel is not executed in the same manner as that of the softer metals: the steel is not

polished until it has been hardened, and the harder it is, the more brilliant will be its polish, Rotten-stone, a kind of very light tripoli, but finer than the other sorts, and found near Bakewell, in Derbyshire, is esteemed for general polishing; but steel, from its extreme hardness, requires to be polished with emery.

Why are various colours produced on heated steel? Because of the oxidation which takes place, as is proved from the circumstance that when steel is heated and suffered to cool under mercury or oil, none of the colours appear; nor do they when it is heated in hydrogen or nitrogen.-Brande.

Why is it customary to judge of the temper of steel by its colours?

Because, the surface being a little brightened, exhibits, when heated, various colours, which constantly change as the temperature increases. Thus, when steel is placed in a bath heated to 600°, the first change is at about 430°, which is very faint; at 460°, the colour is straw, becoming deeper as the temperature is increased; at 500, the colour is brown; this is followed by a red tinge, with streaks of purple, then purple; and at nearly 600°, it is blue. The degrees at which the different colours are produced, being thus known, the workman has only to heat the bath with its contents up to the required point. For example, suppose the blade of a pen-knife, (or a hundred of them,) to require tempering; they are suffered to remain in the bath until the mercury in the thermometer rises to 460°, and no longer, that being the heat at which the knife (supposing it to be made of the best English cast steel) will be sufficiently tempered.

Why is cast steel so called?

Because it is prepared by fusing blistered steel with a flux composed of carbonaceous and vitrifiable ingredients, casting it into ingots, and afterwards by gentle heating, and careful hammering, giving it the form of bars.