Part 1 (2/2)

Even after Watt had secured the aid of Boulton's best workmen, Smeaton gave it as his opinion that the steam-engine could never be brought into general use, because of the difficulty of getting its various parts made with the requisite precision.

The execution by machinery of work ordinarily executed by hand-tools has been a gigantic stride in the path of material civilisation. The earliest phase of the great modern movement in this direction is represented, probably, by the sawmill. A sawmill was erected near London as long ago as 1663--by a foreigner--but was shortly abandoned in consequence of the determined hostility of the sawyers; and more than a century elapsed before another mill was put up. But the sawmill is comparatively a rude structure, and the material it operates upon is easily treated, even by the hand. When we come to deal, however, with such substances as iron and steel, the benefit of machinery becomes incalculable. Without our recent machine-tools, indeed, the stupendous iron creations of the present day would have been impossible at any cost; for no amount of hand-labour could ever attain that perfect exact.i.tude of construction without which it would be idle to attempt fitting the component parts of these colossal structures together.

The first impulse, however, to the improvement of machine-tools for ironwork was given by a difficulty born not of ma.s.s but of minuteness.

Up to the end of the last century, the locks in common use among us were of the rudest description, and afforded scarcely any security against thieves. To meet this universal want, Joseph Bramah set his remarkable inventive faculties to work, and speedily contrived a lock so perfect, that it held its ground for many a day. But Bramah's locks are machines of the most delicate kind, depending for their efficiency upon the precision with which their component parts are finished; and, at that time, the attainment of this precision, at such a price as to render the lock an article of extensive commerce, seemed an insuperable difficulty. In his dilemma, Bramah's attention was directed to a youngster in the Woolwich a.r.s.enal smithy, named Henry Maudsley, whose reputation for ingenuity was already great among his fellows. Bramah was at first almost ashamed to take such a mere lad into his counsels; but a preliminary conversation convinced him that his confidence would not be misplaced. He persuaded Maudsley to enter his employment, and the result was the invention, between them, of the planing-machine, applicable either to wood or metal, as also of certain improvements in the old lathe, more particularly of that known as the 'slide-rest.'

In the old-fas.h.i.+oned lathe, the workman guided his cutting-tool by sheer muscular strength, and the slightest variation in the pressure necessarily led to an irregularity of surface. The rest for the hand is in this case fixed, and the tool held by the workman travels along it.

Now, the principle of the slide-rest is the opposite of this. The rest itself holds the tool firmly fixed in it, and slides along the bench in a direction parallel with the axis of the work. All that the workman has to do, therefore, is to turn a screw-handle, by means of which the cutter is carried along with the smallest possible expenditure of strength; and even this trifling labour has been since got rid of, by making the rest self-acting.

Simple and obvious as this improvement seems, its importance cannot be overrated. The accuracy it insured was precisely the desideratum of the day! By means of the slide-rest, the most delicate as well as the most ponderous pieces of machinery can be turned with mathematical precision; and from this invention must date that extraordinary development of mechanical power and production which is a characteristic of the age we live in. 'Without the aid of the vast accession to our power of producing perfect mechanism which it at once supplied,' says a first-cla.s.s judge in matters of the kind, 'we could never have worked out into practical and profitable forms the conceptions of those master-minds who, during the past half-century, have so successfully pioneered the way for mankind. The steam-engine itself, which supplies us with such unbounded power, owes its present perfection to this most admirable means of giving to metallic objects the most precise and perfect geometrical forms. How could we, for instance, have good steam-engines if we had not the means of boring out a true cylinder, or turning a true piston-rod, or planing a valve-face?'

It would perhaps be impossible to cite any more authoritative estimate of Maudsley's invention than the above. The words placed between inverted commas are the words of James Nasmyth, the inventor of that wonderful steam-hammer which Professor Tomlinson characterises as 'one of the most perfect of artificial machines and n.o.blest triumphs of mind over matter that modern English engineers have yet developed.'

[Ill.u.s.tration: Nasmyth's Steam-hammer.]

This machine enlarged at one bound the whole scale of working in iron, and permitted Maudsley's lathe to develop its entire range of capacity.

The old 'tilt-hammer' was so constructed that the more voluminous the material submitted to it, the _less_ was the power attainable; so that as soon as certain dimensions had been exceeded, the hammer became utterly useless. When the _Great Western_ steams.h.i.+p was in course of construction, tenders were invited from the leading mechanical firms for the supply of the enormous paddle-shaft required for her engines. But a forging of the size in question had never been executed, and no firm in England would undertake the contract. In this dilemma, Mr Nasmyth was applied to, and the result of his study of the problem was this marvellous steam-hammer--so powerful that it will forge an Armstrong hundred-pounder as easily as a farrier forges a horse-shoe, and so delicately manageable that it will crack a nut without bruising its kernel!

BESSEMER STEEL.

In 1722, Reaumur produced steel by melting three parts of cast-iron with one part of wrought iron (probably in a crucible) in a common forge; he, however, failed to produce steel in this manner on a working scale. This process has many points in common with the Indian Wootz-steel manufacture.

As we have seen, to Benjamin Huntsman, a Doncaster artisan, belongs the credit of first producing cast-steel upon a working scale, as he was the first to accomplish the entire fusion of converted bar-iron (that is, blister-steel) of the required degree of hardness, in crucibles or clay pots, placed among the c.o.ke of an air-furnace. This process is still carried on at Sheffield and elsewhere, and is what is generally known as the crucible or pot-steel process. It was mainly supplementary to the cementation process, as formerly blister-steel was alone melted in the crucibles; but latterly, and at the present time, the crucible mode of manufacture embraces the fusion of other varieties and combinations of metal, producing accordingly different cla.s.ses and qualities of steel.

In 1839, Josiah Marshall Heath patented the important application of carburet of manganese to steel in the crucible, which application imparted to the resulting product the properties of varying temper and increased forgeability. He subsequently found out that a separate operation was not necessary to form the carburet--which is produced by heating peroxide of manganese and carbon to a high temperature--but that the same result could be attained by simply in the first instance adding the carbon and oxide of manganese direct to the metal in the crucible.

He unsuspectingly communicated this after-discovery to his agent--by name Unwin--who took advantage of the fact that it was not incorporated in the wording of the patent, and so was unprotected, to make use of it for his own advantage. The result was one of the most remarkable patent trials on record, extending over twelve years, and terminating in 1855 against the patentee--a remarkable instance of the triumph of legal technicalities over the moral sense of right.

A very important development of the manufacture of steel followed the introduction of the 'Bessemer process,' by means of which a low carbon or mild cast-steel can be produced at about one-tenth of the cost of crucible steel. It is used for rails, for the tires of the wheels of railway carriages, for s.h.i.+p-plates, boiler-plates, for shafting, and a mult.i.tude of constructional and other purposes to which only wrought iron was formerly applied, besides many for which no metal at all was used.

Sir Henry Bessemer's process for making steel, which is now so largely practised in England, on the continent of Europe, and in America, was patented in 1856. It was first applied to the making of malleable iron, but this has never been successfully made by the Bessemer method. For the manufacture of a cheap but highly serviceable steel, however, its success has been so splendid that no other metallurgical process has given its inventor so great a renown. Although the apparatus actually used is somewhat costly and elaborate, yet the principle of the operation is very simple. A large converting vessel, with openings called tuyeres in its bottom, is partially filled up with from 5 to 10 tons of molten pig-iron, and a blast of air, at a pressure of from 18 to 20 lb. per square inch, is forced through this metal by a blowing engine. Pig-iron contains from 3 to 5 per cent. of carbon, and, if it has been smelted with charcoal from a pure ore, as is the case with Swedish iron, the blast is continued till only from .25 to 1 per cent.

of the carbon is left in the metal, that is to say, steel is produced.

Sometimes, however, the minimum quant.i.ty of carbon is even less than .25 per cent. In England, where a less pure but still expensive cast-iron--viz. haemat.i.te pig--is used for the production of steel in the ordinary Bessemer converter, the process differs slightly. In this case the whole of the carbon is oxidised by the blast of air, and the requisite quant.i.ty of this element is afterwards restored to the metal by pouring into the converter a small quant.i.ty of a peculiar kind of cast-iron, called _spiegeleisen_, which contains a known quant.i.ty of carbon. But small quant.i.ties of manganese and silicon are also present in Bessemer steel. The 'blow' lasts from 20 to 30 minutes. Steel made from whatever kind of pig-iron, either by this or by the 'basic'

process, is not sufficiently dense, at least for most purposes, and it is accordingly manipulated under the steam-hammer and rolled into a variety of forms. Bessemer steel is employed, as we have said, for heavy objects, as rails, tires, rollers, boiler-plates, s.h.i.+p-plates, and for many other purposes for which malleable iron was formerly used.

Basic steel is now largely made from inferior pig-iron, such as the Cleveland, by the Thomas-Gilchrist process patented in 1878. It is, however, only a modification of the Bessemer process to the extent of subst.i.tuting for the siliceous or 'acid' lining generally used, a lime or 'basic' lining for the converter. Limestone, preferably a magnesian limestone in some form, is commonly employed for the lining. By the use of a basic lining, phosphorus is eliminated towards the end of the 'blow.' Phosphorus is a very deleterious substance in steel, and is present, sometimes to the extent of 2 per cent., in pig-iron smelted from impure ore.

The four inventions of this century which have given the greatest impetus to the manufacture of iron and steel were--the introduction of the hot blast into the blast-furnace for the production of crude iron, made by J. B. Neilson, of the Glasgow Gas-works, in 1827; the application of the cold blast in the Bessemer converter which we have just described; the production of steel direct from the ore, by Siemens, in the open hearth; and the discovery of a basic lining by which phosphorus is eliminated and all kinds of iron converted into steel.

This last was the discovery of G. J. Snelus, of London, and it was made a practical success by the Thomas & Gilchrist process just described. In 1883, Mr Snelus was awarded the Bessemer gold medal of the Iron and Steel Inst.i.tute 'as the first man who made pure steel from impure iron in a Bessemer converter lined with basic materials.'

SIR HENRY BESSEMER.

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