THE MAN OF THE, was the designation applied to a remarkable personage whose real name was unknown. He was so called because of a black mask which completely concealed his face. This mask was of black velvet, and so constructed by means of springs that the wearer never required to take it off, not even while eating. He was kept in the Castle of Pignerol, from which he was taken (in 1681) to Exelles; thence (in 1687) to the island of St Marguerite; and lastly (in 1698) to the Bastile in Paris. At each of these places he was attended by the same keeper, M. de St Mars. When taken from place to place, armed attendants on horseback were ready to despatch him if he made any attempt to escape, or even to show his face. That the prisoner thus guarded was a person of no small importance is evident from the precautions taken to keep him from being recognised. During his captivity in the Bastile, he was always treated with the greatest consideration. His fare was sumptuous, and his accommodation was the best that could be obtained in the prison. He had a great predilection for lace and extremely fine linen, which were supplied him; indeed he got whatever he asked. He read extensively, and was a good player on the guitar. The physician who attended him at the Bastile Iron Mask describes him (according to Voltaire) as remarkably well made, with skin rather brown, and tone of voice interesting from the fact that he never complained of his fate, nor dropped any hint of what he might be. The physician at times examined his tongue and various parts of his body, but never saw his countenance. Besides these incidents, there are some interesting anecdotes, showing how important it was considered to keep the man with the mask unknown. It is related that while he was at St Marguerite, he carved with a knife some words on a silver plate, which he threw from a window upon the shore. The plate was picked up by a fisherman, who brought it to the governor of the jail. The words must have contained a complete revelation of the mystery connected with the prisoner, for the governor of the jail (M. de St Mars) detained the fisherman to ascertain whether he had shown the plate to any one. The fisherman assured him that he had not, but it was equally important that he should not have read the words himself; accordingly, the jailer did not let him go till he was certain of the man's inability to read at all. Upon setting him at liberty, M. de St Mars said to him—"It is well for you that you cannot read!" Upon another occasion, the man with the mask had covered a shirt with writing, and got it somehow thrown into the water. It was picked up by a boy and brought to the governor. The boy was asked if he had read any of the writing, but he denied having done so. However, in a few days afterwards he was found dead in his bed. According to another anecdote, a prisoner confined in the apartment immediately above that of the man with the mask, succeeded in carrying on a conversation with him by means of the chimneys. When pressed to tell who he was, the mask replied that his life, and not only his, but the lives of all who might become possessed of the secret, would be sacrificed should he make known the mystery. When about to be removed from St Marguerite to the Bastile, he inquired of his keeper (M. de St Mars) if his life were in danger. The keeper was overheard to answer, "No, Prince, your life is safe," &c. This accords with the respect uniformly paid him, for his governor never sat in his presence.

The mysterious individual died in the Bastile on the 19th November 1703. He fell suddenly ill one day after attending mass, and at 10 o'clock on the evening of the following day he died. He was buried on the next day, in the cemetery of St Paul, under the false name of Marchialy. He was considered as being about 45 years of age. Everything belonging to him, his clothes, bed, &c., were committed to the flames. The apartment in which he had been kept was searched to see that he had not concealed any notice by which he might be identified. The walls were scraped and whitewashed anew, and the tiles of the floor changed. To discover the person who was for so many years concealed behind the black mask, was one of the most curious problems in history. All the possible (and some impossible) personages have been successively declared to be the same as this great unknown. In an anonymous book published at Amsterdam in 1745, under the title Secret Memoirs for the History of Persia, the man with the mask was declared to be the Duke of Vernandois, the natural brother of the dauphin. The duke was represented as having given a personal affront to the dauphin, for which offence he was condemned to perpetual imprisonment. These so-called secret memoirs were in reality a history of the intrigues of the French court after the death of Louis XIV, and the persons who figured in the book were disguised under imaginary and Persian names. In 1746 another book appeared with the title of The Man with the Iron Mask, and although it contained an enumeration of adventures not at all applicable to the person of whom we have been speaking, yet from this time he became specifically designated by the title as given above. In 1751 Voltaire's work, The Age of Louis XIV, appeared at Berlin, and contained an account of the unknown person with the mask. The leading particulars of this account, relating to treatment in the Bastile, Iron Mask, &c., we have already given. Voltaire takes credit to himself as being the first who had given any proper historical account of the man with the mask. He draws attention to the important fact that, at the time when St Mars and his charge went to St Marguerite, no person of distinction had disappeared in Europe. In 1759, Lagrange-Chancel endeavoured to show that the unknown was the Duke de Bourbon. In 1768, St Foix threw out the conjecture that the person concerned was the Duke of Monmouth, who was openly executed in London! In 1769 some particulars were brought to light by Griffet in the discovery of the manuscript journal of Dijonca, a lieutenant du roi in the Bastile in 1698. These particulars related to the death, burial, &c., of the unknown, which we have given already. In 1770 an attempt was made by Baron d'Hess to identify him as Matthioli, secretary to the Duke of Mantua, who had been seized and confined in the Castle of Pignerol. Voltaire appeared again on the subject in the seventh edition of the Dictionnaire Philosophique, in which he corrected some inaccuracies into which he had formerly fallen, but throws no new light on the subject. It has, indeed, been asked how Voltaire should come to know so much about the man with the mask as he pretends? It has been suggested that he was the author of the Persian Memoirs, although he afterwards called it a contemptible and obscure tract; and that his inaccuracies regarding dates, &c., in his Age of Louis XIV, (which was published under the assumed name of Francheville), were intentional, his purpose being, if possible, to draw out such as might possess any authentic documents on the subject. If so, in failing to clear up one mystery, he seems willing to have created another, for he concludes the notice of which we are speaking by the remarkable sentence—"He who writes this perhaps knows more about the subject than Father Griffet, and will say no more about it." The editor of the said dictionary suggests that the unknown may have been an elder brother of Louis XIV—an illegitimate son of Anne of Austria, by Cardinal Mazarini or the Duke of Buckingham, brought up secretly in order to prevent scandal in the royal family and dissensions in the kingdom. In 1789 a document was passed off as having been discovered at the Bastile, ocularly settling the question by the words, "Fouquet arriving from St Marguerite with an iron mask . . ." in a mass of unintelligible notes. Cuvier suggested that the unknown prisoner was a twin-brother of Louis XIV, and in 1790 appeared the Memoirs of Cardinal Richelieu, which contained a document with the following title—"Account of the birth and education of the unfortunate Prince, abducted from society by the Cardinals Richelieu and Mazarin, and confined by order of Louis XIV; drawn up by the governor of this Prince on his deathbed." According to the narrative the Prince, who was educated secretly by order of his father, one day saw a portrait of Louis XIV, and at once perceived its likeness to himself. He was immediately masked and imprisoned as before described. It need scarcely be added that there is great difficulty in the way of accepting the document under consideration as authentic. In 1800 the claims of Matthioli were revived by Roux-Fazillac, and defended by Delort in 1825. Several of the persons already mentioned were also again successively supposed to be the mysterious prisoner; new names, however, such as that of Henry Cromwell, being added to the list. In 1837 some able researches were published by Jacob, in which the belief is supported that the statesman Fouquet was the prisoner. It is contended that the prosecutions taken in the case of Fouquet were precisely similar to those taken in the case of the prisoner with the mask; and that it was immediately after the pretended death of Fouquet that the prisoner with the mask appeared. More recently Lord Dover took up the subject, and supported the belief that Count Matthioli, before-mentioned, was the mysterious unknown. Iron, on account of its abundance, working qualities, and tenacity, is probably the most useful and valuable of metals. According to Dr. Ure, "it is capable of being cast into moulds of any form, of being drawn into wire of any desired length or fineness, of being extended into plates or sheets, of being bent in every direction, of being sharpened, or hardened, or softened at pleasure. Iron accommodates itself to all our wants and desires, and even to our caprices; it is equally serviceable to the arts, the sciences, to agriculture, and war; the same ore furnishes the sword, the ploughshare, the scythe, the pruning-hook, the needle, the graver, the spring of a watch or of a carriage, the chisel, the chain, the anchor, the compass, the cannon, and the bomb. It is a medicine of much virtue, and the only metal friendly to the human frame." In its primitive position it is commingled with the earth's strata in bountiful profusion; it is found in various combinations and conditions in every formation, and it is a constituent element of both animals and vegetables.

In treating of the manufacture, properties, and production of this most important material, it will conduce to perspicuity to arrange the subject under five distinct heads, viz.—I. The History; II. The Ores and Fuel; III. The Manufacture; IV. The Strength and other Properties of Iron; V. The Statistics of the Iron trade.

I. History of the Iron Manufacture.

Malleable iron appears to have been known from a remote antiquity. Its obvious utility and great superiority over the softer metals, then commonly used, combined with the expense of its reduction, caused it to be highly prized, though the extreme difficulty of working it by the rude methods then employed greatly restricted its application. There are notices in Homer and Hesiod of the arts of reducing and forging iron, but cast-iron was then unknown, an imperfectly malleable iron being produced at once from the ores in the furnace. It is probable that the Greeks obtained most of their iron through the Phoenicians from the shores of the Black Sea, and from Laconia.

It would be interesting to trace the gradual advances which have been made in the reduction of iron from its discovery to the present time; to inquire into the circumstances which led to the successive changes in the processes, and into the principle on which those changes were founded; to examine into the differences in the products which from time to time ensued, and to notice the influence of these conditions on the extent and progress of the manufacture. Our knowledge of these changes, however, is scanty and imperfect, and we can only conjecture what was probably its early progress.

The furnaces which were first employed for smelting iron were probably similar to those now called air-bloomeries. They were probably simple conical structures, with small openings below for the admission of air, and a large one above for the escape of the products of combustion, and would be erected on high grounds in order that the wind might assist combustion. The fire being kindled, successive layers of ore and charcoal would be placed in it, and the heat regulated by opening or closing the apertures below.

The process of reduction would consist of the de-oxidation of the ore and the cementation of the metal by long continued heat. The temperature would never rise sufficiently high to fuse the ore, and the product would therefore be an imperfectly malleable iron, mixed with scoriae and unreduced oxide. It would then be brought under the hammer, and fashioned into a rude bloom, during which process it would be freed from the greater portion of the earthy impurities.

By such a process as this the Romans probably worked the iron ores of our own island; scoriae, the refuse of ancient bloomeries, occur in various localities, in some cases identified with that people by the coincident remains of altars dedicated to the god who presided over iron. Mungo Park saw a rude furnace of this kind used by the Africans, and, indeed, with some modifications, it is still retained in Spain, and along the coasts of the Mediterranean, where rich specular ores are worked.

The advantages of an artificial blast would soon become manifest, and a pair of bellows or a cylinder and piston would soon be applied to the simple construction mentioned above. Homer represents Hephaestus as throwing the materials from which the shield of Achilles was to be forged into a furnace urged by 20 pairs of bellows (πυρα). The inhabitants of Madagascar smelt iron in much the same way, their blowing apparatus, however, consisting of hollow trunks of trees, with loosely fitting pistons.

The furnace corresponds to the blast-bloomery, and has by successive improvements developed into the blast furnace, now almost universally used, and into the Catalan forge, still employed in some districts. The application of the blast would offer considerable advantages; it would obviate the necessity of an elevated site, place the temperature more immediately under the direction of the smelter, and render the whole process more regular and certain. The method of reduction remained the same as before, but the product would differ considerably, for whenever the blast was sufficiently powerful, the iron would be fused, a partial carburation would take place, and the resulting metal would be a species of steel, utterly useless to the workmen of those days; hence, it seems necessary to infer, that a rude process of refining was invented, the metal being again heated with charcoal, and the blast directed over its surface, the carbon would be burnt out, and the iron become tough and malleable. The processes might perhaps form two successive stages of one operation, as at present practised with the Catalan forge.

The increasing demand for iron, and the progress of internal communication, would lead the smelter to increase the size and height of his bloomery, and this probably would lead to a very unexpected result. The greater length through which the ore had to descend would prolong its contact with the charcoal, and a higher state of carburation would ensue, the product being cast-iron—a compound till then perhaps unknown.

From the time that cast-iron became the product of the smelting furnace, the refining would be made a separate process, requiring a separate furnace and machinery. It would soon be found also that, as the furnace increased in height, the pressure of the superincumbent mass would render the materials so dense as to retard the ascent of the blast, and thus cause it to become soft and inefficient; hence the internal buttresses called bosses were

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1 This is shown by the epithet ωλεσμένος (much-wrought), applied to it by Homer—Ilid., vi. 48. The first great improvement on the blowing apparatus was the substitution of large cylinders, with closely fitting pistons, for the bellows. The earliest of any magnitude were probably those erected by Smeaton at the Carron Iron-Works, in 1765.

In 1783-4, Mr Cort of Gosport introduced the processes of puddling and rolling, two of the most important inventions connected with the production of iron since the employment of the blast furnace. (See Addenda, a.)

About this time the steam-engine of James Watt came into use, and along with it commenced a new era in the history of the iron trade and every other branch of industry. Its immense power, economy, and convenience of application, brought it at once into general employment. It was soon applied to pumping, blowing, and rolling; it enabled the mines to be sunk to a greater depth; refractory ores to be reduced with facility, and the processes of rolling, forging, &c., to be effected with a rapidity previously unknown.

Of late years, Scotland has made considerable progress in the iron manufacture. The introduction of railway communication, and the invention of the hot-blast, have given a stimulus to the trade which has raised Glasgow into importance as an iron district, and few towns possess greater facilities for the sale of their produce, than this central depot of the mineral treasures of the country by which it is surrounded.

The hot-blast process, for which a patent was taken out by Mr Neilson in 1824, has effected an entire revolution in the iron industry of Great Britain, and forms the last era in the history of this material. This simple but effective invention has given such facilities for the reduction of refractory ores, that between three and four times the quantity of iron can be produced weekly, with an expenditure of little more than one-third the fuel; and, moreover, the coal does not require to be coked, or the ores to be calcined.

In conclusion, we may add that there appear to have been five distinct epochs in the history of the iron trade.

The first dating from the employment of an artificial blast to accelerate combustion.

The second marked by the employment of coke for reduction, about the year 1750.

The third dating from the introduction of the steam-engine, and on account of the facilities which that invention has given for raising the ores, pumping the mines, supplying the furnace with a copious and regular blast, and moving the powerful forge and rolling machinery, we may safely attribute this era to the genius of James Watt.

The fourth epoch is indicated by the introduction of the system of puddling and rolling, very soon after the employment of the steam-engine.

The fifth, and last—though not the least important epoch in the history of this manufacture—is marked by the application of the hot-blast—an invention which has increased the production of iron fourfold, and has enabled the ironmaster to smelt otherwise useless and unreducible ores; it has abolished the processes of coking and roasting, and has given facilities for a large and rapid production, far beyond the most sanguine anticipations of its inventor. Manufacturers taking advantage of so powerful an agent, have not hesitated to reduce improper materials, such as cinder-heaps and impure ores, and by unduly hastening the process, and attending to quantity more than to quality, have produced an inferior description of iron, that has brought the invention into unmerited obloquy.

II. The Ores.

The ores of iron are found in profuse abundance in every latitude, embedded in or stratified with every formation. They occur both crystallized, massive, and arenaceous, lying deep in strata of vast extent, filling veins and faults in other rocks, and scattered over the surface of the ground. Sometimes, but rarely, found native; usually as oxides, sulphurites, or carbonates, more or less mingled with other substances. Of these ores there are perhaps twenty varieties, many of which are, however, rare; others are combined with substances which unfit them for the manufacture of iron, so that the remainder may be classed under the following general heads: their composition, however, varies greatly:

1. The magnetic oxides, in which the iron occurs, as Fe₃O₄ or Fe₂O₃ + FeO. This is the purest ore which is worked, the best Swedish metal is manufactured from it. It is found in primitive rocks, and is widely diffused over the globe.

2. Specular iron ore, peroxide of iron, Fe₂O₃. This is rich and valuable ore, and has been worked from a remote antiquity in Elba and Spain. It is found chiefly in primary and transition rocks.

3. Red and brown haematites, hydrated peroxide of iron. These ores occur in botryoidal radiating masses, in Cumberland, Ireland, America, and other places.

4. Carbonate of iron. This ore occurs mixed with large quantities of argillaceous, carbonaceous, and siliceous substances, forming the large deposits of clay-ironstone and blackbands, from which most of the iron of this country is obtained. These strata are generally found in close proximity to the coal measures.

All the above ores are more or less mixed with silica, alumina, oxide of manganese, &c., and it may not be uninteresting to glance at their geographical distribution in Europe and America.

This country possesses peculiar and remarkable advantages for the manufacture of iron. The ores are found in exhaustless abundance, usually interstratified with the coal for their reduction, and in close proximity to the mountain limestone, which is used as a flux. In few countries do these three essential materials occur in such abundance, or so near together as to give the necessary facilities for a large and profitable production.

The ores principally employed are the clay-ironstones and carbonates of blackbands, which are found interstratified with the coal fields of Ayrshire, Lanarkshire, Shropshire, South Wales, and other parts, and these vary in richness in different localities, according to position and the amount of silica clay and other foreign matter with which they are associated. The chemical composition of three varieties of the ore used in Lanarkshire is given by Dr. Colquhoun, as follows:

| | No. 1 | No. 2 | No. 3 | |----------------|-------|-------|-------| | Protoside of iron | 53.63 | 47.33 | 35.22 | | Carbonic acid | 35.17 | 33.10 | 32.53 | | Silica | 1.40 | 6.63 | 9.56 | | Alumina | 0.63 | 4.30 | 5.34 | | Lime | 3.33 | 2.00 | 8.62 | | Magnesia | 1.77 | 2.20 | 5.19 | | Peroxide of iron | 0.23 | 0.33 | 1.16 | | Bituminous matter| 3.03 | 1.70 | 2.13 | | Sulphur | 0.00 | 0.22 | 0.92 | | Oxide of Manganese| 0.00 | 0.13 | 0.00 | | Moisture and loss| 1.41 | 2.26 | 0.00 |

The carbonic acid in the above ores may be partly combined with the lime as carbonate of lime, as well as with the protoside of iron.

M. Berthier gives, according to Dr. Ure, the following analyses of the English and Welsh ironstones of the coal measures:

| | Rich Welsh Ore | Poor Welsh Ore | Dudley Rich Ore or Gubbin | |----------------|----------------|---------------|--------------------------| | Loss by ignition | 30.00 | 27.00 | 31.00 | | Insoluble residuum | 8.40 | 22.03 | 7.66 | | Peroxide of iron | 60.00 | 42.66 | 68.33 | | Lime | 0.00 | 6.00 | 2.66 | | | 98.40 | 97.69 | 99.65 |

Calculating the amount of carbonate of iron and metallic iron indicated by the above analyses, we have:

| | Carbonate of iron | Metallic iron | |----------------|-------------------|--------------| | | 88.77 | 65.09 | | | | 85.20 | | | 42.15 | 31.28 | | | | 40.45 |

The richness of the above ironstones would be about 33 per cent of iron. In the process of roasting, 28 per cent of the ore is dissipated.

Mr Mitchell gives also the following assays of clay-ironstone and blackband ore, as under:

| | Clay Ironstone, Leitrim, Ireland | Blackband Carbonate Ore | |----------------|----------------------------------|-------------------------| | Protoside of iron | 51.653 | 20.924 | | Peroxide of iron | 37.42 | 7.41 | | Oxide of Manganese| 9.76 | 1.74 | | Alumina | 1.849 | 14.974 | | Magnesia | 2.84 | 9.87 | | Lime | 4.10 | 8.81 | | Potash | 2.74 | trace. | | Soda | 3.72 | trace. | | Sulphur | 2.14 | 0.98 | | Phosphoric acid | 2.84 | 1.14 | | Nitric acid | 31.142 | 14.990 | | Silica | 6.640 | 26.179 | | Carbonaceous matter| 2.160 | 16.940 | | Loss | 100.000 | 100.000 |

In North Lancashire and Cumberland, the red haematite ores are now extensively worked, and great quantities are yearly shipped from Whitehaven, Ulverstone, &c., to Staffordshire, South Wales, and Scotland, for mixing with the poorer argillaceous and blackband ores. In Cumberland and North Lancashire, no less than 546,998 tons were raised in 1854 for this purpose, and the greater portion was exported from those districts.

In addition to these exports, about 25 to 30,000 tons are smelted by the hot blast at Cleator, in the neighbourhood of Whitehaven. It produces a strong and ductile iron, considered highly valuable for mixing with the weaker irons. These ores have been carefully analysed, and contain:

| | Protoside of iron | Silica | Alumina | Lime | Magnesia | Water | |----------------|-------------------|--------|---------|------|----------|-------| | | 90.3 | 5.0 | 3.0 | trace.| trace. | 6.0 | | | | | | | | 104.3 |

Or about 62 per cent of metallic iron.

In Ireland there are vast deposits of iron ore of great richness, though as yet but little worked. Some of these, such as the ores worked at the Arigna mines, and the Kidney ores of Balcarry Bay, yield as much as 70 per cent of iron. If these mines were worked more extensively, and if peat fuel were used in the smelting opera- The iron ores of France.

France possesses an abundant supply of iron ore, but on account of the scarcity of coal, the manufacture has been greatly restricted in extent. The introduction of railway communication is, however, rapidly removing this difficulty, and the operations of smelting are greatly on the increase. The railroad has enabled the French iron-master to substitute coal for charcoal in the reduction of the iron ores, and in consequence an immense increase has taken place in the production of pig and manufactured iron. The ores are found in beds or strata in the Jura range; accumulated in kidney-shaped concretions in the fissures of the limestone; or dispersed over the surface of the ground, and but slightly covered with sand or clay.

They are found in the departments of the Yonne, the Meuse, and the Moselle, and indeed may be traced from the Pas de Calais on the north to the Jura on the south, indicating throughout an abundant and ample supply.

The present increased production of iron in France is chiefly due to the introduction of coal in smelting, but it may also be traced in some measure to the encouragement given by the Government to that branch of industry, and to the enterprise of such men as M. de Gallois and M. Dufrenoy, who have exerted themselves to extend its manufacture in that country. M. de Gallois resided in England for several years, immediately subsequent to the peace of 1815, and having obtained admission into the different iron-works here, he returned to France and established the works at St. Etienne, now probably the largest and most extensive in that country. The production of crude pig-iron in France is now little short of 1,000,000 tons annually, but the demand for railways, rolling-stock, bridges, iron ships, girders, and other constructions is so great that large quantities of iron are still annually imported from this country.

Valuable deposits of the blackband and clay carbonate ores are found interstratified with the great coal-field of Rohr; and the bog-iron and haematite ores are found in considerable profusion in Rhenish Prussia and other parts. In Upper Silesia, on the Vistula and the Oder, large deposits of coal and iron are found in juxtaposition, and are worked to a considerable extent.

The consumption of iron is not so great as in France, though it is increasing rapidly, as may be seen from returns recently given by the British Chargé d’Affairs at Berlin. These returns show that the amount of iron ore raised in Prussia has increased from 1,495,516 tons in 1853, to 2,144,509 tons in 1854; this has taken place in nearly all the producing districts, but chiefly on the Rhine, where the demand has increased from 719,684 to 1,068,656 tons; in Westphalia, from 146,320 to 330,014 tons; in Silesia, from 563,739 to 650,369 tons; in Lower Saxony and Thuringia, from 51,963 to 70,676 tons; in Prussian Brandenburg, from 8,084 to 12,731 tons; and in the Upper Zollverein from 6,736 to 12,063 tons.

In Austria, all the iron is smelted with charcoal or carbonised peat, and is in consequence of the finest ores of quality; it may be applied to every description of manufacture, from the most ductile wire to the hardest steel, &c. The production is, however, small. The ores are found in Hungary, Styria, Moravia, and Upper Silesia.

In Belgium, both coal and iron are found in equal abundance, and are worked at Charleroi, Liège, and at other places. The ores, which are chiefly haematite, are derived from the limestone at the base of the coal measures. (See Addenda, p.)

The superiority of the Swedish iron has long been acknowledged, and till recently it has been unrivalled. This arises not only from the purity of the ore—the magnetic oxide of iron—but in consequence of its being smelted with charcoal only. The quantity is, however, restricted, as the ironmasters are allowed by law only a certain number of trees per annum, in order that the forests may not be totally destroyed. Coal does not exist in either Sweden or Norway.

In 1844 some experimental researches were undertaken by Mr Fairbairn of Manchester, at the request of the Sublime Porte, in regard to the properties of iron made from the ores of Samakoff in Turkey. The ores were strongly magnetic, and contained, according to Dumas and others, 62 to 64 per cent of iron. They consisted of:

\[ \begin{align*} \text{One atom iron} & : \text{one atom oxygen} = 8 : 36 \\ \text{Two atoms iron} & : \text{three atoms oxygen} = 56 : 80 \\ \text{Iron} & : \text{Oxygen} = 84 : 32 \end{align*} \]

Some of these ores have been smelted with charcoal, and some very fine specimens of iron and steel produced. The manufacture is, however, in a languid state in Turkey, and although smelting furnaces, blowing apparatus, forges, rolling mills, &c., were prepared and sent out from this country, they are to a great extent useless among a people who have deeply rooted prejudices and habitual inactivity to overcome, and everything to learn in all those habits of industry which indicate the rising prosperity of an energetic and an active people.

Both the magnetic, haematite, and clay-ironstones abound in the United States. The magnetic ores of worked in New England, New York, and New Jersey; the haematite in Pennsylvania, New York, New Jersey, and other localities; but the greater part of the manufacture must eventually establish itself in the valley of the Mississippi, west of the Alleghany range, where vast deposits of coal and iron exist, though at present but imperfectly known or developed. The ores in most of these districts are smelted with a mixture of charcoal and anthracite, and the usual limestone flux, and produce a very excellent quality of iron.

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1 The universal exhibition of last year (1855) fully justifies the remarks in reference to the great increase of the iron trade of France. Any person in the least conversant with the imperfect machinery and processes of the iron manufacture as it existed in France some years since, could not have been otherwise than struck with the improved character of those exemplified in the Paris Exhibition. In no country (probably not excepting even this) has so great progress been made in so short a time, in advancing from a state of comparative rudeness to one of considerable perfection, as in France. In Nova Scotia some of the richest ores yet discovered occur in exhaustless abundance. The iron manufactured from them is of the very best quality, and is equal to the finest Swedish metal. The specular ore of the Acadian mines, Nova Scotia, is said by Dr. Ure to be a nearly pure peroxide of iron, containing 99 per cent of the peroxide, and about 70 per cent of iron. When smelted, 100 parts yield 75 of iron, the increase in weight being due to combined carbon. The red ore Dr. Ure states to be analogous to the kidney ore of Cumberland, and to contain:

| Peroxide of iron | 85-8 | 84-4 | |------------------|------|------| | Silica | 8 | 8 | | Water | 6 | 7 |

The Acadian ores are situated in the neighbourhood of large tracts of forests, capable of supplying almost any quantity of charcoal for the manufacture of the superior qualities of iron and steel. Several specimens of iron from these mines have been submitted to direct experiment, and the results prove its high powers of resistance to strain, ductility, and adaptation to all those processes by which the finest description of wire and steel are manufactured.

The difficulties which the Government have had to encounter, during the last two years, in obtaining a sufficiently strong metal for artillery, are likely to be removed by the use of the Acadian pig-iron. Large quantities have been purchased by the War Office, and experiments are now in progress, under the direction of Lieutenant-Colonel Wilmot, Inspector of Artillery, and of Mr Fairhaim, which seem calculated to establish the superiority of this metal for casting every description of heavy ordnance.

There are also some very rich ores at the Nictau mines, as the following analyses by Dr. Jackson shew. They contain impressions of Silian tentaculites, spiroifers, &c.

| Brown Ore, somewhat magnetic. | Red Iron Ore. | |-------------------------------|---------------| | Peroxide of iron | 70-20 | 64-40 | | Silica | 14-40 | 12-20 | | Carbonate of Lime | 5-60 | 5-40 | | Carbonate of Magnesia | 2-80 | 3-20 | | Alumina | 6-80 | 1-20 | | Oxide of Manganese | 4-0 | 4-40 | | Water | 0-0 | 2-40 |

* Gain from oxygen. † Over-run, probably carbonic acid from carbonate of lime.

As our limits are circumscribed, it will not be necessary to extend this section further; suffice it therefore to observe, that in all countries nature has, with a beneficent purpose, interlaid and interstratified the whole surface of the globe with this useful and indispensable material, and it would ill bespeak that high intelligence with which man is endowed if he did not avail himself of, and turn to good account, the immense stores of mineral treasures which are so profusely laid at his feet.

**The Fuel.**

The enquiry into the properties and composition of the ores of iron, and the processes employed for their reduction and subsequent conversion into bars and plates, would be incomplete unless accompanied by descriptive analyses of the fuel by which they are fused. Indeed the results of the operations of smelting, puddling, &c., are so intimately dependent on the quality of the fuel employed, as to render a knowledge of its constituents essential to the manufacture of good iron.

Charcoal was at first universally employed in the manufacture of iron, and on account of its purity compared with other kinds of fuel, and its strong chemical affinities and consequent high combustibility, it is of very superior value where it can be obtained in large quantities at a moderate cost. This, however, is rarely the case, and hence its use is restricted within very narrow limits in most countries. Charcoal is the result of several processes, in each of which the object is to increase the amount of fuel in a given bulk. The wood being cut into convenient lengths, and piled closely together, in a large heap, the interstices being filled with the smaller branches, and the whole covered with wet charcoal powder, is then set on fire. Care is taken that only sufficient air is admitted to consume the gaseous products of the wood, so as to maintain the high temperature without needlessly consuming the carbon. After the whole of the gaseous products have been separated, and the carbon and salts only are left, the access of air is prevented, and the heap allowed to cool.

Another and better process is to throw the wood into a large close oven or furnace, heated either by the combustion within it, or by a separate fire conducted in flues around it. By this process, not only is the yield greater and of better quality, from the slower progress of the operation, but the products of the distillation may be preserved and employed for a great variety of purposes. The following results of some experiments by Karsten, shew the difference in yield of very rapid and very slow processes:

| Wood. | Charcoal produced by quick carbonisation. | Charcoal produced by slow carbonisation. | |-------|------------------------------------------|----------------------------------------| | Young Oak | 16-54 | 25-60 | | Old | 15-91 | 25-71 | | Young Deal | 14-25 | 25-25 | | Old | 14-05 | 25-00 | | Young Fir | 16-22 | 27-72 | | Old | 15-35 | 24-75 | | Mean | 15-38 | 25-67 |

These, on the average, give for the quick process 15-38, and for the slow 25-67, being in the ratio of 1:1.67, or 0-67 in favour of the quick process.

**PEAT.**—This material seems likely to come into use for smelting iron in countries such as Ireland, where neither coal nor wood are found in abundance. It is purer and less objectionable than coal, and if properly dried, compressed, and carbonized, would prove a very valuable fuel for the reduction of such ores as we have already described in the section on the iron ores of Ireland. It is carbonized in the same way as the char-ring of wood.

**COKE.**—Before the introduction of the hot-blast, this material was used to a very great extent in the manufacture of iron; it is prepared from coal in the same way that charcoal is prepared from wood, the operation being called the coking or desulphurizing process. The heaps do not require so careful a regulation of the admission of air as those of charcoal, on account of the comparatively incombustible character of the coke. Sometimes the heaps are made large, with perforated brick chimneys, to increase the draught through the mounds; at other times they are formed into smaller heaps, and the conversion takes place without the intervention of flues. The more usual and economical plan is, however, the employment of close ovens, by which process a great saving is effected, the yield being from 30 to 50 per cent in the one case, The Fuel, and from 50 to 75 in the other, according to the nature and quality of the coal.

Coal.—The hot-blast has enabled the ironmasters to use raw coal in the blast furnaces, the great heat of the ascending current of the products of combustion coking it as it falls in the furnace. The sulphur, however, and other deleterious ingredients, do not appear to be so completely got rid of as when the coal is used in the shape of coke; and it appears probable, that even with the hot blast, the separate process of coking might be advantageously used, on account of the greater purity of the iron produced.

The following tables, selected from various sources, give the composition of the different kinds of fuel, all of which are applicable to the reduction and fusion of the iron ores:

| Fuel | Locality | Specific Gravity | Carbon | Hydrogen | Oxygen and Nitrogen | Ashes in 100 parts | Authority | |---------------|---------------|------------------|--------|----------|---------------------|--------------------|-----------| | Splint Coal | | | | | | | Thomson | | | Newcastle, Wylam | 1.290 | 75.00 | 6.25 | 18.75 | | Ure | | | Glasgow | 1.307 | 82.924 | 6.491 | 10.457 | 1.128 | Richardson | | Cannel Coal | | | | | | | Thomson | | | Lancashire, Wigan | 1.272 | 64.72 | 21.56 | 13.72 | | Ure | | | Edinburgh | 1.318 | 67.597 | 5.405 | 12.432 | 14.566 | Richardson | | Cherry Coal | | | | | | | Thomson | | | Newcastle, Jarrow | 1.263 | 74.45 | 12.40 | 13.15 | | Ure | | | Glasgow | 1.286 | 81.206 | 5.452 | 11.923 | 1.421 | Richardson | | Coking Coal | | | | | | | Richardson | | | Newcastle, Gatesfield | 1.280 | 87.952 | 5.239 | 5.416 | 1.393 | | | | Durham South Hetton | 1.274 | 83.274 | 5.171 | 3.636 | 1.519 | | | Anthracite | | | | | | | Thomson | | | Swansea | 1.348 | 92.56 | 2.330 | 2.330 | 1.720 | Regnault | | | South Wales | 1.270 | 90.58 | 2.600 | 4.100 | | Jacquinin | | | Pennsylvania | 1.462 | 94.05 | 3.380 | 2.570 | | Overman | | | Massachusetts Worcester | 1.462 | 94.05 | 2.430 | 2.450 | 4.670 | Regnault | | Peat | | | | | | | Thomson | | | Vulcaire | 1.337 | 88.068 | 3.432 | 31.760 | | | | | Long | 1.393 | 89.709 | 2.300 | 9.000 | | | | | Camp de Feu | 1.469 | 82.175 | 6.725 | 9.100 | | | | | Cappage | 1.469 | 82.175 | 6.725 | 9.100 | | | | | Kilbeggan | 1.469 | 82.175 | 6.725 | 9.100 | | | | | Kilbagan | 1.469 | 82.175 | 6.725 | 9.100 | | |

According to Knapp, peat contains from 1 to 33 per cent its weight of ash. In coal we have the following from Mr Mushet's analyses:

| Specific gravity | Carbon | Ashes | Volatile matter | |------------------|--------|-------|----------------| | Welsh furnace coal | 1.337 | 88.068 | 3.432 | 31.760 | | Derbyshire furnace coal | 1.264 | 52.882 | 4.288 | 42.830 | | cannel coal | 1.278 | 48.362 | 4.638 | 47.000 |

And again the analyses, from Overman, of the ash of coal, may be quoted, as shewing the constituents contained in the ashes derived from combustion:

| Solphate of lime | 80.3 | 3.6 | | Lime | 3.8 | 2.5 | | Silex | 14.2 | 85.7 | | Oxide of iron | 1.7 | 0.0 | | Alumina | 0.0 | 8.2 |

The following table of the heating power of various kinds of fuel, from Knapp's Chemical Technology, is not without interest; in practice, however, only a portion of the absolute heating power is made available:

| Charcoal—Average | Berthier | 68.0 | | Peat from Allen in Ireland, Upper | Griffith | 62.7 | | Lower Pressed | 56.6 | | Peat charcoal—Esence | Berthier | 50.7 | | Frament and Champ de Feu | Berthier | 58.9 | | Coke—St. Etienne | Berthier | 65.6 | | Besseges | Berthier | 64.3 | | Rive de Gier | Berthier | 58.9 | | Brown coal—Mean of 5 varieties | Berthier | 50.3 | | Cannel coal, Wigan | Berthier | 64.1 | | Cherry, Derbyshire | Berthier | 61.6 | | Cannel, Glasgow | Berthier | 56.4 | | Lancashire | Berthier | 53.2 | | Durham | Berthier | 71.6 | | Gas coke, Paris | Berthier | 50.3 | | Anthracite | Berthier | 69.1 | | Pennsylvania | Berthier | 67.4 |

The specific gravity, carbon, hydrogen, oxygen and nitrogen, and ash content are given for each type of fuel. In concluding the observations on fuel, we may notice that the various kinds of coal are classed by mineralogists as the bituminous, and stone or anthracite coal. The first class is chiefly employed for the purpose of smelting; though, since the introduction of the hot-blast, anthracite is coming largely into use both in this country and America. Mr Crane of South Wales was the first who attempted the reduction of iron ores by anthracite, and Mr Bodd, at his works at Yatalyfera, followed successfully in the same path. To these two gentlemen the public are indebted for having surmounted the obstacles to the employment of this fuel for smelting iron.

On the occasion of a visit to Mr Crane's works, nearly twenty years ago, the writer had an opportunity of inspecting some specimens of anthracite, which had passed through the furnace and been in contact with the minerals at the temperature of fusion for 48 hours, without having suffered decomposition, and were found to be charred to a depth of only three-fourths of an inch, the interior being of a perfectly shining and black colour, and quite unaffected by the heat of the furnace.

THE MANUFACTURE OF IRON.

The processes for the manufacture of iron, as we have already pointed out, are of two distinct kinds, those of cementation and those of smelting; the product of the former is imperfectly malleable iron, that of the latter, cast-iron, or iron combined with more or less carbon.

The first and older process is uncertain in its results, involves considerable expense, and as there are no efficient means of getting rid of the earthy impurities, it necessitates the employment of rich magnetic, specular, or hematite ores; on account of these defects, it is now seldom employed. The ores to be reduced by this process were heated with charcoal in open furnaces like the Catalan hearth, the fire being urged by a blast. The oxygen, water, and volatile substances were driven off, and the iron—carburized and partially fused—sunk to the bottom of the hearth. The blast was then directed downwards, so as to play over the surface of the iron, and oxidized the greater part of the combined carbon; during this operation the iron became tough and malleable, and fit for the hammer.

The annexed section (fig. 1) shows the disposition of the Catalan hearth during the process of reduction. The fuel and ore B, are piled over the hearth A, and ignited; the blast to urge the fire is applied at D, and the gaseous products of combustion pass off by the chimney C.

A similar process has lately been invented by Mr Clay, which appears to reduce the rich ores with great advantage, and to be free from the defects of the older process. Mr Clay mixes the ore with four-tenths of its weight of coal, and grinds it so small that it will pass through a screen of one-eighth of an inch mesh; it is then put into the hopper A (fig. 2), from which it falls upon the preparatory bed B, at the side of the puddling furnace C. While in this position, the ore is heated, and partly decomposed, and the coal coked. The charge is then drawn forward into the reverberatory furnace C, where it is fused by the heat of the gaseous products passing from a fire at D to the chimney F, and is puddled and balled in the ordinary way. The cinder produced contains 50 to 55 per cent of iron, is free from phosphorus, and is very suitable for smelting in the blast furnace. This process is said to produce puddled bars equal to those made by the four operations of calcining, smelting, refining, and puddling, under the old system, and appears to be peculiarly adapted for the reduction of those rich ores which cannot be smelted advantageously in the blast furnace, because the small quantity of slag which is formed does not protect the metal from the oxidizing effects of the blast.

The process of smelting in the blast furnace is now almost universally adopted for the reduction of iron ores, and for the cheapness and working qualities of the metal. The manufacture of iron produced, as well as for the rapidity of the manufacture, it is decidedly superior to all others.

Ores which contain much carbonic acid, water, or volatile matter, were at one time invariably subjected to a preparatory process of calcination, but, since the introduction of the hot-blast, they are now frequently employed in the raw state. The calcination is sometimes effected in the open air, by stacking the ore with coal, setting fire to it, and allowing it to burn out; but this method is liable to serious objection. It is impossible to keep the temperature uniform throughout the heap, and in consequence, while some portions are scarcely affected, others are fused together into large masses, which cannot be smelted without difficulty, even when broken up. Apart from the irregularity and uncertainty of the open air process, it appears to be more expensive than the calcination in kilns, when the admission of air is entirely under command. These ovens or kilns are usually built of masonry, and are placed, if possible, on a level with the charging platform of the smelting furnace. The argillaceous ores lose, during this process, 20 to 30 per cent; the carbonaceous, 30 to 40 per cent of their weight. (See Addenda, C).

The blast furnace consists of a large mass of masonry, usually square at the base, from which the sides are carried up in a slightly slanting direction, so as to form, externally, a truncated pyramid. In the sides there are large arched recesses, in which are the openings into the furnace for the admission of the blast, and for running out the metal and cinder; at the top of the furnace is a cylindrical erection of brickwork, called the tunnel-head, for protecting the workmen from the heated gases rising from the furnace, and having one or more doors through which the charges of ore, fuel, and flux are thrown into the furnace. In front, protected by a roof, is the casting-house, where the metal is run from the furnace into moulds.

Fig. 3 is a vertical section, and fig. 4 a plan of one of the furnaces at the Dowlais Iron Works, which belong to the representatives of the late Sir J. Guest. Mr Truran, in a recently published and elaborate work on iron, has figured and described it. He states that it is one of the largest class, 38 feet square at the base, diminishing upwards 3 inches for every vertical foot, till it attains a height of 25 feet, where the square form ends with a moulded cap; above this, the form is circular, diminishing in diameter at a similar rate, and finishing at top with a plain moulded cornice, as a support for the charging platform. In the section and plan A is the hearth, 8 feet high and 8 feet in diameter. BB the boches, rising to a height of 15 feet, and 18 feet wide at their greatest diameter. From the top of the boches the body of the furnace contracts, in a barrel-shaped curve, so that at the charging platform D, at a height of 50 feet, it is only ten feet in diameter; E is the tunnel-head, with doors of iron, to admit the charges of ore and fuel; FFF the tuyere-houses, arched over and spread outwards, with the openings into the furnace for admitting the blast. G, the opening through which the iron is run from the furnace. The exterior is generally built of stone, and requires to be strongly bound with iron hoops, to prevent fracture from the expansion of the interior by the heat. The interior is lined with fire-brick set in fire-clay, a space of 2 or 3 inches being left between the two courses, to allow the expansion of the inner course. The hearth and boches were usually constructed of refractory sandstone grit, or conglomerate, but fire-bricks are now chiefly used, and although they do not last so long, they are, in the end, more economical, and may be replaced whenever the furnace is blown out. The proper inclination of the boches is a point of much importance, so that the materials, whilst smelting, may neither press too heavily downwards, nor yet be so retarded as to adhere in a half-liquid state to the brickwork, and cool there, thus forming what are known by the name of scaffolds, the removal of which is a source of great inconvenience.

Another form of furnace is occasionally used for smelting, called the cupola, and built much more slightly than the blast furnace. Its form is circular, and from the boches upwards it is constructed of fire-brick, one, or sometimes two, courses in thickness. It is strongly bound together with wrought-iron hoops, and pillars of cast-iron, bolted at each end to imbedded rings of the same metal, rise through the foundation to the summit of the tuyere arches, giving considerable firmness and stability to the structure. Cheapness and facility of construction are much in its favour, and although objections have been made to the thinness of its sides, as permitting great loss of heat by radiation, it has met with very general adoption.

In addition to the cupola furnace, another of the same character has of late years been introduced. It consists The manufacture of a truncated cone, composed entirely of boiler-plates rivetted together, as per annexed fig. 5. On the four opposite sides recesses are cut to admit the tuyeres and the opening from the hearth into the casting-house. The interior of the furnace is lined with fire-brick and fire-clay in the usual way, and this plate furnace is not only perfectly secure, as regards the expansion and contraction, but it is found to be economical and to answer every purpose in common with the large stone and iron-bound furnaces.

Fig. 5 exhibits a plan and elevation of this description of furnace, the parts AAA being the tuyere-houses, and B for the discharge of the metal from the furnace.

The blast. The blast is usually created by a steam engine; a piston being attached to the extremity of the beam, working in a cylinder of large diameter, and forcing the air through proper valves into a large spherical reservoir, constructed of boiler-plate, whence its own elasticity causes it to flow in a regular unintermitting stream into the furnace. A cylindrical vessel, open at bottom, and immersed in a pit of water, has sometimes been used to regulate the pressure of the blast, but the water evaporated is detrimental to the working of the furnace. The nozzles by which the blast is directed into the furnace are made of cast or wrought-iron, and sometimes a current of water is conveyed round their extremities to keep them cool. The number of blow-pipe nozzles to each furnace varies at different works; the usual number is three, one for each of the tuyere houses, but sometimes six, eight, or twelve are employed; it, however, appears questionable whether this is not objectionable, as the density and penetrating power of the blast is considerably diminished by this system of diffusion. This, however, is a point which can only be decided by practice, and must be left to the judgment of the smelter. The usual pressure of the blast as it enters the furnace is about 3½ lbs. per square inch, but in some cases it is as much as 5 lbs. per square inch. (See Addenda, D).

The communication between the ground and the tunnel-head is effected in various ways. In South Wales the furnaces are usually built on a declivity, which affords ready means of access from behind; sometimes an incline is constructed, or other contrivances, such as the balance and pneumatic lifts, are resorted to for the elevation of the materials.

The dimensions and form of the blast furnace vary greatly, according to the fashion of the district, and the notions of the builder. Yet so much does the quantity and quality of the iron depend upon the size of the furnace and strength of the blast, that we may venture to assert that the production varies in the ratio of the cubic contents of the furnace, and the volume of air admitted. Mr Truran gives the following particulars of the Dowlais Foundry iron furnace:—“The capacity is 275 cubic yards. It is blown with a blast of 5390 cubic feet of [cold] air per minute. The materials charged at the top consist of calcined argillaceous ore, coal, and limestone. The yield or consumption averages 48 cwt. of calcined ore, 50 cwt. of coal, and 17 cwt. of broken limestone, to 20 cwt. of crude iron obtained. The weekly make of iron is occasionally over 130 tons. The weekly product of cinder amounts to 250 tons. For the production of white iron for the forge, in furnaces of the same capacity as the foregoing, a larger volume of blast is employed, along with a different burden of materials. The blast averages 7370 feet per minute. The consumption of materials to one ton of iron averages 28 cwt. of calcined argillaceous ore, 10 cwt. of hematite, 10 cwt. of forge and finery cinders, 42 cwt. of coal, and 14 cwt. of limestone. With these materials the weekly produce amounts to 170 tons of crude iron, and 310 tons of cinder.”

The action which takes place in the blast furnace is as follows:—The contents being raised to an intense heat by the combustion of the fuel, are brought into a softened state; the limestone parts with its carbonic acid, and combining with the earthy ingredients of the ironstone, forms, with them, a liquid slag, whilst the separated metallic particles, descending slowly through the furnace, are deoxidized and fused; in their passage they imbibe a portion of carbon, and at last settle down in the hearth, from whence they are run off into pigs about every twelve hours; the slag, being lighter, floats upon the surface of the liquid metal, and is constantly flowing out over a notch in the dam-plate, level with the top of the hearth. This slag indicates, by its appearance, the manner in which the furnace is working; thus, if the cinder is liquid, nearly transparent, or of a light greyish colour, and has a fracture like limestone, a favourable state of the furnace is indicated. Tints of blue, yellow, or green are caused by a portion of oxide of iron passing into the slag, and show that the furnace is working cold. The worst appearance of the cinder is, however, a deep brown or black colour, the slag flowing in a broad hot rugged stream, and indicating that the supply of coke is not sufficient to deoxidize the whole of the iron.

During the process of smelting, the interior of the furnace requires to be very carefully watched. The stream of air constantly rushing in at the tuyeres, exerts a chilling agency on the melted matter directly opposed to it at its entrance. The consequence of this is the formation of rude perforated cones of indurated scoriae, stretching from either side horizontally into the furnace, each one having its base directly over the embouchure of a blast-pipe. When these project only to a certain The manufacture of iron.

Extent; they are favourable to the working of the furnace, as the blast is thrown right into the centre, and prevented from passing up the sides and burning the brickwork. Sometimes, however, when the furnace is driving cold and slow, these conduits of slag become so strong, and jut out so far as to meet in the middle, and thus cause a great obstruction to the entrance and ascent of the blast. When this happens, there is usually no remedy but to increase the burden, that is, to increase the quantity of mix or ore to the charge. This causes an intense heat, the furnace is said to work hot, and the conduits of slag drop off from the sides. This, however, is followed by bad as well as good consequences; the brickwork is frequently melted, and, for a time, the iron produced is small in quantity, and of the worst quality. To bring the furnace again to its proper state, the burden must be reduced; the sides then become cool, new tubes of slag are formed, and the iron produced is good.

At the end of every twelve hours, more or less, the furnace is tapped, that is to say, the aperture in the dam-stone, which, at the commencement, had been stopped up with a mixture of loam and sand, is re-opened, and the metal contained in the hearth allowed to flow out into moulds, made in the sand of the cast-house floor, thus forming a cast or sough of pigs. When this operation ceases, the dam-stone is again secured, and the work proceeds as before. In this manner a furnace is kept continually going, night and day, and never ceases to work until repairs are necessary. Incessant action has even been thought necessary to the successful carrying on of an iron-work, but the example of perhaps the largest ironmaster in South Wales has shewn, contrary to general practice in that district, that smelting may be discontinued for at least one day in the week without any very serious derangement of operations.

Thus far we have confined our observations to the production of iron by the cold-blast process; we have now to consider the changes introduced by the employment of a heated blast.

In the year 1828, Mr J. Beaumont Neilson, a practical engineer at Glasgow, took out a patent for an "improved application of air to produce heat in fires, forges, and furnaces, where bellows or other blowing apparatus are required." Mr Neilson proposed to pass the current of air through suitably shaped vessels, where it was to be heated before it entered the furnace. In this simple substitution of a hot-blast, heated in a separate apparatus, for a cold-blast heated in the furnace itself, consists the whole invention.

Like most other improvements, the progress of this was at first slow. Retarded by practical difficulties, which beset all new processes in their first use—stopped every now and then by the prejudices of custom and ignorance, which cling with inveterate tenacity to maxims of established practice, and repel indiscriminately innovations which improve and those which modify without improving,—the invention was more than once on the point of being abandoned. A great part of the interest in its possible remuneration was transferred by the inventor to strangers, whose combined efforts and influence were necessary to insure its success. But though thus tardy in its first steps and feeble in its early efforts, the hot-blast process is now adopted at the greater number of the iron-works of Great Britain, and other parts of Europe and America.

It is perhaps not generally known that practical men, previous to Mr Neilson's invention, universally believed that the colder the blast the better was the quality and quantity of the iron produced; and this opinion appeared to be confirmed by the fact that the furnaces worked better in winter than in summer. Acting on such views, the ironmaster actually resorted to artificial means of refrigeration, to reduce the temperature of the blast before it entered the furnace. The fact of the improved action of the furnace in winter may perhaps be explained as a consequence of the diminished amount of aqueous vapour contained in the atmosphere in cold weather; and the opinion that the low temperature is the cause of the alleged increase of production has been shewn to be wrong by the success of Mr Neilson's invention. This simple invention affects only the transit of the air from the blowing cylinder to the furnace, an oven or stove being interposed, through which, in appropriately shaped vessels, the air is conducted, and in which it is heated to 600° or 800° Fahr., or to any other temperature adapted for the purpose of smelting.

The earliest and simplest plan by which the blast was heated is shown in the sketch, fig. 6 (p. 547). In an oven of brickwork 0000, with a fire-pan by the door D, a large cylindrical tube or receiver h h, made of rivetted boiler-plate, about 3 feet in diameter, and 8 or 10 feet long, was placed. The pipes, B and S, attached to the receiver h h at the opposite ends, communicated with the blowing-cylinder and smelting-furnace respectively. Lunular-shaped partitions p p p, projecting from opposite sides on the interior of the receiver, caused the air passing through it to impinge alternately first on one side and then on the other, in order that the temperature might be uniformly and effectively communicated from the metal to the blast. By this means a moderate current of air has been heated up to 300° or 400° Fahr.

Figs. 7, 8, 9, and 10, show the apparatus first employed, Calder we believe, by Mr Dixon at Calder, and hence generally called the Calder pipes. As erected at the Butterley Iron-Works, Derbyshire, the apparatus consists of two parallel horizontal pipes LL, fig. 7, called technically the "lying pipes," one communicating with the cold-blast inlet pipe B, the other with the hot-blast outlet pipe S, fig. 9. Into sockets formed in these, the ends of the arched heating pipes h h fit tightly, as shown in fig. 7 and in fig. 10, upon a larger scale. The air, therefore, entering the inlet pipe B, figs. 8 and 9, passes over the transverse arched pipes h h, where it is exposed to the action of a large surface of heated metal, and is delivered into the hot-blast pipe S, which conveys it at the required temperature into the blast furnace. The whole apparatus is enclosed in the oven or furnace 0000, as shown in the figs. 7, 8, and 9.

The figures of the transverse pipes vary considerably at different iron-works. Sometimes they rise up and form a large semicircular arch over the fire, 8 or 10 One other form of apparatus, represented in the preceding figures, Nos. 11, 12, and 13, demands notice, on account of its great heating power. The cold air enters by the pipe M into one side of the lying pipe AA, which is divided down the centre by a partition or diaphragm, and then passes up one side of the heating pipes, which are also divided by partitions; it then turns round at the top, as shown at D (fig. 12), and descends in the direction of the arrows into the lying pipe AA on the other side to that which it entered. It is thence conveyed by the arched pipe E, fig. 13, into the second divided pipe BB, through another series of heating pipes, and ultimately escapes by the outlet pipe C, at a high temperature, to the smelting furnace. The diaphragm pipes are, however, not generally used.

The ordinary arrangement is exhibited in the drawing of one of the furnaces and heating apparatus of the Coltness Works, kindly furnished by Mr Hunter, the intelligent managing partner of that establishment. The drawings, figs. 14 and 15, represent a sectional elevation and plan of a very successful and regular working furnace, but the size and form, as already observed, require to be governed by the quality and nature of the materials that are to be used.

The more difficult the reduction of the ironstone the smaller must be the diameter of the hearth, so as to enable the blast to penetrate and circulate throughout the whole of its contents. In other conditions, where the ores are easily reduced, hearths of 9 feet diameter have been introduced with great advantage, and that without detriment to the quality of the iron produced. The diameter of the body of the furnace is likewise regulated by the quality of the materials used, and in cases where the coal is not bituminous and the ore hard, a large diameter is found to work very irregularly; and the results have been, where furnaces have been erected 18 feet diameter, to have them reduced to only 6 feet.

The height of the furnace is also regulated by the nature of the materials and the strength of the blast by which they are reduced. Sometimes, when the coal is soft and crushes by the superincumbent pressure, it is bound or compressed to such an extent as to prevent the blast penetrating the mass, and causes an irregular working of the furnace; and, moreover, under these conditions, it makes what is called white or silvery iron.

The pressure of the blast requires also to be regulated to suit the materials, and, according to the workings at Coltness (shown in figs. 14 and 15), the pressure is about 4 lbs. on the square inch, and as much as 10,000 cubic feet of air is discharged into the furnace per minute. The temperature of the blast is 594° Fahr., and the area of the heating surface of the apparatus for raising that temperature is 3500 square feet.

The quantity of materials to make a ton of iron at these works varies in some relative proportion to their densities; but the following may be taken as a fair average of the consumption of fuel, ore, limestone, &c.:— With the above charges the furnaces will produce from 168 to 170 tons per week, or 8,700 tons of good iron per annum.

Fig. 16 shews the general arrangement and the disposition of a hot-blast furnace, and the apparatus connected with it. J is the blowing cylinder, from which the air is forced into the receiver K, made of wrought-iron boiler plate; from this it passes by the pipe L into the heating ovens, one of which is shewn in section at M, and the pipe N conducts it, when heated, to the furnace. PPP are the tuyeres, FF the charging doors, E the tunnel-head.

With regard to the advantages and defects of the hot-blast process, much has been said on both sides, and the question does not appear by any means exactly settled. It is asserted, on the one hand, that iron reduced by the hot-blast loses much of its strength, whilst, on the other, it is contended that the quality of the iron is richer, more fluid, and better adapted for general purposes than that produced by the cold-blast. The advocates of the hot-blast say that the process has increased the production and diminished the consumption of coal three or four fold; and the upholders of the cold-blast maintain that the same effects may be produced, to almost the same extent, by a judicious proportion of the shape and size of the interior of the furnace, a denser blast, and greater attention on the part of the superintendent to the process.

On these points it appears to us that although the hot-blast has enabled the manufacturer to smelt inferior ores, cinder-heaps, and other improper materials, and to send into the market an inferior description of iron; this is no reason for its rejection, but rather an argument in its favour. It is true that when a strong rigid iron is required for such works as bridges or artillery, the somewhat uncertain character of hot-blast metal renders it objectionable, but this appears to be due rather to the carelessness or want of attention in the manufacture than to the use of heated air or defects in the process. On the other hand, the hot-blast, by maintaining a higher temperature in the furnace, ensures more effectually the combination of the carbon with the iron, and produces a fluid metal of good working qualities, generally superior to cold-blast iron, in cases where great strength is not required; and, moreover, we have yet to learn why even the strongest and most rigid iron cannot be made by this process. The comparative strength of hot and cold blast iron will, however, be given in another part of this article; for the present it is sufficient to observe that the results of the experiments are not unfavourable to the hot-blast. In 1830, the weekly produce of three furnaces, coke, and air at 300° Fahr. being used, was 162 tons 2 cwt.; and the average of coal to one ton of iron was reduced to 5 tons 3 cwt. 1 qr.

In 1833, the weekly produce of four furnaces, near coal, and air heated to 600° being used, was 245 tons; and the average of coal to one ton of iron was reduced to 2 tons, 5 cwt. 1 qr.

"On the whole then, the application of the hot-blast has caused the same fuel to reduce three times as much iron as before, and the same blast twice as much."

This decrease in the amount of fuel and blast required for the reduction of iron, Dr. Clark accounts for by showing, that in an ordinary furnace, "2 cwt. of air a minute or 6 tons an hour are injected into the furnace." This he considers "a tremendous refrigeratory passing through the hottest part of the furnace," and to a great extent repressing the temperature which is necessary for the complete and rapid reduction of the iron.

Mr Truran considers that "writers on the hot-blast have greatly exaggerated the effects of this invention on the iron manufacture of this country. If we are to believe the majority of them, the great reductions which have been effected within the last 25 years, in the quantities of fuel and flux to smelt a given weight of iron, and the large increase of make from the furnaces, is entirely owing to the use of this invention. That the hot-blast, under certain circumstances, has effected a saving in the consumption of fuel, and also augmented the weekly make, we freely admit. But the saving of fuel, and increase of make due to its employment, is not generally one-fourth of the quantity, which writers have asserted." Here Mr Truran is at issue with Dr Clark, and denies the cooling effect of a cold-blast. He attributes the effects of a heated-blast, "first, to the caloric thrown into the furnace along with the blast, enabling a corresponding quantity of coal to be withdrawn from the burden of materials, with a proportionate reduction in the volume of blast, the effects of which are seen in an augmentation of the make, but do not result in any saving of fuel; secondly, to the reduced volume of blast and large proportion of caloric which it carries into the furnace, causing a diminished consumption of fuel in the upper parts of the furnace." Although we do not agree with all Mr Truran's strictures on the hot-blast, the consumption of fuel in the throat is, nevertheless, a question well worthy of investigation. The combustion The manufacture of iron is of course largely increased by the narrow form of throat given to furnaces, which greatly increases the effect of the blast there, and accounts for the difficulty of using those kinds of coal, in the raw state, which splinter if rapidly heated. If Mr. Trurnit's conjectures be correct, and it be found, that by increasing the area of the throat, raw coal and anthracite can be advantageously used with a cold-blast, the superiority of the hot-blast will not be so decidedly marked. This must, however, be determined by practice; as at present, certainly, it is well known that the anthracite and splint coal can be used most effectively and economically with the hot-blast.

We quote from one more authority on this subject. M. Dufrenoy, in his report to the Director-General of Mines in France, states, that upon heating the air proceeding from the blowing cylinder up to $612^\circ$ Fahr., a considerable saving in fuel was effected by the use of raw coal instead of coke, and that this caused no derangement of the working of the furnace or deterioration of the iron produced. On the contrary, "the quality of the metal was improved, and a furnace which, when charged with coke, produced only about half No. 1 and half No. 2 pig-iron, gave a much larger proportion of No. 1 after the substitution of raw coal. Besides this, the quantity of limestone was considerably diminished." This last circumstance, according to M. Dufrenoy, is due to the increased temperature of the furnace, which fuses more readily the earthy matter and other impurities in combination with the ores.

To show the saving effected, M. Dufrenoy gives the quantities used in each of the experiments at the Clyde Iron-Works:

In 1832, the combustion being produced by cold air, the consumption for one ton of iron was—

| Coal—for fusion | Tons. Cwt. | Tons. Cwt. | |----------------|-----------|-----------| | corresponding with | 6 | 13 | | for blowing engine | 1 | 0 |

Total coal used | 7 | 13

Limestone | 0 | 104

In 1831, the furnaces being blown with air heated to $450^\circ$ Fahr.—

| Coal—for fusion | Tons. Cwt. | Tons. Cwt. | |----------------|-----------|-----------| | corresponding with | 4 | 6 | | for the hot air apparatus | 0 | 5 | | for blowing engine | 0 | 7 |

Total coal used | 4 | 18

Limestone | 0 | 9

In July 1833, the temperature of the blast being raised to $612^\circ$ Fahr., and the fusion effected by raw coal, the consumption per ton of iron was—

| Coal—for fusion | Tons. Cwt. | Tons. Cwt. | |----------------|-----------|-----------| | corresponding with | 2 | 0 | | for the hot air apparatus | 0 | 8 | | for blowing engine | 0 | 11 |

Total coal used | 2 | 19

Limestone | 0 | 7

Since that time, the employment of a blast heated to $800^\circ$ or $900^\circ$ has still further increased the weekly production and saving of fuel.

The Waste Gases.—From the description that we have given of the smelting operations, it is evident that a large volume of gaseous products are constantly escaping at the top of the blast-furnace. These are found to contain a large proportion of unconsumed inflammable gas, capable of developing heat, and in countries where fuel is expensive, it is of great importance that these should be applied to useful purposes, and not be wasted in the atmosphere. Various contrivances have been adopted for this purpose, and in some places, particularly on the Continent, they have been utilized with great economy.

To enable the waste gases to be collected and applied to raising steam, heating hot-blast stoves, &c., without detriment to the working of the blast-furnace, it is necessary to withdraw them at an elevation where they have completed their work, yet at such a distance from the mouth of the furnace that they may be extracted in a dry state, and before they come into contact with the atmosphere, so as to cause combustion. This may be effected, either by increasing the height of the blast-furnace, withdrawing a portion of the gases through apertures in the side, or, if the furnace be not too large, by closing the top of the furnace with a moveable door.

Fig. 17 shows the first plan; AA are the apertures through which the gases escape by the chamber BB into the pipe C, which conveys them to the place where they are burnt. The requisite pressure for causing the gases to escape at AA is obtained by heaping the charges of fuel and ore to some height above them. Fig. 18 shows another contrivance for the same purpose; a casting AA in the shape of a truncated cone is fixed at the top of the furnace, the small diameter downwards; the aperture in the bottom of this is closed by another conical casting B, supported by a chain and counterpoise weight; this evidently shuts the mouth of the furnace, and the gases pass off by the pipe C. When a charge is to be thrown in, it is emptied into the cone hopper AA. When the charge is complete, the moveable cone B is lowered so as to enable the charge to pass between it and the edges of the hopper when it is again raised, and the operations of the furnace proceed as before.

In this country sufficient attention has not been paid to this economical practice, as compared with what has been done in other countries where fuel is expensive. It is no excuse that fuel is cheap, as in most cases the gases can be applied with economy, and their combustion tends to abate the serious nuisance of smoke.

The Conversion of Crude into Malleable Iron.

The conversion of the carburised crude iron, obtained converting from the blast furnace, into malleable or wrought iron is operations. The manufacture of iron is effected by several operations of an oxidising character, in which it is sought to separate, in the gaseous state, the carbon contained in the iron, by combining it with oxygen, whilst the other metals alloyed with the iron and the phosphorus pass into the slag.

In reference to subsequent operations, the iron produced in the smelting furnace may be divided into two kinds—that reduced by charcoal and that reduced by coke or raw coal. When charcoal iron has to be converted by charcoal, as in Sweden, it is decarburised in the charcoal refinery, with or without an intervening process. Where coal can be obtained, however, it is now usually converted by the process of puddling. Pig iron produced by coke or coal is converted into malleable iron either by decarburisation in the refinery or oxidising hearth, and subsequent puddling, or it is converted at once in the puddling furnace by the process of boiling, which is equally effective, and is now more generally practised.

This last process, as the one most generally adopted in this country, deserves a special notice, and we are fortunate in having before us the particulars of the manner in which it is conducted by Messrs Rushton and Eckersley of Bolton, kindly furnished by Mr Rushton, the senior partner of the firm. This establishment is probably one of the most modern and complete of the kind in the kingdom; it is one that has spared no expense in the application of useful inventions, and has kept pace with every improvement that has taken place in the manufacture of bar and plate iron for the last fifteen years.

The machinery and appliances at these works consist of:

- 6 Steam engines, of 180 total nominal HP. - 2 Five-ton and 2 fifty-cwt. steam hammers. - 3 Elev. hammers. - 1 Set of heavy iron rolls. - 1 Set of boiler plate rolls. - 1 Merchant train and balling mill. - 16 Puddling furnaces. - 14 Balling and scrap furnaces.

And other machinery, such as plate and bar shears, lathes, &c.

At Messrs Rushton and Eckersley's works, a small proportion of the Cumberland haematite ore is mixed with the crude pig iron to be converted, as it is found to assist in the process of boiling in the puddling furnace, and in other respects to facilitate the process and improve the quality of the iron.

The crude pig iron is assorted according to the degree and uniformity of its carburisation, and classed as Nos. 1, 2, 3, &c.; No. 1 being most highly carburised, No. 2 less so, and so on to No. 4, which contains much more oxygen than the others. The carbon combined with iron gives it fusibility and fluidity, but deprives it of ductility. To render it malleable and capable of being welded, it must be deprived as far as possible of all the extraneous substances which have been mixed with it in the blast furnace, more especially of the carbon. Primâ facie, therefore, it would appear that the highly carburised pig iron is the most suitable for casting, whilst that containing least carbon is best adapted for conversion into malleable iron; hence, in the trade, the crude iron is divided into foundry and forge pigs.

The pigs, however, in which carbon most predominates, and which, as a rule, have been most effectually separated from all other impurities during the process of smelting, are in many respects preferable for the manufacture of wrought iron; up to this time, however, great practical difficulties have attended the decarburisation of iron containing so much carbon, and the white or forge iron is almost always preferred, measures having been taken for depriving it of the metals and earthy impurities not separated in the blast furnace.

With regard to the process of refining, we may observe, that the crude iron is melted in a hollow fire, and partly decarburised by the action of a blast of air forced over its surface by a fan or blowing engine. The carbon having a greater affinity for the oxygen than for the iron, combines with it, and passes off as gaseous carbonic oxide or carbonic acid. During this process, a portion of the silicon, &c., is fused out, and separated from the iron. It is obvious from the above that the iron to be refined, being placed in contact with fuel at a high temperature, is liable to be deteriorated by the admixture of sulphur and other impurities of the fuel; and as the iron is only partially exposed to the action of the blast, the operation is necessarily, under these circumstances, imperfect. From the refinery the metal is run out into large moulds, and is then broken up into what is technically distinguished as "plate metal."

The process of puddling succeeds that of refining; and in this operation the reverberatory furnace is employed, with the fire separated by a partition or bridge from the cesse hearth, on which is placed the metal to be puddled. By this arrangement the flame is conducted over the surface of the metal, creating an intense heat, though the deleterious portions of the fuel cannot mix with the iron. In this furnace the iron is kept in a state of fusion, whilst the workman, called the "puddler," by means of a rake or rabble, agitates the metal so as to expose, as far as he is able, the whole of the charge to the action of the oxygen passing over it from the fire. By this means the carbon is oxidised, and the metal is gradually reduced to a tough, pasty condition, and subsequently to a granular form, somewhat resembling heaps of boiled rice, with the grains greatly enlarged. In this condition of the furnace the cinder or earthy impurities yield to the intense heat, and flow off from the mass over the bottom in a highly fluid state.

The iron at this stage is comparatively pure, and quickly becomes capable of agglutination; the puddler then collects the metallic granules or particles with his rabble, and rolls them together, backwards and forwards, over the furnace bottom, into balls of convenient dimensions (about the size of thirteen-inch shells), when he removes them from the furnace to be subjected to the action of the hammer or mechanical pressure necessary to give to the iron homogeneity and fibre. These processes of refining and puddling have universally been employed till recently; but improvements have rendered it simpler, and the refining process is now very generally abolished.

Shortly after the employment of the puddling process, most of it was found advantageous to mix a portion of crude iron with the refined plate metal, the expense of the puddling process of refining being saved upon the iron used in the crude state; and trusting to the decarburising effects of the puddling furnace, it was found that the refining process might be altogether dispensed with, if the crude iron containing a proportion of oxygen and very little carbon was employed. In this single process it is to be observed, that as all the carbon has to be got rid of in the puddling furnace, the evolution of gas is much more violent, the fluid iron boiling and bubbling energetically during the period of its disengagement, and hence the operation has acquired the popular name of the "boiling" process.

---

1 See Mr. Blackwell's paper, On the Iron Industry of Great Britain, read before the Society of Arts. In this operation the pig iron when melted is more fluid, on account of containing a greater proportion of carbon than the metal from the refinery, and requires more labour in stirring it about and submitting it to the action of the current of air; the process, moreover, is attended by a greater waste of iron than puddling either plate, or crude iron and plate mixed, but not so great a loss as in the two operations of refining and puddling. It must, however, be admitted that the superior fluidity of the iron in the boiling process has a more injurious action on the furnace. Notwithstanding these objections, the system of boiling without the intermediate process of refining has been gaining ground for the last ten years, and in many places has entirely superseded the use of the refinery; recent events have therefore led to the conclusion, that in a short time the refining process will have become a thing of the past.

Numerous attempts have been made to secure a more scientific and perfect decarburisation of the crude iron, but without success. One improvement, however, recently patented by Mr James Nasmyth, gives promise of making the boiling process as nearly perfect as we may hope to see it. It has been in use for two years at the Bolton Iron Works, and from its constant employment in the puddling furnaces of that establishment, it has given direct proof of its utility, and is gradually extending itself among the large manufacturers as its advantages become known.

The invention consists of the introduction of a small quantity of steam at about 5 lbs. pressure per square inch, into the molten metal as soon as it is fused, as the oxygen of the steam has at that high temperature a greater affinity for carbon than for the hydrogen with which it is combined or for the iron, the carbon is rapidly oxidised off. The liberated hydrogen has no affinity for the iron, but unites with sulphur, phosphorus, arsenic, &c.—substances very injurious to the quality of the iron, if present even in minute quantities, and yet frequently found in the ores and fuel.

The mode of operating is as follows:—The steam is conveyed from the boiler to a vertical pipe fixed near the furnace door, having at its lower end a small tap or syphon, to let off the condensed steam, and prevent its being blown into the furnace. A cock with several jointed pieces of pipe are fastened to the flange of the vertical pipe, so as to form, as it were, jointed bracket pipes, somewhat similar to those of gas pipes, which allow free motion in every direction, as in the annexed sketch, in which A, fig. 19, is the reverberatory furnace, a the vertical steam pipe communicating with the boiler, b the tap or steam cock, cc the elbow-jointed tubes, CC the handle, and DD the steam tube or rabble, bent at the end, so as to inject the steam on the liquid metal GG.

This apparatus is introduced into the furnace immediately the iron is melted, the puddler moving it slowly about in the molten iron, while the steam pours upon it through the bent end of the tube. In the course of from five to eight minutes the mass begins to thicken, the steam pipe is withdrawn, and the operation finished in the ordinary way with the common iron rabble. The time saved by this process in every operation or heat, as it is technically called, averages from ten to fifteen minutes, and that during the hottest and most laborious part of the operation.

By means of this apparatus, the highly carburized pig iron, which is the most free from impurities, is rendered malleable in one furnace operation, without the deteriorating adjuncts of the refining and puddling process as ordinarily practised; in this operation no deleterious substance can combine with the iron, whilst in the refinery process the mixture of the fuel and metal is liable to deteriorate the latter with sulphur, silicon, &c. This new process, it is affirmed, has a beneficial effect in purifying the iron with greater economy and rapidity than any other process with which we are acquainted.

Irrespective of the improvements just described, there is another which is extensively used on the Continent, denominated the Silesian gas furnace. For a drawing and explanation of this furnace we are indebted to Mr Anderson, inspector of machinery at the arsenal, Woolwich. The following drawing, fig. 20, will explain the new Silesian furnaces which are used in the manufacture of iron in that country, in place of our reverberatory air furnaces, and are said, on good authority, to be a very great improvement, not only in regard to the entire prevention of smoke and the economy of fuel, but also in simplifying the wrought-iron manufacture, and enabling a less skilled class of workmen to manage the furnaces.

The chief feature is the gas generator, which may be described as a close brick chamber with an opening at the bottom for the admission of air from a fan, by means of which the gases are driven out of the chamber into the furnace amongst the iron to be heated. At the point where the gases enter the furnace, a series of tuyeres are provided for the admission of air from the same fan. The pipes that convey the air and the gas from the retort to the tuyeres are both provided with valves, in order that the attendant may modify the quantity from either source, so as to produce any intensity of flame the work may require, and also to produce perfect combustion, thus placing the entire action of the furnace under complete control. It is about eleven years since these furnaces were first introduced, and notwithstanding the prejudices that were naturally raised against them, they are said to be now extensively adopted in the Silesian district, and in great favour with both the master and the workmen.

In this description of furnace there appear to be three great advantages over the air furnace—

1st. The entire absence of smoke, in consequence of complete combustion.

2d. The saving of upwards of 33 per cent in fuel, from the whole of the gaseous products being made available, and there being no necessity for the flame to pass up the chimney to produce draught, as in the case of the reverberatory furnace, which requires an inordinate supply of fuel as compared with what is wanted to work the fan.

3d. The absolute control the attendant has over the The manufacture of iron.

The furnace, as regards the temperature and the simplicity with which it can be worked. Its operations in this respect are, according to those who have seen it at work, so perfect as to be as precise in its action as a machine.

From this description it would appear that the iron-masters of this country have not made themselves acquainted with these improvements, but having some knowledge of the efficiency and existence of this process,

we would earnestly recommend it to their attention, as an invention in more respects than one entitled to consideration.

Since the above was written, an apparently new light has been thrown on the conversion of iron, by a paper read by Mr H. Bessemer at the last meeting of the British Association for the advancement of science, held at Cheltenham in August last (1856). In this paper the author announces to the world the discovery of an entirely new system of operations for the manufacture of malleable iron and steel. The crude metal is converted, by one simple process, directly as it comes from the blast furnace. We should detract from its clearness did we attempt to curtail the lucid description in which Mr Bessemer has recommended his invention to the manufacturers and the public; we therefore give the account in his own words:

Mr Bessemer states that "for the last two years his attention has been almost exclusively directed to the manufacture of malleable iron and steel, in which, however, he had made but little progress until within the last eight or nine months. The constant pulling down and rebuilding of furnaces, and the toil of daily experiments with large charges of iron, had begun to exhaust his patience, but the numerous observations he had made during this very unpromising period, all tended to confirm an entirely new view of the subject, which at that time forced itself upon his attention, viz.—that he could produce a much more intense heat, without any furnace or fuel, than could be obtained by either of the modifications he had used, and consequently, that he should not only avoid the injurious action of mineral fuel on the iron under operation, but that he would, at the same time, avoid also the expense of the fuel. Some preliminary trials were made on from 10 lbs. to 20 lbs. of iron, and although the process was fraught with considerable difficulty, it exhibited such unmistakable signs of success, as to induce him at once to put up an apparatus capable of converting about 7 cwt. of crude pig iron into malleable iron in thirty minutes. With such masses of metal to operate on, the difficulties which beset the small laboratory experiments of 10 lbs. entirely disappeared. On this new field of inquiry, he set out with the assumption that crude iron contains about 5 per cent. of carbon; that carbon cannot exist at a white heat in the presence of oxygen without uniting therewith, and producing combustion; that such combustion would proceed with a rapidity dependent on the amount of surface of carbon exposed; and, lastly, that the temperature which the metal would acquire would be also dependent on the rapidity with which the oxygen and carbon were made to combine, and consequently, that it was only necessary to bring the oxygen and carbon together in such a manner that a vast surface should be exposed to their mutual action, in order to produce a temperature hitherto unattainable in our largest furnaces. With a view of testing practically this theory, he constructed a cylindrical vessel of three feet in diameter, and five feet in height, somewhat like an ordinary cupola furnace, the interior of which was lined with fire-bricks, and at about two inches from the bottom of it he inserted five tuyere pipes, the nozzles of which were formed of well-burnt fire clay, the orifice of each tuyere being about three-eighths of an inch in diameter; they were put into the brick lining from the outside, so as to admit of their removal and renewal in a few minutes, when they were worn out. At one side of the vessel, about half way up from the bottom, there was a hole made for running in the crude metal, and in the opposite side was a taphole, stopped with loam, by means of which the iron was run out at the end of the process. In practice, this converting vessel may be made of any convenient size, but he prefers that it should not hold less than one or more than five tons of fluid iron at each charge. The vessel should be placed so near to the blast furnace as to allow the iron to flow along a gutter into it; a small blast cylinder is required, capable of compressing air to about 8 lbs. or 10 lbs. per square inch. A communication having been made between it and the tuyeres before mentioned, the converting vessel will be in a condition to commence work; it will, however, on the occasion of its first being used, after relining with fire-bricks, be necessary to make a fire in the interior with a few baskets of coke, so as to dry the brickwork. The manufacture of iron.

and heat up the vessel for the first operation, after which the fire is to be carefully raked out at the tapping hole, which is again to be made good with loam. The vessel will then be in readiness to commence work, and may be so continued until the brick lining, in the course of time, is worn away, and a new lining is required. The tuyeres, as before stated, were situated nearly close to the bottom of the vessel, the fluid metal therefore rose some eighteen inches or two feet above them. It was therefore necessary, in order to prevent the metal from entering the tuyere holes, to turn on the blast before allowing the fluid crude iron to run into the vessel from the blast furnace. This having been done, and the fluid iron run in, a rapid boiling up of the metal was heard going on within the vessel, the iron being tossed violently about, and dashed from side to side, shaking the vessel by the force with which it moved. Flame, accompanied by a few bright sparks, immediately issued from the throat of the converting vessel. This state of things lasted for about fifteen or twenty minutes, during which time the oxygen in the atmospheric air combined with the carbon contained in the iron, producing carbonic acid gas, and at the same time evolving a powerful heat. Now as this heat is generated in the interior of, and is diffusive in innumerable fiery bubbles throughout the whole fluid mass, the vessel absorbs the greater part of it, and its temperature becomes immensely increased, and by the expiration of the fifteen or twenty minutes before named, that part of the carbon which appears mechanically mixed and diffused through the crude iron has been entirely consumed. The temperature, however, is so high that the chemically combined carbon now begins to separate from the metal, as is at once indicated by an immense increase in the volume of flame rushing out of the throat of the vessel. The metal in the vessel now rises several inches above its natural level, and a light frothy slag makes its appearance, and is thrown out in large foam-like masses. This violent eruption of cinder generally lasts about five or six minutes, when all further appearance of it ceases, a steady and powerful flame replacing the shower of sparks and cinders which always accompanies the boil. The rapid union of carbon and oxygen which thus takes place, adds still further to the temperature of the metal, while the diminished quantity of carbon present allows a part of the oxygen to combine with the iron which undergoes a combustion, and is converted into an oxide. At the excessive temperature that the metal has now acquired, the oxide, as soon as formed, undergoes fusion, and forms a powerful solvent of those earthy bases that are associated with the iron. The violent ebullition which is going on mixes most intimately the scoriae and metal, every part of which is thus brought into contact with the fluid oxide, which will thus wash and cleanse the metal most thoroughly from the silica and other earthy bases, which are combined with the crude iron, while the sulphur and other volatile matters, which cling so tenaciously to iron at ordinary temperatures, are driven off, the sulphur combining with the oxygen, and forms sulphurous acid gas. The loss in weight of crude iron during its conversion into an ingot of malleable iron, was found, on a mean of four experiments, to be $12\frac{1}{2}$ per cent, to which will have to be added the loss of metal in the finishing rolls. This will make the entire loss probably not less than 18 per cent instead of about 25 per cent which is the loss on the present system. A large portion of this metal is, however, recoverable by treating with carbonaceous gases the rich oxides thrown out of the furnace during the boil. These slags are found to contain innumerable small grains of metallic iron, which are mechanically held in suspension in the slags, and may be easily recovered. It has already been stated that after the boil has taken place, a steady and powerful flame succeeds, which continues without any change for about ten minutes, when it rapidly falls off. As soon as this diminution of flame is apparent, the workman knows that the process is completed, and that the crude iron has been converted into pure malleable iron, which he will form into ingots of any suitable size and shape, by simply opening the tap hole of the converting vessel, and allowing the fluid malleable iron to flow into the iron ingot moulds placed there to receive it. The masses of iron thus formed will be perfectly free from any admixture of cinder oxide, or other extraneous matters, and will be far more pure, and in a forwarder state of manufacture, than a pile formed of ordinary puddle bars. And thus, by a single process, requiring no manipulation or particular skill; and with only one workman, from three to five tons of crude iron passes into the condition of several piles of malleable iron, in from thirty to thirty-five minutes, with the expenditure of about one-third part the blast now used in a finery furnace with an equal charge of iron, and with the consumption of no other fuel than is contained in the crude iron. To those who are best acquainted with the nature of fluid iron, it may be a matter of surprise that a blast of cold air forced into melted crude iron is capable of raising its temperature to such a degree as to retain it in a perfect state of fluidity, after it has lost all its carbon, and is in the condition of malleable iron, which, in the highest heat of our forges, only becomes a pasty mass. But such is the excessive temperature that may be arrived at, with a properly shaped converting vessel, and a judicious distribution of the blast, that not only may the fluidity of the metal be retained, but so much surplus heat can be created as to remelt the crop ends, ingot, runners, and other scrap, that is made throughout the process, and thus bring them, without labour or fuel, into ingots of a quality equal to the rest of the charge of new metal. For this purpose, a small arched chamber is formed immediately over the throat of the converting vessel, somewhat like the tunnel-head of the blast furnace. This chamber has two or more openings in the sides of it, and its floor is made to slope downwards to the throat. As soon as a charge of fluid malleable iron has been drawn off from the converting vessel, the workman will take the scrap intended to be worked into the next charge, and proceed to introduce the several pieces into the small chamber, piling them up round the opening of the throat. When this is done, he will run in his charge of crude metal, and again commence the process. By the time the boil commences, the bars ends or other scrap will have acquired a white heat, and by the time it is over, most of them will have melted and run down into the charge. Any pieces, however, that remain, may then be pushed in by the workman, and by the time the process is completed, they will all be melted and intimately combined with the rest of the charge; so that all scrap iron, whether cast or malleable, may thus be used up without any loss or expense. As an example of the power that iron has of generating heat in this process, Mr Bessemer mentions that when trying how small a set of tuyeres could be used, the size he had chosen proved too small, and after blowing into the metal for one hour and three-quarters, he could not get up heat enough with them to bring on the boil. The experiment was therefore discontinued, during which time two-thirds of the metal solidified, and the rest was run off. A larger set of tuyere pipes were then put in, and a fresh charge of fluid iron run into the vessel, which had the effect of entirely remelting the former charge; and when To persons conversant with the manufacture of iron, it will be at once apparent that the ingots of malleable metal which are produced by this process, will have no hard or steely parts, such as are found in puddled iron, requiring a great amount of rolling to blend them with the general mass, nor will such ingots require an excess of rolling to expel the cinder from the interior of the mass, since none can exist in the ingot, which is pure and perfectly homogeneous throughout, and hence requires only as much rolling as is necessary for the development of fibre; it therefore follows that instead of forming a merchant bar or rail by the union of a number of separate pieces welded together, it will be far more simple, and less expensive, to make several bars or rails from a single ingot; doubtless this would have been done long ago had not the whole process been limited by the size of the ball which the puddler could make.

The facility which the new process affords, of making large masses, will enable the manufacturer to produce bars that, on the old mode of working, it was impossible to obtain; while, at the same time, it admits of the use of some powerful machinery, whereby a great deal of labour will be saved, and the process be greatly expedited. Mr Bessemer merely mentions this in passing, without entering into details, as the patents he has obtained for improvements in this branch of the manufacture are not yet specified. He next points out the perfectly homogeneous character of cast-steel—its freedom from sand cracks and flaws—and its greater cohesive force and elasticity, compared with the blister steel from which it is made, qualities which it derives solely from its fusion and formation into ingots—all of which properties malleable iron acquires in like manner, by its fusion and formation into ingots in the new process. Nor must it be forgotten that no amount of rolling will give to blistered steel (although formed of rolled bars) the same homogeneous character that cast-steel acquires, by a mere extension of the ingot to some ten or twelve times its original length.

One of the most important facts connected with the new system of manufacturing malleable iron is, that all the iron so produced will be of the quality known as charcoal iron, not that any charcoal is used in its manufacture, but because the whole of the processes following the smelting of it are conducted entirely without contact with, or the use of any mineral fuel; the iron resulting therefrom will, in consequence, be perfectly free from those injurious properties which that description of fuel never fails to impart to iron that is brought under its influence. At the same time, this system of manufacturing malleable iron offers extraordinary facility for making large shafts, cranks, and other heavy masses; it will be obvious that any weight of metal that can be founded in ordinary cast-iron, by the means at present at our disposal, may also be founded in molten malleable iron, and be wrought into the forms and shapes required, provided that we increase the size and power of our machinery to the extent necessary to deal with such large masses of metal. A few minutes' reflection will shew the great anomaly presented by the scale on which the processes of iron-making are at present carried on. The little furnaces originally used for smelting ore have, from time to time, increased in size, until they have assumed colossal proportions, and are made to operate on 200 or 300 tons of material at a time, giving out 10 tons of fluid metal at a single run. The manufacturer has thus gone on increasing the size of his smelting furnaces, adapting to their use the blast apparatus of the requisite proportions, and has by this means lessened the cost of production, in every way ensuring a cheapness and uniformity of production, that could never have been secured by a multiplicity of small furnaces. While the manufacturer has shewn himself fully alive to these advantages, he has still been under the necessity of leaving the succeeding operations to be carried out on a scale wholly at variance with the principles he has found so advantageous in the smelting department. It is true that, hitherto, no better method was known than the puddling process, in which from 4 cwt. to 5 cwt. of iron is all that can be operated upon at a time, and even this small quantity is divided into homoeopathic doses of some 70 lbs. or 80 lbs., each of which is moulded and fashioned by human labour, carefully watched and tended in the furnace, and removed therefrom, one at a time, to be carefully manipulated and squeezed into form. The vast extent of the manufacture, and the gigantic scale on which the early stages of its progress is conducted, it is astonishing that no effort should have been made to raise the after processes somewhat nearer to a level commensurate with the preceding ones, and thus rescue the trade from the trammels which have so long surrounded it. Mr Bessemer then adverts to another important feature of the new process, the production of what he calls semi-steel. At the stage of the process immediately following the boil, the whole of the crude iron has passed into the condition of cast-steel of ordinary quality; by the continuation of the process the steel so produced gradually loses its small remaining portion of carbon, and passes successively from hard to soft steel, and from softened steel to steely iron, and eventually to very soft iron; hence, at a certain period of the process, any quality may be obtained; there is one in particular, which, by way of distinction, he calls semi-steel, being in hardness about midway between ordinary cast-steel and soft malleable iron. This metal possesses the advantage of much greater tensile strength than soft iron; it is also more elastic, and does not readily take a permanent set, while it is much harder, and is not worn or indented so easily as soft iron. At the same time it is not so brittle or hard to work as ordinary cast-steel. These qualities render it eminently well adapted to purposes where lightness and strength are specially required, or where there is much wear, as in the case of railway bars, which, from their softness and lamellar texture, soon become destroyed. The cost of semi-steel will be a fraction less than iron, because the loss of metal that takes place by oxidation in the converting vessel is about $\frac{2}{3}$ per cent, less than it is with iron, but as it is a little more difficult to roll, its cost per ton may fairly be considered to be the same as iron, but as its tensile strength is some thirty or forty per cent greater than bar iron, it follows that for most purposes a much less weight of metal may be used, so that taken in that way the semi-steel will form a much cheaper metal than any we are at present acquainted with.

In conclusion, Mr Bessemer observes that the facts he has discovered have not been elicited by mere laboratory experiments, but have been the result of operations on a scale nearly twice as great as is pursued in the largest iron-works, the experimental apparatus converting 7 cwt. in thirty minutes, while the ordinary puddling furnace makes only $4\frac{1}{2}$ cwt. in two hours, which is made into six separate balls; while the ingots or blooms are smooth, even prisms ten inches square by thirty inches in length, weighing about as much as ten ordinary puddle balls." (See Addenda, E). MACHINERY OF THE MANUFACTURE.

The mechanical operations connected with the manufacture of wrought-iron consist of shingling, hammering, rolling, &c., to which we may add the forging of "noses," that is, the forging of those peculiar forms so extensively in demand for steam-engines, railway carriages, and other works, which has lately become a large and important branch of trade.

In tracing the whole of the processes in the manufacture of wrought iron bars and plates, it will not be necessary to enlarge on those practices which have been superseded by more modern and improved machinery. Suffice it then to observe, that formerly the puddled balls or plates, &c., were shingled or fashioned into oblong slabs or blooms by the blows of a heavy forge hammer; during this operation, the scoriae and impurities which adhered to the balls were separated from the blooms by the force of impact, and then by a series of blows the iron was rendered malleable, dense, and compact. The blooms were then passed through a series of grooved iron rollers, which reduced them to the form of long, slender iron bars. These were cut up and piled regularly together or forged, and brought to a welding heat in the heating or balling furnace, when they were again passed several times through grooved rollers, and by this latter process were made into bars or plates ready for the shears.

In order to arrive at a clear conception of the mechanical operations employed in the manufacture of iron, it will be necessary to describe more at length the processes as at present practised, with the improved and powerful machinery now employed; and as much depends upon the application of the motive power, the steam-engine claims the first notice. Until of late years, the vertical steam-engine was invariably used for giving motion to the forge hammer and rolling mill, which were placed on one side of the fly-wheel and the crank on the other; but the high-pressure, non-condensing engine is found to be decidedly preferable, as the waste heat passing off with the products of combustion from the puddling and heating furnaces, is quite sufficient to raise the steam for working the rolls and one of Brown's bloom squeezers, as shown in the following drawing.

In this arrangement the cylinder A (figs. 21, 22, 23), is placed horizontally, and is supplied with steam from boilers near the puddling furnaces. The piston and slides B, and connecting rod C, give motion to the crank shaft D, on which is fixed a heavy fly-wheel E. The puddling rolls F are driven direct from the end of the fly-wheel shaft, and the bloom squeezers H, by a train of spur wheels GG. Under the lower rolls of the squeezers a Jacob's ladder or elevator I is fixed, for raising the block which has been deprived of its impurities, and reduced to an oblong shape by passing between the rollers of the squeezer. The block, on leaving the rollers, is carried in front of one of the projecting divisions of the ladder and thrown on to the platform in front of the rolls; The puddled iron, K, thrown on the top, is gradually compressed between the revolving rollers as it descends and at last emerges at the bottom, where it is thrown onto the moveable "Jacob's ladder," by which it is elevated to the rolls, as already described. This machine effects a considerable saving of time; will do the work of 12 or 14 furnaces, and may be kept constantly going as a feeder to one or two pairs of rolls. There are two distinct forms of this machine, one as shown in fig. 24, where the bloom receives only two compressions; and the other, which is much more effective, where it is squeezed four times before it leaves the rolls and falls upon the Jacob's ladder, as exhibited in figs. 25, and 26.

There are two other machines for preparing the blooms horizontally by compression. One is a table firmly embedded in masonry, as shown at AA, in fig. 27, with a ledge rising up from it to a height of about two feet, so as to form an open box. Within this is a revolving box C, of a similar character, much smaller than the last and placed eccentrically in regard to it. The ball or bloom D is placed between the innermost revolving box C and the outer case AA, where the space between them is greatest, and is carried round till it emerges at E, compressed and fit for the rolls.

Another instrument, fig. 28, used for the same purpose, acts as a pair of pliers, and squeezes the iron between two flat blades AA. This machine is called the Alligator, and is probably more effective than the horizontal machine, but it requires an attendant to keep the bloom rolling about under the jaws AA, and is, in other respects, inferior to Brown's patent squeezer.

We have stated that the horizontal, non-condensing steam-engine, from its compact form and convenience of tongs of handling, is admirably adapted for giving motion to the horizontal machinery of iron-works. For this object, it is superior engine to the beam-engine, as its speed can be regulated with the The manufacture of iron requires the greatest nicety, by opening or shutting the valve, so as to suit all the requirements of the manufacture, under the varied conditions of the pressure of the steam, and the power required for rolling heavy plates and bars, or those of a lighter description. It is also much cheaper in its original cost, and all its parts being fixed upon a

Fig. 25.

Fig. 26.

large bed-plate, require a comparatively small amount of masonry to render it solid and secure.

In regard to the manufacture of the rollers for the puddling, boiler-plate, and merchant train, the greatest care must be observed in the selection of the iron and the mode of casting. In Staffordshire there are roller-makers, but in general the manufacturer casts his own, and as much depends upon the metal, the strongest qualities are carefully selected and mixed with Welsh No. 1 or No. 2, and Staffordshire No. 2. This latter description of iron, when duly prepared, exhibits great tenacity, and is well adapted, either in the first or second melting, for such a purpose. In casting, the moulds are prepared in loam, and when dry are sunk vertically into the pit to a depth of about 5 feet below the floor. The moulding box is surrounded by sand firmly consolidated by beaters, and a second mould or head is placed above it, which receives an additional quantity of iron to supply the space left by shrinking, and keep the roll under pressure until it solidifies, and thus secures a great uniformity and density in the roller. The metal is run into the mould direct from the air furnace by channels cut in the sand, and immediately the mould is filled, the workman agitates the metal with a rod, in order to consolidate the mass and get rid of any air or gas which may be confined in the metal. This stirring with iron rods is continued till the metal cools to a semi-fluid state, when it is covered up and allowed slowly to cool and crystallize. This slow rate of cooling is necessary to favour an uniform degree of contraction, as the exterior closes up like a series of hoops round the core of the casting, which is always the most porous and the last to cool. In every casting of this kind it is essential to avoid unequal contraction, and this cannot be accomplished unless time is given for the arrangement of the particles by a slow process of crystallization. Rollers for The manufacture of boiler-plates and thin sheet-iron are difficult to cast sound iron. They are subjected to very great strain, and require to be cast from the most tenacious metals. The bearings or neck should be enlarged, or turned to the shape shewn at AA, and the cylindrical part B should be slightly concave, because, when the slab is first passed through the rollers, it comes in contact with a small portion only of the revolving surface. The central parts of the roller thus become highly heated, whilst their extremities are perfectly cool; the consequence is, that the expansion of the roller is greatest in the middle, and unless this be provided for by a concavity in the barrel, the plates become buckled, that is, both warped and uneven in thickness, and consequently imperfect and unfit for the purposes of boiler making. Bar rolls are generally cast in chill, and great care is required to prevent the chill penetrating too deep so as to injure the tenacity of the metal and render it brittle.

There are different kinds of rolling mills used in the iron manufacture, and they vary considerably in their dimensions according to the work they have to perform. The first, through which the puddled iron is passed, we have already described as puddling rolls. There are others for roughing down which vary from 4 to 5 feet long, and are about 18 inches diameter; those for merchant bars, about 2 feet 6 inches to 3 feet long and 18 inches in diameter, are in constant use. The boiler-plate and black sheet-iron rolls are generally of large dimensions; some of them for large plates are upwards of 6 feet long and 18 to 21 inches in diameter; these require a powerful engine and the momentum of a large fly-wheel to carry the plate through the rollers, and not unfrequently when thin wide plates have to be rolled, the two combined prove unequal to the task, and the result is, the plates cool and stick fast in the middle. The greatest care is necessary in rolling plates of this kind, as any neglect of the speed of the engine or the setting of the rolls results in the breakage of the latter, or bringing the former to a complete stand.

The speed of the different kinds of rolling mills varies according to the work they have to perform. Those for merchant bars make from 60 to 70 revolutions a minute, whilst those of large size for boiler-plates are reduced to 28 or 30. Others, such as the finishing and guide rollers, run at from 120 to 400 revolutions a minute. In Staffordshire, where some of the finer kinds of iron are prepared for the manufacture of wire, the rollers are generally made of cast-steel, and run at a high velocity; such is the ductility of this description of iron, that in passing through a succession of rollers, it will have elongated to 10 or 15 times its original length, and when completely finished, will have assumed the form of a strong wire \( \frac{3}{8} \) to \( \frac{1}{4} \) of an inch in diameter, and 40 to 50 feet in length.

A high temperature is an indispensable condition of success in rolling. The experience of the workman enables him to judge, from the appearance of the furnace, when the pile is at a welding heat, so that when compressed in the rolls the particles will unite. Sometimes it is necessary to give a fine polish or skin to the iron as it leaves the rolls, but this can only be done when the iron cools down to a dark-red colour, and by the practised eye of an intelligent workman.

The above operations would still be incomplete unless the ironmaster had means of cutting the bars and plates to any required size and shape. The machinery for this purpose has of late been brought to a high degree of perfection, both in regard to power and precision.

The circular saw has been successfully applied for squaring and cutting the larger descriptions of bars, and does its work, particularly in railway bars, with almost mathematical precision. This machine consists of a cast-iron frame or bed AA, fig. 29, bolted down to a solid foundation, on the ends of which slide two frames BB to support the bar to be cut. The two circular saws or cutters CC are driven by straps passing over the pulleys DE, and rotate at the rate of 800 to 1000 revolutions per minute. The machine is set in motion by transferring the straps from the loose pulley D to the fast pulley E, and as soon as the required speed is attained, the frame BB is carried forward, and the bar FF along with it, by a lever G or eccentric motion, till the bar is cut through. The rate of cutting or pressure upon the saws may be regulated either by hand or weight; care must, however, be taken not to allow the saws to become too hot, and this is provided against by running them in a trough of water. By this process it is evident that the bar must always be cut square at the ends and correctly to the same length. (See Addenda, F).

A great variety of shears are used for cutting iron. Some driven by cams or eccentrics, and some by connecting-rods and a crank on the revolving shaft. In large iron-works it is necessary to have two or three kinds, some for cutting up scrap iron and bars for piling, and others for boiler-plates. Of the first we may notice two, one shewn in fig. 30, cuts on both sides at AA, and is driven by a crank and connecting-rod B. This machine is chiefly used for cutting puddled bars from the puddling rolls, or any work required for shingling. The next machine, fig. 31, receives motion in the same manner, and also cuts on both sides, the cutters being fixed on the lever and moving with it. This is used for the same purpose as the last, and likewise for cutting scrap iron. These machines are extensively used in the manufacture of iron, and before the introduction of the plate shears, they were used, with some modifications, to cut boiler-plates, but the work was very imperfectly executed.

The demand for plates of large dimensions and greatly increased weight, such as those for the front and tube plates of locomotive and marine boilers, and those for tubular and plate bridges, created great difficulties, not only in piling, heating, and rolling, but also in cutting the plates accurately to the required size. To meet these demands, and more particularly for the manufacture of the large plates employed in the cellular top of the Britannia and Conway tubular bridges, Messrs G. B. Thorneycroft and Co. constructed a large shearing machine which cut upwards of 10 feet at one stroke. These shears have now come into general use, and are of great importance, on account of the accuracy with which they cut plates of large dimensions, square and even. Figs. 32 and 33 represent this machine; \(a a a\) is the standard and table on which the plate is fixed. This table slides forward at right angles to the shears or cutters \(A A A^* A^*\). The top cutter descending by the action of three eccentrics \(c c c\), which press upon the top of the frame \(B\) as it revolves, and force it down, and by one stroke, the knife \(A A\) cuts through the whole length of the plate, perfectly clean and straight. The plate is then reversed, the newly cut edge being held against the slopes, and the sliding frame again moved forward to the required width of the plate, when another stroke cuts the other side as before. The rapidity with which the plates are cut is another advantage of this machine, as great as the precision of its cut, and when the immense quantity of plates daily produced at Messrs Thorneycroft and Co.'s works are considered, its importance becomes evident.

At the Paris Universal Exhibition of last year (1855), a plate-cutting machine was exhibited, from the United States of America, which appears to effect the same operation as Messrs Thorneycroft and Co.'s. It consists of a strong cast-iron frame, nine or ten feet wide, having inserted along its face a steel plate, on which the iron to be cut rests and is held firmly by a faller, which descends on the upper side of the plate. On the same side of the frame a revolving steel cutter, about nine inches in diameter, traverses the whole length of the frame, and in its passage cuts the plate, by compression, in a perfectly straight line, corresponding with the steel edge below. Cutting and shaving plates by a revolving disc has been long in use, but the traversing motion in this machine is certainly new, and its application very creditable to the ingenuity of the inventor. The travelling cutter, which requires great power when cutting thick plates, is driven by a strap over a pulley at one end of the machine, and looking at the work it has to perform and the complexity of its parts, we should consider it less effective and more liable to derangement than the simple and powerful machine of Messrs Thorneycroft.

Having thus traced the processes for the conversion of crude into malleable iron, and the machinery employed, it only remains to give a general summary of the whole. As regards the arrangement of large iron-works, the general principle should be for the machinery to be classed and fixed in the order of the different processes, so that the products of one machine should pass at once to the next, and, in fact, the crude iron should be received at one end, and having passed through all the processes, delivered at the other in the manufactured state.

The crude iron from the smelting furnace is either refined and puddled, or subjected to the boiling process, to get rid of the combined carbon and render the iron malleable; it is then shingled by the forge hammer, by the "alligator," by Brown's squeezer, or by the other machines which have been invented for this purpose. It is then at once passed through the puddling rolls, where it is reduced to the form of a flat bar, and is then cut into convenient lengths by the shears. These pieces are again piled or faggoted together into convenient heaps and re-heated in the furnace. As soon as a fagot thus prepared has been heated to the welding temperature, it is passed through the roughing rolls to reduce it to the form of a bar, and then through the finish- ing rolls, where the required form and size is given to it, either round or square bars, &c. These are straightened and cut to the required sizes, and are then ready for delivery. In most large works all these operations are carried on simultaneously with the smelting process, and in some with extensive mining operations for procuring the coal, ore, and limestone required to supply a production of several thousand tons of manufactured iron per month.

THE FORGE.

The forging of iron has entered, of late years, so largely into the constructive arts, that the manufactures, however perfect in the rolling-mill, would be very imperfect indeed without the forge. To the discussion of this part of the subject there are many inducements, and we cannot but wonder at the many devices, and the numerous contrivances which present themselves for the attainment of the operations of the forge. In effecting these objects, Mr Nasmyth's steam-hammer is evidently the most effective, and to that instrument we are indebted for the formation and welding of iron upon a scale previously unknown to the workers in that metal.

Mr Nasmyth took out his patent for this invention and heavy blows, operates severely upon the piston-rod. The hammer-block FF is guided in its vertical descent by two planed guides or projections, extending the length of the side-standards AA, between which the hammer-block slides. The attendant gives motion to the hammer by admitting steam from the boiler to act upon the under side of the piston, by moving the regulator I by the handle d. The length of stroke is regulated by increasing or diminishing the distance between the cam N and the valve lever O o, by turning the screws P and U by the bevil wheels q q. The lever O o operates by the cam N coming in contact with the roller o. As soon as this contact takes place, the further admission of steam is not only arrested, but its escape is at the same time effected, and the hammer, left unsupported, descends by its gravity upon the work on the anvil with an energy due to the height of the fall. From this description, it will be seen that the movement of the roller o causes the shoulder of the rod P to get under the point of the trigger catch U; the valve is by these means kept closed till the whole force of the blow is struck. The instant the operation is effected, the concussion of the hammer causes the latch X to knock off the point of the trigger from the shoulder on the valve-rod P, by means of the bent lever s r, and the instant this is accomplished, the valve is re-opened to admit the steam below the piston, by the pressure of steam on the upper side of the small piston in the cylinder M, forcing down the valve rod, which, in this respect, is the active agent for opening the valve.

To arrest the motion of the hammer, it is only necessary to shut the steam-valve; during the process of forging, it is, however, desirable to give time between the blows, to enable the workman to turn and shift his work on the anvil; and to effect this reduced motion, the trigger U is held back from the shoulder of the valve-rod P, by the handle y, which at the same instant opens the valve in the case J, and thus the action of the steam in the cylinder D retards the downward motion of the hammer. The result of these changes is an easy descent of the hammer, which vibrates up and down without touching the anvil, but ready for blows of any severity the instant the trigger is elevated above the shoulder of the valve lever P. From this description, it will appear evident that Mr Nasmyth's invention is one of the most important that has occurred in the art of forging iron. It has given an impetus to the manufacture, and affords facilities for the welding of large blocks of malleable iron that could not be accomplished by the tilt and helve hammers formerly in use; and we have only to instance the forging of the stern-posts and cutwaters of iron ships; the paddle-wheel and screw-shafts of our ocean steamers, some of them weighing upwards of 20 tons, to appreciate the value, as well as the intensity of action of the steam-hammer.

In addition to the machinery of the forge, the V anvil, the natural offspring of the steam-hammer, came into existence from the same fertile source. It is chiefly employed for forging round bars and shafts, and may be thus described, A being a section of the round bar or shaft to be forged, B the anvil-block, and C the hammer. From this, it is obvious that, in place of the old plan, where the work is forged upon flat surfaces, as shewn in the annexed figure, and where the blows are diverging, the effect of the V anvil is a converging action, thus consolidating the mass, and enabling the forger to retain his work directly under the centre of the hammer. This is the more strikingly apparent, as the blows of the hammer upon a round shaft have the effect of causing the mass to assume the elliptical form, forcing out the sides as at AA in every successive blow, and this again, when turned, produces a spongy, porous centre, as shewn in fig. 38. This process is, however, more clearly exemplified in Ryder's forging machines, where all the anvils are of the V form, for the forging of spindles, round bars, and bolts.

The next important discovery in the art of forging, is that of Mr Ryder's machine, patented some years since, for forging small articles, which, on account of the rapidity and precision of its operations, demands a notice in passing. It consisted essentially of a series of small anvils about three inches square, supported from below by large screws passing through the frame of the machine. This screw was employed in order that the distance between the hammer and anvil might be accurately adjusted. Between the screw and the anvil, a stuffing of cork was introduced to deaden the effect of the blow. The hammers were arranged over the anvils, and slid up and down in the frame of the machine. The blow was effected by the revolution of an eccentric, acting... The manufacture of iron by means of a cradle on the hammer-head, the hammer, however, being lifted again by a strong spiral spring. The hammers made about 700 strokes a-minute. At the side of the machine was a cutter or shears worked by a long lever; with this the articles were cut to the required length as they were finished.

Figs. 39 and 40 represent this machine as improved by Messrs Platt Brothers of Oldham. A A A A are the anvils supported on a wedge B, instead of the screw, as in Ryder's. This substitution was made because the blows of the hammer tore off the threads of the screw, and the machine soon got out of order. The distance between the hammer and anvil is regulated by forcing forwards the wedge B by the rack and pinion C. The cork was then found insufficient as a stuffing, and an immensely strong spring D was substituted; this spring is formed of a band of steel 1\(\frac{3}{4}\) inches broad, and \(\frac{5}{8}\) thick, coiled in a close spiral 2 inches in diameter, and 6\(\frac{1}{2}\) long; it answers its purpose admirably. The hammers are shewn at HHHH, supported by springs, one of which is seen in the section at E. The eccentrics GGGG, driven by the shaft FF, in their revolution force down the cradles KKKK, which in their turn act on the tops of the hammers, the springs (E) keep the hammer-head always in contact with the cradles KK. The shaft FF is driven by a strap on the pulley LL. It is evident that, by the revolution of the shaft, the eccentric forces down the hammer, and then allows the spring to lift it again; the rapidity of the strokes is only limited by the power of the spring E to keep the hammer in contact with the cradle; if the eccentric revolves too fast, a violent jerking motion is produced. In Mr Ryder's machine, The manufacture of iron.

700 strokes a-minute was the maximum; but Messrs. Piatt Brothers, by increasing the strength of the spring, run as high as 1100. A pair of knife edges, worked by the machine itself, has also been substituted for the hand-iron into the malleable state, it assuredly is not unreasonable to look forward to still greater improvements in the manipulations of the forge.

IV.—THE STRENGTH AND OTHER PROPERTIES OF IRON.

In this section we have to consider the tensile and transverse strengths and powers of resisting compression of cast and malleable iron, as determined by direct experiment upon specimens of the material; and also to examine whether, as has been alleged, the hot-blast process injures the tenacity of the metal.

Cast Iron.—The following tables give the results of experiments undertaken by Mr Hodgkinson and Mr Fairbairn at the request of the British Association, to determine the tensile and transverse strengths of cast-iron derived from the hot and cold blast. The castings for ascertaining the tensile strain were made very strong at the ends, with eyes for the bolts to which the shackles were attached; the middle part, where it was intended that the specimen should break, was cast of a cruciform + transverse section. The four largest castings were broken by the chain-testing machine belonging to the corporation of Liverpool, the others by Mr Fairbairn's lever.

Table I.—Results of the Experiments on the tensile strength of Cast-Iron.

| Description of Iron | Number of Experiments | Mean strength per square inch of section | |---------------------|-----------------------|----------------------------------------| | Carron iron, No. 2, hot-blast | 3 | 15,505 tons, c.wts. | | " " " cold-blast | 2 | 16,883 " " | | " " No. 3, hot-blast | 2 | 17,755 " " | | " " " cold-blast | 2 | 14,200 " " | | Devon (Scotland) iron, No. 3, hot-blast | 1 | 21,907 " " | | Buffery iron, No. 1, hot-blast | 1 | 13,484 " " | | " " " cold-blast | 1 | 17,466 " " | | Coed Talon (North Wales) iron, No. 2, hot-blast | 2 | 16,676 " " | | Do. do. cold-blast | 2 | 18,855 " " |

From the same series of experiments we select the following tables, giving the results obtained in regard to the resistance opposed to compression by cast-iron. The specimens employed were cylinders and prisms of various dimensions, and having their faces turned accurately parallel to each other and perpendicular to the axis of the specimen. They were crushed by a lever between parallel steel discs.

Table II.—Weights required to crush cylinders, &c., of Carron Iron, No. 2, Hot-Blast.

| Diameter of Cylinder in parts of an inch | Number of Experiments | Mean Crushing Weight per square inch | General Mean per square inch | |------------------------------------------|-----------------------|-------------------------------------|----------------------------| | 3 | 3 | 6,426 lbs. | 121,685 lbs. | | 4 | 4 | 14,542 lbs. | 131,065 lbs. | | 5 | 5 | 22,110 lbs. | 112,605 lbs. | | = 64" | 1 | 25,888 lbs. | 111,260 lbs. | | Prism base 50 inch square | 3 | 25,104 lbs. | 100,728 lbs. | | Prism base 1' 00 x 26" | 2 | 20,276 lbs. | 101,062 lbs. | The manufacture of iron.

Table III.—Weights required to crush cylinders, &c., of Carron Iron, No. 2, Cold-Blast.

| Diameter of Cylinder in parts of an inch | Number of Experiments | Mean Crushing Weight per square inch | General Mean per square inch | |------------------------------------------|-----------------------|-------------------------------------|-------------------------------| | Equilateral triangle side 865 inches | 2 | 14,190 | 128,478 | | Squares—side ½ inch | 2 | 24,250 | 123,708 | | Rectangles base 1½ × 2½ inches | 3 | 26,237 | 107,971 | | Cylinders 4½ inch diameter and 7½ inch high | 2 | 15,369 | 96,634 |

Table IV.—Results of Experiments to ascertain the forces necessary to crush short cylinders, &c., of Cast-Iron from various parts of the United Kingdom.

| Description of Iron | Number of Experiments | Mean Crushing Weight per square inch | |---------------------|-----------------------|-------------------------------------| | Devon (Scotch) iron, No. 2, hot-blast | 2 | 145,455 | | Buffery iron, No. 1, hot-blast | 4 | 86,397 | | Coed Talon, No. 2, hot-blast | 4 | 93,385 | | Coed Talon, No. 2, cold-blast | 4 | 82,734 | | Carron Iron, No. 2, hot-blast | 2 | 108,540 | | Carron Iron, No. 2, cold-blast | 2 | 106,375 | | Carron Iron, No. 3, hot-blast | 4 | 133,440 | | Carron Iron, No. 3, cold-blast | 4 | 115,462 |

The specimens of Carron iron in table IV. were prisms, whose base was \( \frac{3}{4} \times \frac{1}{2} = \frac{3}{8} \) inch, and whose height varied from \( \frac{1}{2} \) inch to 1 inch. The other specimens were cylinders, whose diameter was about \( \frac{1}{2} \) inch, and height varied from \( \frac{1}{2} \) inch to 2 inches.

From the above experiments, Mr Hodgkinson concludes that "where the length is not more than about three times the diameter, the strength for a given base is pretty nearly the same." Fracture took place either by wedges sliding off, or by the top and bottom forming pyramids, and forcing out the sides; and the angle of the wedge is nearly constant, a mean of 21 cylinders being 55° 32'.

From the same series of experiments we give the results obtained by Mr Fairbairn, in regard to the effects of time and temperature. The bars employed were cast to be 1 inch square, and 4 feet 6 inches long, and were loaded with permanent weights as under; the deflections being taken at various intervals during a period of fifteen months. Coed-talon hot and cold-blast iron was employed.

Table V.—The Effects of Time on loaded bars of Hot and Cold-blast Iron in their resistance to a transverse strain.

| Permanent load in lbs. | Increase of deflection of cold-blast bars | Increase of deflection of hot-blast bars | |------------------------|-------------------------------------------|-----------------------------------------| | 280 | .033 | .043 | | 336 | .046 | .077 | | 392 | .150 | .088 | | 440 | .047 | | | Mean | .066 | .069 |

It has been assumed by most writers on the strength of materials, that the elasticity of cast-iron remained perfect to the extent of one-third the weight that would break it. This is, however, a mere assumption, as it has been found that the elasticity of cast-iron is injured with less than one-half that weight, and the question to be solved in the above experiments was, to what extent the material could be loaded without endangering its security; or how long it would continue to support weights, varying from one-half to one-tenth of the load that would produce fracture. These experiments were continued from six to seven years, and the results obtained were, that the bars which were loaded to within \( \frac{1}{4} \) of their breaking weight, would have continued to have borne the load, in the absence of any disturbing cause, ad infinitum; but the effect of change, either of the same or a lighter load led ultimately to fracture.

From these facts it is deduced, that so long as the molecules of the material are under strain (however severe that strain may be), they will arrange and accommodate themselves to the pressure, but with the slightest disturbance, whether produced from vibration or the increase or diminution of load, it becomes, under these influences, only a question of time when rupture ensues.

In the following experiments on the relative strengths of coed-talon hot and cold-blast iron to resist transverse strain at different temperatures, the results are reduced to those of bars 2 feet 3 inches between supports, and 1 inch square, as follows:

Table VI.

| Temperature, Fahr. | Specific Gravity | Modulus of Elasticity | Breaking weight | Ultimate deflection | Power of resisting impact | |--------------------|------------------|----------------------|-----------------|---------------------|--------------------------| | Cold Blast, No. 2 | | | | | | | Hot Blast, No. 2 | | | | | | | Cold Blast, No. 2 | | | | | | | Hot Blast, No. 2 | | | | | | | Hot Blast, No. 2 | | | | | | | Hot Blast, No. 3 | | | | | | | Hot Blast, No. 2 | | | | | |

From the above it will be seen "that a considerable failure of the strength took place after heating the No. 2 iron from 26° to 190°. At 212°, we have in the No. 3 a much greater weight sustained than by No. 2 at 190°; and at 600° there appears, in both hot and cold-blast, the anomaly of increased strength as the temperature is increased." The above results are, with one exception, in favour of the cold blast, as far as strength is concerned; and in favour of the hot blast, with one exception, as regards power of resisting impact.

The following table gives the results of Mr Fairbairn's experiments on the transverse strength of rectangular cast-iron bars, each bar being reduced to exactly one inch square.

"In the following abstract, the transverse strength, which may be taken as a criterion of the value of each iron, is obtained from the mean of the experiments first on the long bars, 4 feet 6 inches between the supports, and next on those of half the length, or 2 feet 3 inches between supports."

"All the other values are deduced from the 4 feet 6 inches bars."

This probably arises from the greater ductility of the bars at an increased temperature. ### Table VII.—General Summary of Results on rectangular bars, as obtained from nearly the whole of the British Iron-Works.—Manchester Memoirs, New Series, Vol. V.

| Description of Iron | No. of Experiments | Specific Gravity | Modulus of Elasticity in lbs per square inch or stiffness | Breaking weight in lbs per sq. in., reduced by 4 ft. 6 in. between supports | Ultimate deflection of 4 ft. 6 in. bars in inches | Power of the 4 ft. 6 in. bars to resist impact | Colour | Quality | |---------------------|--------------------|------------------|-------------------------------------------------------------|-----------------------------------------------------------------|-----------------------------------------------|-----------------------------------------------|--------|---------| | Penkney, No. 3, cold-blast | 4 | 7.122 | 17,211,000 | 567 | 595 | 581 | 1.747 | 992 | Whitish grey | Hard | | Devon, No. 3, hot-blast* | 2 | 7.251 | 22,472,650 | 537 | ... | 537 | 1.090 | 589 | Whitish grey | " | | Oldberry, No. 3, hot-blast* | 5 | 7.500 | 22,733,400 | 543 | 537 | 530 | 1.005 | 549 | Whitish grey | " | | Coed-Talun, No. 3, hot-blast* | 2 | 7.056 | 17,873,100 | 520 | 524 | 527 | 1.365 | 710 | Whitish grey | " | | Eglington, No. 4, from prepared coke† | 6 | ... | ... | 515 | ... | 515 | 1.469 | 751 | Light grey | Rather hard | | Beamfort, No. 5, hot-blast* | 7 | 7.069 | 16,802,000 | 505 | 519 | 517 | 1.999 | 807 | Dullish grey | Hard | | Butterley | 4 | 7.038 | 15,579,500 | 489 | 515 | 502 | 1.815 | 889 | Dark grey | Soft | | Bute, No. 1, cold-blast | 4 | 7.066 | 15,163,000 | 483 | 487 | 491 | 1.764 | 872 | Bluish grey | " | | Windmill End, No. 2, cold-blast | 4 | 7.049 | 14,907,000 | 441 | 529 | 485 | 1.621 | 718 | Grey | Hard | | Old Park, No. 6, cold-blast | 10 | 7.108 | 16,261,000 | 478 | 470 | 474 | 1.512 | 729 | Dull grey | Soft | | Beamfort, No. 4, hot-blast* | 4 | 7.055 | 14,569,500 | 462 | 483 | 472 | 1.832 | 855 | Dark grey | " | | Low Moor, No. 2, cold-blast | 4 | 7.079 | 15,381,200 | 463 | ... | 463 | 1.559 | 721 | Dark grey | Rather hard | | Buffery, No. 1, cold-blast* | 5 | 7.017 | 14,911,566 | 465 | 458 | 459 | 1.748 | 815 | Light grey | " | | Brimble, No. 2, cold-blast* | 3 | 7.017 | 14,852,000 | 457 | 455 | 455 | 1.811 | 822 | Dark grey | Rather soft | | Apsdale, No. 2, hot-blast* | 4 | 7.059 | 14,307,500 | 455 | 457 | 455 | 1.484 | 650 | Bluish grey | Hard | | Oldberry, No. 2, cold-blast | 4 | 7.038 | 15,160,000 | 458 | 473 | 455 | 1.957 | 886 | Dark grey | Rather soft | | Pentwyn, No. 2 | 4 | 7.028 | 14,043,000 | 453 | 455 | 454 | 1.336 | 593 | Grey | Hard | | Maerdy, No. 2, cold-blast* | 4 | 7.113 | 14,003,550 | 443 | 464 | 453 | 1.734 | 770 | Bright grey | Fluid | | Adelphi, No. 2, cold-blast | 4 | 7.080 | 13,815,500 | 441 | 457 | 449 | 1.759 | 777 | Light grey | Soft | | Blaina, No. 2, cold-blast | 5 | 7.159 | 14,281,468 | 433 | 464 | 448 | 1.726 | 747 | Bright grey | Hard | | Blaina, No. 3, cold-blast | 4 | 7.285 | 22,907,700 | 448 | ... | 448 | 1.790 | 830 | Light grey | " | | Gartsherrie, No. 3, hot-blast | 4 | 7.017 | 13,894,000 | 427 | 467 | 447 | 1.870 | 768 | Dull grey | " | | Eglington, No. 4, common coke | 6 | ... | ... | 447 | ... | 447 | 1.825 | 841 | Light grey | Open | | Frood, No. 2, cold-blast | 5 | 7.031 | 13,112,666 | 460 | 434 | 447 | 1.414 | 629 | Dark grey | Soft | | Lane End, No. 2 | 5 | 7.028 | 15,787,566 | 444 | ... | 444 | 1.336 | 593 | Grey | " | | Carree, No. 3, cold-blast | 4 | 7.087 | 14,043,000 | 444 | 443 | 443 | 1.336 | 593 | Grey | Rather soft | | Dandyvan, No. 3, cold-blast | 4 | 7.087 | 16,534,000 | 456 | 450 | 443 | 1.469 | 674 | Dull grey | Fluid | | Maesteg (marked red) | 5 | 7.038 | 13,971,500 | 440 | 444 | 442 | 1.887 | 830 | Bluish grey | " | | Corbyns Hall, No. 2 | 5 | 7.007 | 13,845,866 | 430 | 454 | 442 | 1.587 | 727 | Grey | " | | Level, No. 2 | 5 | 7.080 | 13,126,500 | 439 | 441 | 440 | 1.857 | 816 | Dull blue | Rather soft | | Wallbrook, No. 3 | 5 | 6.979 | 15,394,766 | 432 | 449 | 440 | 1.843 | 825 | Light grey | Rather hard | | Milton, No. 3, hot-blast | 4 | 7.051 | 15,832,500 | 427 | 449 | 438 | 1.868 | 855 | Grey | " | | Buffery, No. 1, hot-blast* | 3 | 6.998 | 13,730,500 | 436 | ... | 436 | 1.840 | 721 | Dull grey | Soft | | Level, No. 1, hot-blast | 5 | 7.080 | 15,432,500 | 461 | 463 | 432 | 1.516 | 699 | Light grey | " | | Level, No. 2, hot-blast | 5 | 7.031 | 15,241,000 | 419 | 439 | 429 | 1.358 | 570 | Dull grey | Soft | | W. S. S., No. 2 | 5 | 7.041 | 14,933,333 | 413 | 446 | 429 | 1.339 | 554 | Light grey | " | | Eagle Foundry, No. 2, hot-blast | 4 | 7.038 | 14,211,000 | 408 | 446 | 427 | 1.512 | 618 | Bluish grey | " | | Bonnyrigg, No. 2, cold-blast | 4 | 6.928 | 12,586,500 | 446 | 408 | 427 | 2.224 | 992 | Grey | " | | Varberg, No. 2, hot-blast | 4 | 7.007 | 15,012,000 | 422 | 430 | 426 | 1.450 | 652 | Grey | Hard | | Coltham, No. 1, hot-blast | 5 | 7.128 | 15,510,065 | 464 | 385 | 424 | 1.221 | 530 | Grey | Rather soft | | Carroll, No. 2, cold-blast | 4 | 7.069 | 17,036,000 | 490 | 486 | 418 | 1.570 | 656 | Bluish grey | Soft | | Maerdy, No. 1, hot-blast* | 4 | 6.958 | 13,294,400 | 417 | ... | 418 | 1.222 | 494 | Dark grey | " | | Brierley, No. 2 | 5 | 7.183 | 15,510,000 | 461 | 464 | 418 | 1.222 | 494 | Dark grey | " | | Coed-Talun, No. 2, hot-blast* | 4 | 6.969 | 13,522,500 | 469 | 424 | 416 | 1.882 | 771 | Bright grey | " | | Coed-Talun, No. 2, cold-blast* | 5 | 6.965 | 14,304,000 | 463 | 418 | 413 | 1.470 | 600 | Grey | " | | Monkland, No. 2, hot-blast | 3 | 6.916 | 12,559,500 | 402 | 404 | 403 | 1.762 | 700 | Bluish grey | " | | Lee's Works, No. 1, hot-blast | 3 | 6.957 | 11,539,333 | 392 | ... | 392 | 1.890 | 742 | Grey | " | | Hilton, No. 1, hot-blast | 4 | 6.976 | 11,974,500 | 358 | 386 | 369 | 1.595 | 532 | Grey | Soft and fluid | | Plas Kynaston, No. 2, hot-blast | 5 | 6.916 | 13,341,633 | 378 | 357 | 357 | 1.566 | 517 | Light grey | Rather soft |

* The irons with asterisks are taken from the experiments on hot and cold blast iron made by Mr Hodgkinson and Mr Fairbairn for the British Association for the Advancement of Science. See Seventh Report, vol. vi.

The modulus of elasticity was usually taken from the deflection caused by 112 lbs. on the 4 feet 6 inch bars.

To find from the above the breaking weight in rectangular bars generally: calling b and d the breadth in inches, and l the distance between the supports in feet; and putting 4.5 for 4 feet 6 inches, we have \( \frac{4.5 \times b^2 \times S}{l} = \text{breaking weight in lbs.} \). The value of S being taken from the table above.

For Example:—What weight would be necessary to break a bar of Lowmoor iron, 2 inches broad, 3 inches deep, and 6 feet between the supports? According to the rule given above we have \( b=2 \) inches, \( d=3 \) inches, \( l=6 \) feet, \( S=472 \), by the table. Then \( \frac{4.5 \times 2^2 \times 472}{6} = 6372 \) lbs.

† This iron was melted in the cupola, from coke entirely freed from sulphur, by Mr C. Calvert’s process. With regard to the comparative strengths of hot and cold-blast iron, the following extracts from Mr Hodgkinson's report, read before the British Association, give the general results of his experiments:

**Table VIII.—Carron Iron, No. 2.**

| Tensile strength in lbs. per inch square | Cold-blast | Hot-blast | Ratio representing Cold-blast by 1000 | |-----------------------------------------|-----------|----------|-------------------------------------| | Compressive do. (lbs. per inch) | 16685 (2) | 13205 (3)| 1000 : 809 | | Do. from prisms of various forms | 10637 (5) | 106540 (2)| 1000 : 1020 | | Do. from cylinders | 10063 (4) | 100788 (3)| 1000 : 1003 | | Transverse strength from all the experi-| 13546 (3) | 131685 (3)| 1000 : 970 | | ments | (11) | (13) | | | Power to resist impact | 476 (2) | 465 (3) | 1000 : 978 | | Ultimate deflection do. of bars 1 in. | 1313 (3) | 1337 (3) | 1000 : 1018 | | Modulus of elasticity do. | 1727000 (2)| 1605000 (2)| 1000 : 931 | | Specific gravity | 7065 | 7046 | 1000 : 997 |

**Table IX.—Devon Iron, No. 3.**

| Tensile strength | Cold-blast | Hot-blast | Ratio representing Cold-blast by 1000 | |-----------------------------------------|------------|-----------|-------------------------------------| | Compressive do. | | | | | Transverse do. | | | | | Power to resist impact | | | | | Transverse strength of bars 1 in. | | | | | Ultimate deflection do. | | | | | Modulus of elasticity do. | | | | | Specific gravity | | | |

**Table X.—Buffery Iron, No. 1.**

| Tensile strength | Cold-blast | Hot-blast | Ratio representing Cold-blast by 1000 | |-----------------------------------------|------------|-----------|-------------------------------------| | Compressive do. | 17465 (1) | 13434 (1) | 1000 : 769 | | Transverse do. | 93366 (4) | 86397 (4) | 1000 : 925 | | Power to resist impact | (2) | (5) | 1000 : 931 | | Transverse strength of bars 1 in. | 463 (3) | 436 (3) | 1000 : 942 | | Ultimate deflection do. | 155 (3) | 164 (3) | 1000 : 1058 | | Modulus of elasticity do. | 15381200 (2)| 13736500 (2)| 1000 : 893 | | Specific gravity | 7079 | 6998 | 1000 : 989 |

**Table XI.—Coed Talon Iron, No. 2.**

| Tensile strength | Cold-blast | Hot-blast | Ratio representing Cold-blast by 1000 | |-----------------------------------------|------------|-----------|-------------------------------------| | Compressive do. | 18855 (2) | 16676 (2) | 1000 : 884 | | Specific gravity | 6955 (4) | 6968 (3) | 1000 : 1002 |

**Table XII.—Carron Iron, No. 3.**

| Tensile strength | Cold-blast | Hot-blast | Ratio representing Cold-blast by 1000 | |-----------------------------------------|------------|-----------|-------------------------------------| | Compressive do. | 14200 (2) | 17755 (2) | 1000 : 1250 | | Specific gravity | 7135 | 7056 (1) | 1000 : 989 |

"In the irons of the quality No. 2, the case seems in some degree different; in these the advantages of the facture of rival kinds seem to be more nearly balanced. They are still, however, rather in favour of the cold-blast.

"So far as my experiments have proceeded, the irons of No. 1 have been deteriorated by the hot-blast; those of No. 2 appear also to have been slightly injured by it, while the irons of No. 3 seem to have benefited by its mollifying powers. The Carron iron No. 3 hot-blast, resists both tension and compression with considerably more energy than that made with the cold-blast; and the No. 3 hot-blast iron from the Devon works, in Scotland, is one of the strongest cast irons I have seen, whilst that made by the cold-blast is comparatively weak, though its specific gravity is very high, and higher than in the hot. The extreme hardness of the cold-blast Devon iron above prevented many experiments that would otherwise have been made upon it, no tools being hard enough to form the specimens. The difference of strength in the Devon irons is peculiarly striking.

"From the evidence here brought forward, it is rendered exceedingly probable that the introduction of a heated blast in the manufacture of cast iron has injured the softer irons, whilst it has frequently mollified and improved those of a harder nature, and, considering the small deterioration that the irons of quality No. 2 have sustained, and the apparent benefit to those of No. 3, together with the great saving effected by the heated blast, there seems good reason for the process becoming as general as it has done."

The following table gives a general summary of the results of Mr Fairbairn's experiments on the strength of iron after successive meltings. The iron used was Eglinton No. 3 hot-blast, and was melted eighteen times, three bars being cast at each melting. These bars, which were about 1 inch square and 5 feet long, were placed upon supports 4 feet 6 inches apart, and broken by a transverse strain. Cubes, from the same irons, exactly 1 inch square, were then crushed between parallel steel bars, by a large wrought-iron lever.

In the following Table XIII., the results on transverse strain are reduced to those on bars exactly 1 inch square and 4 feet 6 inches between supports.

In the above results it will be observed that the maximum of strength, elasticity, &c., is only arrived at after the metal has undergone twelve successive meltings. It is probable that other metals and their alloys may follow the same law, but that is a question that has yet to be solved, probably by a series of experiments. The manufacture of iron requiring a considerable amount of time and labour to accomplish, but which we may venture to hope will be shortly forthcoming from the same author.

In the resistance of the different meltings from the same iron, to a force tending to crush them, we have the following results.

**Table XIV.**

| Number of meltings | Resistance to compression per square inch, in tons. | |--------------------|-----------------------------------------------------| | 1 | 44.0 | | 2 | 43.6 | | 3 | 41.1 | | 4 | 40.7 | | 5 | 41.1 | | 6 | 41.1 | | 7 | 40.9 | | 8 | 41.1 | | 9 | 55.1 | | 10 | 57.7 | | 11 | Mean 69.8 | | 12 | 73.1 | | 13 | 66.0 | | 14 | 95.9 | | 15 | 76.7 | | 16 | 70.5 | | 18 | 88.0 |

Nearly the whole of the specimens were fractured by wedges which split or slid off diagonally at an angle of from 52° to 58°.

**Malleable Iron.**—The greatly extended application of wrought iron to every variety of construction renders an investigation of its properties peculiarly interesting. It is now employed more extensively than cast iron; and, on account of its ductility and strength, nearly two thirds of the weight of material may in many cases be saved by its employment, while great lightness and durability are secured. Its superiority is especially evident in constructions where great stiffness is not required, but on the other hand any degree of rigidity may be obtained by the employment of a tubular or cellular structure, and this may be seen in the construction of wrought iron tubular bridges, beams, and iron ships. The material of malleable iron which is making such vast changes in the forms of construction, cannot but be interesting and important, and considering that the present is far from the limit of its application, we shall endeavour to give it that degree of attention which the importance of the subject demands.

From the forge and the rolling mill we derive two distinct qualities of iron, known as "red short" and "cold short." The former is the most ductile, and is a tough fibrous material which exhibits considerable strength when cold; the latter is more brittle, and has a highly crystalline fracture almost like cast-iron; but the fact is probably not generally known, that the brittle works as well and is as ductile under the hammer as the other when at a high temperature.

Mr Charles Hood, in a paper read some time ago before the Institute of Civil Engineers, went into the subject of the change in the internal structure of iron, independently of and subsequently to the processes of its manufacture. After adducing several instances of tough fibrous malleable iron becoming crystalline and brittle during their employment, he attributes these changes to the influence of percussion, heat, and magnetism, but questions whether either will produce the effect per se. Mr Hood continues, "The most common exemplification of the effect of heat in crystallizing fibrous iron is, by breaking a wrought-iron furnace bar, which, whatever quality it was of in the first instance, will in a short time invariably be converted into crystallized iron, and by heating and rapidly cooling, by quenching with water a few times any piece of wrought iron, the same effect may be far more speedily produced. In these cases we have at least two of the above causes in operation—heat and magnetism. In every instance of heating iron to a very high temperature, it undergoes a change in its electric or magnetic condition; for at very high temperatures iron loses its magnetic powers, which return as it gradually cools to a lower temperature. In the case of quenching the iron with water, we have a still more decisive assistance from the electric and magnetic forces; for Sir Humphrey Davy long since pointed out that all cases of vaporization produced negative electricity in the bodies in contact with the vapour; a fact which has lately excited a good deal of attention in consequence of the discovery of large quantities of negative electricity in effluent steam."

Mr Hood then proceeds to the subject of percussion. "In the manufacture of some descriptions of hammered iron, the bar is first rolled into shape, and then one-half the length of the bar is heated in a furnace, and immediately taken to the tilt hammer and hammered, and the other end of the bar is then heated and hammered in the same manner. In order to avoid any unevenness in the bar, or any difference in its colour where the two distinct operations have terminated, the workman frequently gives the bar a few blows with the hammer upon that part which he first operated upon. That part of the bar immediately becomes crystallized, and so extremely brittle that it will break to pieces by merely throwing it on the ground, though all the rest of the bar will exhibit the best and toughest quality imaginable. This change, therefore, has been produced by percussion (as the primary agent) when the bar is at a lower temperature than the welding heat. Here it must be observed that it is not the excess of hammering which produces the effect, but the absence of a sufficient degree of heat, at the time that the hammering takes place; and the evil may probably be all produced by four or five blows of the hammer if the bar happens to be of a small size. In this case we witness the combined effects of percussion, heat, and magnetism. When the bar is hammered at the proper temperature, no such crystallization takes place, because the bar is insensible to magnetism; but as soon as the bar becomes of that lower degree of temperature at which it can be affected by magnetism, the effect of the blows it receives is to produce magnetic induction, and that magnetic induction and consequent polarity of its particles, when assisted by further vibrations from additional percussion, produces a crystallized texture."

The crystallization of perfectly fibrous and ductile wrought-iron has long been a subject of dispute, and although we agree with most of Mr Hood's views, we are not altogether prepared to admit that the causes assigned are the only ones concerned in producing the change, or that more than one is necessary. On the occasion of the accident on the Versailles Railway some years since, the whole array of science and practice were brought to bear upon the elucidation of the cause. Undoubtedly the broken axle presented a crystalline fracture, but it has never been ascertained how far heat and magnetism were in operation as in the case of an axle, and more especially a crank-axle, the constant vibration caused by irregularities in the way and the weight of the engine appears to be quite sufficient to occasion the breakage without aid from the other forces. Undoubtedly in almost all cases of the sudden fracture of axles or The manufacture of wrought-iron bars, during employment, the fracture presents a crystalline structure, but we believe that any molecular disturbance, such as impact, can effect this, the only question being, how long will the material sustain the action before it breaks. This question has been attempted to be decided by direct experiment under the direction of the Commission on Railway Structures. It was found that with cast-iron bars subjected to long continued impacts, "when the blow was powerful enough to bend the bars through one-half of their ultimate deflection (that is to say, the deflection which corresponds to their fracture by dead pressure), no bar was able to stand 4000 of such blows in succession. But all the bars (when sound) resisted the effects of 4000 blows, each bending them through one-third of their ultimate deflection." These results were confirmed by experiments with a revolving cam which deflected the bars.

"In wrought-iron bars no very perceptible effect was produced by 10,000 successive deflections by means of a revolving cam, each deflection being due to half the weight which, when applied statically, produced a large permanent flexure." These results agree with those obtained by Mr Fairbairn in regard to the effects of time on loaded bars of cast-iron, already given.

Arago and Wollaston have paid considerable attention to this subject; the latter having been the first to point out that native iron is disposed to break in octahedra and tetrahedra, or combinations of these forms. The law which leads to fracture in wrought-iron from changes in the molecular structure operates with more or less intensity in other bodies; repeated disturbances, in turn destroying the cohesive force of the material by which they are held together. A French writer of eminence, Arago, appears to consider the crystallization of wrought-iron to be due to the joint action of time and vibration, but we think with Mr Hood that time and its duration depends entirely upon the intensity of the disturbing forces, and, moreover, that the time of fracture is retarded or accelerated in a given ratio to the intensity with which these forces are applied.

From the above statements we may safely deduce the fact, that it is essential to the use of this material to consider the purposes to which it is applied, the forms to which it may be subjected, and the conditions under which it may be placed, in order to arrive at just conclusions as to the proportions, in order to afford to the structure (whatever that may be), ample security in its powers of resistance to strain.

On the subject of the strength of wrought-iron, we have before us the researches of Mr Fairbairn, in a paper entitled, "An Inquiry into the Strength of wrought-iron plates and their riveted joints, as applied to Shipbuilding and Vessels exposed to severe strains." In that communication it is shewn, from direct experiments, that in plates of rolled iron there is no material difference between those torn asunder in the direction of the fibre, and those torn asunder across the fibre. This uniformity of resistance arises probably from the way in which the plates are manufactured, which is generally out of flat bars, cut and piled upon each other, as at A, one-half transversely and the other half longitudinal in the line of the pile. From this it will be seen that in preparing the bloom or shingle for the rollers, the fibre is equally divided, and the only superiority that can possibly be attained is in the rolling which draws the shingle rather more in the direction of the length of the plate than its breadth.

In the following table we have the results of the experiments:

| Quality of Plates | Mean breaking weight in the direction of the fibre, in tons per square inch | Mean breaking weight across the fibre, in tons per square inch | |-------------------|-------------------------------------------------|-------------------------------------------------| | Yorkshire plates | 25,770 | 27,490 | | Yorkshire plates | 22,760 | 26,637 | | Derbyshire plates | 21,680 | 18,650 | | Shropshire plates | 22,836 | 22,000 | | Staffordshire plates | 19,563 | 21,010 | | Mean | 22,519 | 23,037 |

Or as 22.5, 23.0, equal to about \( \frac{1}{3} \) in favour of those torn across the fibre.

From the above it is satisfactory to know, so far as regards uniformity in the strength of plates, that the liability to rupture is as great when drawn in one direction as in the other; and it is not improbable, that the same properties would be exhibited, and the same resistance maintained, if the plates were drawn in any particular direction obliquely across the fibrous or laminated structure.

From the same author we select the results of a series of experiments on the tensile strength of S C bars of different lengths, and about 1\(\frac{1}{2}\) in diameter. The following table gives the strains required for each of four successive breakages of the same pieces of iron. These experiments are highly interesting, as they not only confirm those made upon plates, but they indicate a progressive increase of strength, notwithstanding the elongation and the reduced sectional area of the bars. These facts are of considerable value, as they distinctly show that a severe tensile strain is not seriously injurious to the bearing powers of wrought-iron, even when carried to the extent of or increased four times repeated, as was done in these experiments. In practice it may not be prudent to test bars and chains to their utmost limit of resistance; it is nevertheless satisfactory to know that in cases of emergency those limits may be approached without incurring serious risk of injury to the ultimate strength of the material.

The following abstract gives the results of the experiments:

| Length between the nippers | Breaking Strain in tons | Mean Elongation in inches | |----------------------------|-------------------------|--------------------------| | Inches | | | | 120 | 32.24 | 260 | | 42 | 32.125 | 98 | | 36 | 32.35 | 88 | | 24 | 32.60 | 62 | | 10 | 32.29 | 42 |

"As all these experiments were made upon the same description of iron, it may be fairly inferred that the length of a bar does not in any way affect its strength."

Reduction of the above Table:

| Length of bar | Elongation | Elongation per unit of length | |---------------|------------|------------------------------| | Inches | | | | 120 | 260 | 216 | | 42 | 98 | 233 | | 36 | 88 | 244 | | 24 | 62 | 258 | | 10 | 42 | 420 |

1 Philosophical Transactions, part ii., 1850, p. 677. Here it appears that the rate of elongation of bars of wrought-iron increases with the decrease of their length; thus while a bar of 120 inches had an elongation of 216 inch per unit of its length, a bar of ten inches has an elongation of 42 inch per unit of its length, or nearly double what it is in the former case. The relation between the length of and its maximum elongation per unit, may be approximately expressed by the following formula, viz.—

\[ l = \frac{18 + 25}{L} \]

where \( L \) represents the length of the bar, and \( l \) the elongation per unit of the length of the bar.

The above results are not without value, as they exhibit the ductility of wrought-iron at a low temperature, as also the greatly increased strength it exhibits with a reduced sectional area under severe strain.

On the transverse strength of wrought-iron it will not be necessary to enlarge, as we have numerous examples before us in the experiments undertaken to determine the strength and form of the Britannia and Conway Tubular Bridges. In these experiments will be found an entirely new description of form and construction, which have emanated from them, and which have led to a new era in the history of bridges, and the application of wrought iron to other purposes besides those in connection with buildings, and its greatly extended application to the useful arts. For further information on this subject we refer the reader to Mr Fairbairn's and Professor Hodgkinson's works, in both of which will be found data sufficient to establish the great superiority of malleable over cast iron, or any other material, either as regards strength or economy in its application.

On the resistance of wrought-iron plates to a force tending to burst them, Rondelet has shewn that it requires a force of 70,000 lbs. per square inch to produce fracture, and Mr Fairbairn's experiments proved that a wrought-iron plate of one-quarter of an inch thick resisted a pressure from a hall 3 inches in diameter, equal to that required to rupture a 3 inch oak plank.

At the request of the British Association, Dr. Thomson of Glasgow examined the chemical constitution of hot-blast iron, and he gives the following as the result of his inquiry:

"(1.) The specific gravity of hot-blast iron is greater than that of cold-blast.

"The following are the specific gravities of eight specimens of cold-blast iron:

1st. Muirkirk . . . 6.410 (5th Ditto) . . . 6.775 2d. Ditto . . . 6.435 (6th From pyrites) . . . 6.944 3d. Ditto . . . 6.493 (7th From Carron) . . . 6.888 4th. Ditto . . . 6.579 (8th Clyde Iron-Works) . . . 7.002

"The specific gravity of the Muirkirk iron is considerably less than that smelted at Carron and the Clyde Iron-Works; the mean of the eight specimens is 6.7034.

"It has been hitherto supposed that the difference between cast-iron and malleable iron consists in the presence of carbon in the former, and its absence from the latter; in other words, that cast-iron is pure iron of iron. But in all the specimens of cast-iron which we analysed we constantly found several other ingredients besides iron and carbon. Manganese is pretty generally present in minute quantity, though in one specimen it amounted to no less a quantity than 7 per cent.; its average amount is 2 per cent. Silicon is never wanting, though its amount is exceedingly variable, the average quantity is about 1 per cent.; some specimens contained 31 per cent. of it, while others contain less than a half per cent. Aluminium is also generally present, though its amount is more variable than that of silicon. Its average amount is 2 per cent.; sometimes it exceeds 44 per cent., and sometimes it is not quite 1-5000th part of the weight of the iron.

"Calcium and magnesium are sometimes present, but very rarely. The manganese does not much exceed 1-5th per cent. In the manufacture of cast-iron which I got from Mr Nisbet, and which he had iron-smelted from pyrites, there was a trace of copper, showing that the pyrites had been treated quite free from copper; and in a specimen from the Clyde Iron-Works there was a trace of sulphur. The following table exhibits the composition of six different specimens of cast-iron, No. 1, analysed in my laboratory, either by myself or by Mr John Tennent.

| Constituent | Mean | |-------------|------| | Iron | 90.98 | | Copper | 0.288 | | Manganese | 0.288 | | Sulphur | 0.288 | | Carbon | 0.288 | | Silica | 0.288 | | Aluminium | 0.288 | | Calcium | 0.288 | | Magnesium | 0.288 |

"The constant constituents of cold-blast cast-iron, No. 1, are iron, manganese, carbon, silicon, and aluminium. The occasional constituents are copper, sulphur, calcium, and magnesium. These occur so rarely, and in such minute quantity, that we may overlook them altogether.

"The constant constituents occur in the following mean atomic proportions:

- 22 atoms iron - 1 atom manganese - 4-36 atoms carbon - 1 atom silicon - 14 aluminum

"(2.) I examined only one specimen of cast-iron, No. 2. It was an old specimen, said to have come from Sweden, but I have no evidence of the correctness of this statement. Its specific gravity was 7.1653 higher than any specimens of cold-blast iron, No. 1. Its constituents were,

| Constituent | Mean | |-------------|------| | Iron | 95.594 | | Manganese | 0.708 | | Carbon | 0.680 | | Silicon | 0.680 | | Aluminium | 0.680 | | Sulphur | 0.680 |

"The presence of sulphur in this specimen leads to the suspicion that it is not a Swedish specimen; for as the Swedish ore is magnetic iron, and the fuel charcoal, the presence of sulphur in the iron is very unlikely.

"In this specimen, the atoms of iron and manganese are to those of carbon, silicon, and aluminium, in the proportion of 4:1 to one, instead of 3:1 to one, as in cast-iron No. 1.

"The atoms of carbon, silicon, and aluminium, approach the proportions of 7:2, and 1:4, as in cast-iron, No. 2, judging from one specimen there is a greater proportion of carbon, compared with the silicon and aluminium, than in cast-iron, No. 1.

"Mr. Tennent analysed a specimen of hot-blast iron, No. 2, from Garthistle. Its specific gravity was 6.9156, and its constituents,

| Constituent | Mean | |-------------|------| | Iron | 90.542 | | Manganese | 0.78 | | Carbon | 0.680 | | Silicon | 0.680 | | Aluminium | 0.680 | | Sulphur | 0.680 |

So that it resembles cast-iron, No. 1, in the proportion of its constituents. The carbon is almost the same as in cold-blast iron, No. 2, but the proportion of manganese is four times as great, while the silicon is about three times as much. The atomic ratios are, carbon: silicon: aluminium = 2:2:3.

"(3.) Five specimens of hot-blast cast-iron, No. 1, were analysed. Two of these were from Carron, and three from the Clyde Iron-Works, where the hot-blast originally began; and where, of course, it has been longest in use. The specific gravity of these specimens was found to be as follows:

1 See Mr Fairbairn's and Mr Edwin Clark's works on the Conway and Britannia Tubular Bridges. 2 "On the application of cast and wrought iron to building purposes," and "Useful Information for Engineers." 3 I have been told by Mr Mushet that the Swedes add sulphur to their iron No. 2. It appears from this, that the hot-blast increases the specific gravity of cast-iron by about 1-23 part. It approaches nearer the specific gravity of cast-iron, No. 2, smelted by cold air, than to that of No. 1.

The following table exhibits the constituents of these four specimens:

| | Clyde | Carron | Carron | Clyde | Clyde | |--------|-------|--------|--------|-------|-------| | Iron | 95.796| 95.422 | 96.02 | 94.266| 94.345| | Manganese | 0.332| 0.355 | 0.411 | 0.160 | 0.120| | Carbon | 2.460 | 2.400 | 2.480 | 1.560 | 1.416| | Silicon | 0.280 | 1.820 | 1.49 | 0.322 | 0.320| | Aluminum | 0.383| 0.488 | 0.26 | 1.374 | 0.599| | Magnesium | 0.422| 0.337 | | 0.792 | |

The mean of these analyses gives us,

Iron: 95.584 or 27.31 Manganese: 0.871 or 0.249 Carbon: 2.099 or 2.79 Silicon: 1.086 or 1.086 Aluminum: 0.422 or 0.337

Or, in the proportion of 64 atoms of iron and manganese to 1 atom of carbon, silicon, and aluminum. In the cold-blast cast-iron we have,

In No. 1: 31 atoms 1 atom. In No. 2: 44 1 In hot-blast: 64 1

Thus it appears, that when iron is smelted by the hot-blast its specific gravity is increased, and it contains a greater proportion of iron, and a smaller proportion of carbon, silicon, and aluminum, than when smelted by the cold-blast.

V. THE STATISTICS OF THE IRON TRADE.

This article has already extended so much beyond the limits of our inquiry, that we must confine ourselves to an exceedingly brief notice of the statistics of this important manufacture. In 1740 the iron trade suffered a sudden check from a falling off in the supply of charcoal, coal or coke not having been employed at that time for smelting. The annual production seems to have decreased from 180,000 to about 17,350 tons per annum. This comparatively small quantity was smelted in the following counties, viz.—

| Counties | Furnaces | Tons. | Furnaces | Tons. | |----------|----------|------|----------|------| | Breccon | 2 | 600 | Nottingham| 1 | | Glamorgan| 2 | 400 | Salop | 6 | | Carmarthenshire | 1 | 100 | Stafford | 2 | | Cheshire | 3 | 3,700| Worcester| 2 | | Denbighshire | 2 | 3,500| Warwick | 10 | | Gloucester | 6 | 2,850| York | 2 | | Hereford | 3 | 1,350| Derby | 4 | | Hampshire| 1 | 200 | Kent | 4 | | Kent | 4 | 400 | Monmouth| 2 | | | | | | 59 | | | | | | 17,350|

Annual average for each furnace: 294 1 Weekly do. do. 5 13 0

Soon afterwards the difficulties in the way of using coal were overcome, and the manufacture extended rapidly. The number of charcoal furnaces decreased, but the quantity produced by each was considerably increased. The following table shows the state of the trade in 1788:

| Counties | No. of Furnaces | Excise Return of Iron made | Suspended quantity by the Trade | Actual Return | |----------|-----------------|----------------------------|--------------------------------|---------------| | Chester | 2 | 4,710 | 2,200 | 1,958 | | Cumberland | 4 | 5,144 | 3,000 | 2,034 | | Derby | 3 | 2,138 | 2,138 | 2,107 | | Gloucester | 2 | 380 | 380 | 380 | | Hereford | 5 | 2,850 | 2,850 | 2,229 | | York | 22 | 21,984 | 21,987 | 17,947 | | Shropshire | 23 | 68,129 | 43,560 | 32,969 | | Wales | 28 | 45,994 | 42,606 | 35,485 | | Stafford | 14 | 15,820 | 15,256 | 13,210 | | Sussex | 1 | 172 | 173 | 173 | | | | 167,321 | 133,350 | 108,793 |

About the year 1796 it was contemplated by Mr Pitt to add to the revenue by a tax on coal. This met with a powerful opposition on the part of the manufacturers and consumers, especially those in the iron trade. A committee was appointed, witnesses were examined, and the measure abandoned as unwise and impracticable. The following table exhibits an abstract of the facts collected, and shows the rapid progress of the iron trade in the eight preceding years: The manufacture of iron.

The return from Scotland exhibited a list of 17 furnaces, and an exact return of pig iron manufactured, of 16,086 tons. Making an annual total of 124,879 tons. Annual average produce from each furnace, including charcoal furnaces, 1,032 tons. Increase of annual average since 1788, 232 tons.

The following table shows the comparative make of pig iron in 1820 and 1827:

| Counties | 1820 Tons | Furnaces | 1827 Tons | |----------------|-----------|----------|-----------| | North Wales | 150,000 | 12 | 24,000 | | South Wales | | | 272,000 | | Shropshire | 180,000 | 31 | 78,000 | | Staffordshire | | | 216,000 | | Yorkshire | 50,000 | 24 | 43,000 | | Derbyshire | | | 20,500 | | Scotland | 20,000 | 18 | 35,000 | | | 400,000 | 284 | 690,500 |

From that time to the present the manufacture has steadily increased. The following table gives the state of the trade in 1854; the particulars are extracted from the Mining Records, published under the direction of Mr R. Hunt, in connection with the Museum of Practical Geology, London. The importance which Scotland has assumed in reference to the iron manufacture is especially worthy of notice.

| Counties | No. of Works | No. of Furnaces erected | No. of Furnaces in blast | Total produce in tons | |----------------|--------------|-------------------------|-------------------------|-----------------------| | England | | | | | | Northumberland| 37 | 106 | 80 | 248,444 | | Durham, and | | | | | | Yorkshire | | | | | | Derbyshire | 13 | 33 | 25 | 127,500 | | Lancashire and | 2 | 5 | 3 | 20,000 | | Cumberland | | | | | | Staffordshire | 72 | 203 | 166 | 847,600 | | Shropshire | 13 | 34 | 23 | 124,800 | | Gloucestershire| 4 | 7 | 5 | 21,999 | | Wales | | | | | | Flintshire, Denbighshire | 7 | 11 | 9 | 32,900 | | Glamorganshire, Anthracite district | 14 | 35 | 21 | | | Glamorganshire and Monmouthshire, Blaenavon district | 34 | 134 | 100 | 750,000 | | Scotland | | | | | | Ayrshire | 9 | 41 | 30 | 249,600 | | Lanarkshire | 13 | 88 | 72 | 468,000 | | Other Counties | 10 | 27 | 16 | 79,040 | | | 228 | 724 | 555 | 3,059,874 |

In connection with the above, we insert the following table from Mr Kenyon Blackwell's paper on the Iron Industry of Great Britain, read before the Society of Arts. It gives the estimated production of crude iron in the various countries.

| Countries | Tons | |----------------|------------| | Great Britain | 3,000,000 | | France | 750,000 | | United States | 750,000 | | Prussia | 300,000 | | Austria | 250,000 | | Belgium | 200,000 | | Russia | 200,000 | | Sweden | Various German | 150,000 | | States | 100,000 | | Other Countries| 300,000 | | | 6,000,000 |

In referring to the above, it will be seen that Great Britain produces as much crude iron as all other countries put together; and a great portion of that iron being converted into bars and plates, indicates a large and important article of production. An article of immense value to the country—of great demand at home and abroad—and justly entitled not only to improvements and economy in its manufacture, but to the generous support of a liberal and an enlightened Government.

(W. F.)

ADDENDA.

(A) Mr Cort's inventions.—It would be a difficult task to enumerate all the services rendered by Mr Cort to the iron industry of this country, or sufficiently to express our sympathies with the descendants and relatives of a man to whose mechanical inventions we owe so much of our national greatness. It is, perhaps, not generally known that Mr Henry Cort expended a fortune of upwards of £20,000 in perfecting his inventions for puddling iron, and rolling it into bars and plates; that he was robbed of the fruit of his discoveries by the villany of officials in a high department of the Government, and that he was ultimately left to starve by the apathy and selfishness of an ungrateful country. When these facts are known, and it has been ascertained that Mr Henry Cort's inventions have conferred an amount of wealth upon the country equivalent to six hundred millions sterling, and have given maintenance and employment to six hundred thousand of the working population of our land for the last three or four generations, we are surely justified in referring to services of such vast importance, and in advocating the principle that substantial proofs of the nation's gratitude should be afforded to rescue from penury and want the descendants of such a benefactor.

(B) The Ores of Belgium.—A species of bog ore is also used, which is said to be found near the surface, and is washed to free it from impurities.

(C) The Calcination of the Ores.—We are informed that the black and clay bands are all calcined in Scotland, even for the hot-blast process, the coaly matter in the black bands being sufficient to effect the calcination without other fuel. The carbonaceous ores lose as much as 40 to 50 per cent. of their weight in the process.

(D) The blast.—The Scotch iron smelters allege that the diffusive power of the blast is increased rather than diminished by increasing the number of blowpipe nozzles, and give as a reason for the use of so many that they can be so much more easily repaired, the stoppage of one not materially affecting the working of the furnace.

(E) Mr Bessemer's process.—Such is the discovery which Mr Bessemer has communicated to the world, but having reason to believe that the products of this new process of conversion have not been sufficiently tested, we deem it essential to reserve our opinion on this important question until further experiments have proved the efficiency of the process. We are the more convinced of the propriety of exercising precaution from the fact that experiments have been made by competent authorities, in which it has been ascertained that the ingots derived from Mr Bessemer's process are not malleable, but highly refined metal, deprived of its carbon, but still requiring to be puddled to fit it for the hammer or the rolls. Should this be the case, the saving effected will not be so great as at first anticipated, and, moreover, it has been asserted by practical men that the heat produced is so intense, as not only to deprive the metal of its carbon, but to injure the tenacity of the iron. Should these statements be correct, it will seriously detract from the merits of a discovery, which at first sight promised the commencement of a new era in the manufacture of iron. It is, however, due to Mr Bessemer to wait for further developments, which may lead to important results before coming to any conclusion as to the merits of his invention. We trust, however, that the practical difficulties which appear to surround the process will ultimately be overcome by a series of well conducted experiments, calculated to determine the full value of the invention.

(F) We are informed that the circular saw for cutting railway bars is frequently driven by the Eolipile or Hero's engine, by which a speed of 2000 revolutions a minute may be attained.

(W. F.) The exclusive use of iron in the construction of bridges is of modern date, though no other material is so peculiarly adapted to such a purpose; it was not, however, from any want of appreciation of these advantages, so much as from the great cost, and even impossibility, of obtaining iron in large masses, that its use was so long delayed. It is now most extensively employed in bridge construction, and though, in elegance or durability, iron cannot compete with stone, where the span is moderate, yet there are numberless cases where its adoption has been the means of solving many of the great problems of modern engineering; and its use has more especially become an absolute necessity in Railway Bridge construction, where headway is so frequently of paramount importance,—and where rapidity of execution is often a more necessary consideration than even economy or durability,—while the defective foundations that have so often to be contended with, render the lightness, the independent strength, and pliable character of iron for such structures of the utmost value.

We shall confine our remarks in this article to rigid structures. It is not, therefore, our intention to treat of the Suspension Bridge, which is, moreover, not of such modern date, nor have any important modifications been made in its construction since the first magnificent specimen of its class was erected by Telford over the Menai Straits in 1820. It will, however, come within our scope to describe the attempts that have been lately made to render the suspension bridge sufficiently rigid for railway traffic, and to point out the difficulties of such a problem.

Previous, however, to any description of the various forms of iron bridges now in use, we shall give a brief history of their introduction.

According to Gauthey,¹ the history of iron bridges

¹ "Traité de la Construction des Ponts," par M. Gauthey. commences in the sixteenth century, when such structures were first proposed in some Italian works. In 1719 the subject was again revived by Desaguliers, but nothing like an attempt at construction appears to have been made till 1755, when an iron bridge was proposed at Lyons, which was to consist of three arches of eighty-two feet span; one of these arches was actually put together in the builder's workyard, but this project was subsequently abandoned from motives of economy, and a timber bridge was substituted.

The first iron bridge actually erected was the semi-circular cast-iron arch across the Severn, near the village of Broseley, at Coalbrookdale, in Shropshire, which was commenced in 1777, and completed in 1779. It must be remarked that some few years prior to this date, the smelting of iron with coke was first successfully accomplished by Mr Abraham Darby of Coalbrookdale, when the use of cast-iron began at once to supersede that of timber in numerous details of construction. This bridge was designed by Mr Thomas Farnolls Pritchard, an architect of Shrewsbury, and was erected by Mr Abraham Darby, and his partner Mr Reynolds, the proprietors of the Coalbrookdale Iron Works; the expense of its construction being defrayed by a joint-stock company. This arch is nearly semicircular, the span being 100 feet 6 inches, and is composed of five ribs, placed 4 feet 10 inches apart; the ribs (see fig. 1) are formed of three concentric arcs of cast-iron, of which the lower one only is entire, and made in two castings united at the crown, the other two being deficient at their summits, in consequence of the superstructure resting on the crown of the lower and entire arc; the outer arcs are 5\(\frac{1}{2}\) inches. square, and the lower arc is $8\frac{1}{2}$ inches deep, by $5\frac{1}{2}$ inches thick; they are connected by means of radial bars fastened with bolts and nuts; and, at their springings, they are supported upon cast-iron plates 4 inches thick, bedded upon masonry. The roadway is supported on open cast-iron spandrels, and is formed of a bed of clay mixed with foundry slag laid upon cast-iron plates. The total weight of iron-work is $378\frac{1}{2}$ tons.

If we consider that the manipulation of cast-iron was then completely in its infancy, a bridge of such dimensions was doubtless a bold, as well as an original undertaking, and the efficiency of the details is worthy of the boldness of the conception. It is to be regretted that, from a defect in the abutments, and the error committed in treating the arch as one of equilibrium, the abutments were thrust inwards at the approaches, and the ribs partially fractured.

Shortly after the completion of this arch, some bold propositions for an extension of the principle originated with some French engineers. M. Callipe, in 1779, made a design for a wrought-iron arch of 656 feet span; proposing to form the ribs of plate-iron placed edgewise, the thrust being resisted by wrought-iron tie-bars, connected with the ribs by vertical and inclined bars; and M. Montepetit, in 1782, presented a plan for an arch of 213\(\frac{1}{4}\) feet span over the Seine at Paris, of which the ribs were to consist of two concentric arcs 5 feet 4 inches apart, composed of double plates of wrought-iron, 12\(\frac{1}{2}\) inches deep by \(\frac{3}{8}\) inch thick; the segments were shewn with return ends, fixed together by bolts and nuts. An engraving of this bridge was given in the *Encyclopédie Methodisée*. M. Gayton published, in 1782, some observations upon these designs, proposing to form the ribs of three plates, breaking joint with each other, and united by a species of stirrup at each joint, with distance-pieces between the arcs, and bolts passing through all. M. Gayton, moreover, was of opinion that no difficulty would be found in constructing such arches of double the span of the largest existing stone arches.

In 1783, M. Racle gave a design for an iron bridge of three arches of 84 feet span, to be erected at Lyons on the site of the Pont de la Mulatière. In this bridge, as well as in other designs by that gentleman, the arcs consisted of panel work, or of cast-iron hollow voussoirs.

M. Aubry also proposed a bridge at Cordoue, of six wrought-iron arches of 75 feet span each. The ribs were to be formed of two arcs of different radii united by bars, the roadway being suspended from the arches. He also gave a design, in 1786, for the construction of a wrought-iron arch of 318\(\frac{1}{4}\) feet span,\(^2\) the ribs being formed of two tiers of arcs, united by radial bars, and the spaces between them filled with diagonal bracing.

Though our neighbours were thus so prolific in design, not one of these various bridges was constructed; a circumstance to be accounted for partly from want of enterprise, and partly from the fact that, with the exception of M. Racle's cast-iron voussoir bridge, they were all formed of wrought-iron, a material which, though more employed in France than cast-iron, was not obtainable in plates or bars of sufficient dimensions for these bold projects; accordingly, after a lapse of some years, the next actual construction of iron bridges took place in England.

In 1795, the old stone bridge which crosses the Severn at Buildwas, about ten miles below Shrewsbury, was washed away by a flood, and Mr Telford being engineer to the county, was called upon to replace it, which he did, in 1796, by a single arch 130 feet span (see fig. 2). The arch, which is very flat, is formed of five cast-iron ribs, having a rise of only 14 feet; there are also two outer ribs with a rise of 30 feet, their springings being lower than those of the inner ribs, and their summits reaching to the top of the iron railing of the superstructure. The platform is 18 feet wide, and formed of cast-iron plates, with deep flanges, which, in themselves, serve as an additional stiffness to the system.

The expansion and contraction of the outer arcs being different from that of the remainder, to which they were dovetailed, was, as might be expected, found rather to derange their action than to add any strength to the structure.

The total weight of iron in the above bridge is about 174 tons, and it was constructed by the Coalbrookdale ironmasters at a cost of £26,034.

We now come to a description of probably one of the boldest examples of arch construction in existence—the bridge over the Wear at Wearmouth, which was completed also in 1796.

About the year 1790, Thomas Payne, the well known political author, proposed to construct cast-iron arches of framed open panels, in the form of voussoirs, and with characteristic energy, he put his views to the test by constructing an experimental arch of 88 feet 6 inches span, which was made by Messrs Walker of Rotherham, and exhibited in a bowling-green at Paddington; this experiment was completely successful. It was intended to have shipped this arch to America, but Payne not being able to defray the expenses, the manufacturers took it back, and the malleable iron was afterwards worked up in the construction of the Wearmouth Bridge.\(^3\)

It was in the same year that a committee was appointed for investigating the inconvenient and dangerous state of the ancient ferry in the middle of the harbour at Wearmouth; and it being decided that a bridge should be substituted, arrangements were made for erecting one of stone, although this intention was subsequently abandoned, and at the instigation of Mr Burdon, the Member for the county, who had gone into the subject with the Messrs Walker of Rotherham, it was determined to construct a cast-iron arch. Mr Thomas Wilson was accordingly entrusted with the design, and the ideas of Payne with regard to open voussoirs of cast-iron were adopted.

The history of this extraordinary structure is thus as remarkable as its execution. If we are to consider Payne as its author, his daring in engineering certainly does full justice to the fervour of his political career; for successful as the result has undoubtedly proved, want of experience, and consequent ignorance of the risk, could have alone induced so bold an experiment; and we are led rather to wonder at, than to admire, a structure which, as regards its proportions and the small quantity of material employed in its construction, will probably remain unrivalled.

This bridge (see fig. 3) consists of a single circular arch of 256 feet span, with a rise of 34 feet; the springings commence at 95 feet above the bed of the river, and the whole height above low water is about 100 feet, admitting vessels of from two to three hundred tons burthen to pass under it without striking their masts. The ribs, which are six in number, are placed 6 feet apart, and they are composed of cast-iron panels acting as voussoirs. In elevation these panels form three concentric arcs, distanced by radial bars, each arc being 6 inches broad by 34 inches thick, and the radial bars 1 foot 9 inches long by 2 inches broad. Each separate panel contains two radial bars, and a series of 105 of these panels or voussoirs form a rib, and they are connected by wrought-iron arcs fitting in grooves on their faces, to which they are finally secured by screws. The ribs were united with each other by transverse horizontal struts 5 feet 11 inches long, formed of cast-iron pipes, which are placed at every joint of the voussoirs, with flanges at their extremities, by which they are attached to them.

The spandrels are filled in with cast-iron circles, following the contour of the roadway, and thus diminishing in diameter from the springing to the crown of the rib. The superstructure is a strong frame-work of timber planking, which was first coated with a mixture of tar and chalk, to preserve it from rot, and then covered with a road of marl, limestone, and gravel.

The whole width of the bridge is 32 feet. The abutments, which are almost of solid masonry, are 24 feet thick, 42 feet broad at their base, and 37 feet at top. The south abutment has a foundation upon a solid rock, and rises from about 22 feet above the bed of the river. On the north side, the ground not being so favourable, it was necessary to carry the foundations 10 feet below the bed.

---

\(^2\) "Mémoires sur Différentes Questions de la Science des Constructions publiques et économiques, par M. Aubry, 1re partie."

\(^3\) See Roe's Cyclopaedia, art. Bridge, sec. Iron Bridges. The total weight of iron is 260 tons, 46 tons of which are wrought-iron, and 214 cast.

The mode of putting together the ribs was so simple and expeditious, that it was accomplished, and the whole bridge thrown over the river, in ten days; the scaffolding, which was very light, was immediately removed, and the bridge was opened for general use on the 9th of August 1795, the total cost being £27,000.

The construction of the bridge, however, occupied altogether a period of three years, and to complete its singular history, it was sold for £30,000 in a lottery, in October 1816.

Independent of the want of material, the most remarkable deficiencies in this bridge are the want of diagonal bracing, and the bad construction of the spandrels which are filled in with light cast-iron circles.

Soon after the centres were removed, the arch, indeed, was found to have deflected laterally eastward to the extent of 12 or 18 inches, but by means of wedges, tie-bars, and braces, it was partially restored to its original form, and the light cross-bracing, as it at present exists, was inserted. The stability of the bridge has been at all times, however, extremely precarious, and ordinary prudence cannot much longer delay its entire removal.

We must here mention that, in 1794, a small bridge was erected at Laason, in Lower Silesia, by M. le Comte Burghaus, consisting of a cast-iron arch, 43 feet span and 20 feet wide, composed of five ribs, formed of several arcs of different radii united by radial bars; the platform was of cast-iron plates. A similar bridge of 20 feet span was constructed at Berlin, upon the Kupfergraben.

In 1797, Mr John Nash took out a patent for some improvements in cast-iron arches, the ribs being formed of plates. Two bridges upon his system were erected over one of the canals in St. Petersburgh. Nash also proposed to form an arch of a series of cast-iron boxes, bolted together, and filled with earth or cement.

About the same period M. Gaston Rosnay obtained a brevet d'invention for some improvements in iron bridges; and suggested the employment of the railing itself, in conjunction with arcs of double or triple wrought-iron plates, for ensuring rigidity. The platform was also made instrumental in strengthening the bridge, and consisted of a double layer of cross planks, which formed an arch resting against the ribs, which were tied together by cross tie-bars.

In the same year, M. Gauthey proposed for the Pont de la Cité at Paris, two outer arches of cast-iron, somewhat similar to the Wearmouth Bridge arches, although not put together in the same manner, the platform being supported between the ribs. Although the Council of Ponts et Chaussées had approved of this design, it was not carried out, two wooden ribs being substituted for the iron ones, which ultimately failed.

In chronological order, we next come to the remarkable design by Messrs Telford and Douglas, in 1801, for replacing London Bridge by a single cast-iron arch of 600 feet span, with a clear headway of 65 feet above high water. The ribs were to be of cast-iron, in segments as large as possible, and they were to be connected by diagonal cross-bracing, disposed in such a manner that any part of the ribs or braces could be taken out and replaced without injuring the stability of the whole, or stopping the traffic. On plan the superstructure was arranged to spread in width from the centre of the arch to the approaches. The scheme was investigated by a select committee of the House of Commons, the works were put in hand, and the river was contracted to the necessary width. But this design was at length abandoned, owing more immediately to the difficulties, with such a headway, of constructing the approaches, which would have involved the formation of extensive inclined planes from the adjoining streets.

Iron bridges now began to be generally adopted. Mr Wilson constructed a cast-iron arch of a span of 180 feet, at Staines-upon-Thames, in 1802 (see fig. 4). Its radius was 260 feet, its rise being 16 feet. It was in many respects similar to the bridge of Wearmouth, being formed of cast panelled voussoirs 4 feet 10 inches long; the concentric arcs were 6 inches deep by 4½ thick. The ribs were connected by cast-iron rectangular bracing-frames. and the voussoirs were united by dowels; the wrought-iron arcs, such as were used to connect the voussoirs of Wearmouth Bridge, being dispensed with. The spandrels are filled with cast-iron circles, and the platform is formed of cast-iron plates stiffened by curved webs, so arranged as to cover two and three ribs alternately. The abutments of this bridge were insufficient to resist the thrust of the arch, and one of them actually moved horizontally on its base without disturbing its joints.

The first iron bridge constructed in France was the Pont du Louvre at Paris, in the year 1803. The plans were prepared by M. Cessart, and modified by M. Dillon, who had charge of its erection. The castings were made near Tournon, at the iron-works of MM. Baudry and Mercin.

The arches of this bridge, which are nine in number (see fig. 5), and 57 feet span, are composed of five cast-iron ribs placed 8 feet apart. Each rib or arc is $6\frac{1}{2}$ inches deep by 3 inches thick, and is made in two parts connected at the crown.

The springings rest on cast-iron plates let into the masonry of the piers, which at top are 6 feet 4 inches thick. The rise of the arcs is 10 feet 8 inches. Other arcs of lesser dimensions, striding over the piers, and resting on each pair of large arcs, are employed to fill in the depth of the spandrels or haunches. This bridge was intended for foot passengers;—its total length between the abutments is 516 feet, and its width between the railings 30 feet. The total weight of iron in the nine arches is about 263 tons. The stone piers, which are eight in number, rest on pile foundations. An interesting experiment was made on one of the ribs by M. Dillon; it was placed upon a substantial timber platform, and weighted with several boxes suspended from the same points of the rib from which the platform itself was to be ultimately suspended, the boxes were gradually loaded until the weight they contained amounted to double that which the rib could possibly have to sustain, the rib was found to be depressed at the summit, and raised at the haunches; but on the removal of the weights the arch

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1 The original design for this bridge consisted of eleven arches of 514 feet span, and the ribs were somewhat similar in external appearance to those of the modified design by M. Dillon, although differently put together. M. Cessart, who was eighty-two years old when he made this design, had the satisfaction of obtaining the approval of the Comité des Ponts et Chaussées; and to show the gratification he experienced by that honour, it may be as well to quote his own words: "Si le Pont des Artes (now the Pont du Louvre) n'a pas été entièrement exécute comme je l'ai proposé dès le principe, je m'en tiens pas moins à l'honneur d'avoir conçu la première tête de cet ouvrage et d'en avoir ainsi préparé le succès, à l'âge de quatre-vingt-deux ans, et sous les auspices du Conseil général des Ponts et Chaussées."—Description des Travaux Hydrauliques, de Louis Alexandre de Cessart, tom. i. Iron regained its original figure, which circumstance was strictly in accordance with the theoretical views upon the subject.

We now come to the Pont d'Austerlitz, erected at Paris by M. Lemaître. This light and elegant structure consists of five equal arches (see fig. 6), which are segments of circles 138 feet radius, the span being 106 feet, and the rise 10 feet 8 inches. Each arch is composed of seven ribs placed 6 feet 8 inches apart, each consisting of three concentric arcs, which form panels by their intersection with radial bars. The arcs are 5½ inches deep by 2½ inches thick, and the radial bars are 2½ inches broad; the spandrels are filled up by arcs and radial bars, forming panels similar to those of the voussoirs. The ribs are united by cast-iron cross-frames, formed of bars 2½ inches square by 6 feet 5 inches long, with a double return at their extremities, by which they are attached to the voussoirs; the depth of the entire arc is 5 feet 3 inches. The platform is of timber, near the abutments, caused by the bridge having been overloaded immediately after its completion. The arcs themselves remained, however, uninjured, and the radial bars having been repaired by bands of wrought-iron placed over the broken parts, the arcs were rendered secure from further distortion.

This bridge was commenced in 1800, and finished in 1806.

In January 1805, some discussion took place as to the best method of widening the waterway of the Pont de l'Archevêché at Lyons, and it was proposed to replace

the five stone arches of that edifice by three cast-iron arches, of which the centre one would have had a span of 239½ feet. (See fig 7.) The proposed construction was novel; the arcs being formed of cast-iron pipes, two tiers of which were proposed for each rib, connected by radial bars, having their ends flattened and bolted between the flanges of the pipes. A similar arrangement was proposed for the bracing-frames between the ribs. The haunches of the arches were also composed of pipes.

The proposed advantages in the use of pipes arose from the consideration that the same quantity of metal in a hollow section was much more advantageously disposed of than in a solid section, and this first application of that principle to bridge structure is deserving of notice.

Some novelty in construction next occurs in a small wrought-iron arch, constructed in 1808, by William Bruyère near St. Denis; the span is only 37 feet 5 inches, with a rise of 3 feet 3 inches, and the ribs are composed

of 2½ inches. The settlement which took place on the striking of the centres varied in different arches from ¼ inch to ½ inch—but it ultimately increased to 2½ inches and 2½ inches. The deflection was partly caused by the fracture of some of the radial bars, particularly those of panel voussoirs, but the angles of each panel are con-

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Fig. 6.—Section.

Fig. 7.—Section.

Fig. 8.—Pont St. Denis.

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M. Bruyère had applied this arrangement with advantage to the construction of wrought-iron sluice gates. Iron Bridges.

Connected in a peculiar manner (see fig. 8); the ends of each radial bar have four quadrant plates forged to them, and the arcs and the diagonal cross-bars, which all meet in the same point, have similar quadrant plates, that halve in thickness with those of the radial bars, and circular plates are made to cover the whole, which are fastened together by four bolts and nuts.

Such a method of construction is extremely costly, and evidently inapplicable to large structures.

A similar bridge was, however, proposed in 1810, by M. Bruyère for the prolongation of the centre portion of the Hotel des Invalides, at Paris; it was to be a single arch of 426½ feet span, the ribs being divided into voussoirs similar to that just described, and tied together by cross-bars and diagonal tie-bars at every interval of the voussoirs. The execution of this structure, which was only meant for foot passengers, had been decided upon, but was afterwards suspended.

Another bridge for the same site was proposed by M. Lamanjé, in 1811. This design presented some additional peculiarities, inasmuch as it consisted of a combination of wrought and cast-iron; the bridge was to consist of three arches, but the centre one only was to have been of iron, with a span of 262½ feet. In this instance there was to have been a carriage-way. The arcs of the voussoirs were to be of cast-iron, but the radial bars and the diagonal braces, and in fact, all those pieces which were exposed to tensile strains, were to have been of wrought-iron. Each upright served to connect two adjoining voussoirs, and carried, at its extremities, ears or lugs, forming a kind of fork which clasped the voussoirs; the cast-iron arcs of the voussoirs were further bound together by wrought-iron arcs.

Improvements in the manufacture of iron kept pace with the rapid increase of its application to construction, while the ingenuity of inventors in this, as in all other developments of new principles, far outstripped the requirements of practice. Mr Pope proposed to construct bridges of 1000 feet span, on a new principle, named by him the "Flying Lever Pendant Bridge." However impracticable the construction may have been, the idea is worthy of notice, inasmuch as it involved new and important applications of action and reaction. These bridges were to consist of a combination of struts and ties acting with various leverages—and so arranged that the entire half-bridge was one huge cantilever, with the abutments sufficiently massive to bear all the overhanging weight and strain of the half-arch. By this means, there was no dependent pressure at the crown or meeting of the two half-arches—each of which was to be built progressively from the abutments to the centre, without the aid of centering or scaffolding; with this arrangement, any portion of the middle could be removed without disturbing the rest.

These bridges were to be constructed of iron or timber. M. Wilbeking, Director-General of Ponts et Chaussées, in Bavaria, published, in 1812, a treatise upon Iron Bridges, in which he proposed to construct arches of iron pipes, fastened together by means of sockets attached to the extremities of the cross-bars into which the ends of the pipes entered; the cross-bars themselves were also formed of cast-iron pipes. M. Wilbeking gave a design for two bridges upon this principle, one of 111½ feet span, by 11 feet 2 inches rise, and the other of 292 feet span by 29 feet rise. The pipes were to be about 8½ inches diameter, by 1½ inch thick, those forming the cross-bars being something less. Each of these bridges was composed of three parallel ribs, with two diagonal or bracing ribs. The spandrels were filled in with radial supports, stiffened by diagonal bars, or by wrought-iron rings. The possibility of keeping such a system in form is extremely problematical.

Messrs Jessup erected two iron bridges of great simplicity at Bristol, the spans being 100 feet, and the rise 15 feet. The arch is composed of six ribs made of two castings united at the crown, and each rib is provided with seven uprights, which sustain the platform of the roadway; the ribs are held together by nine wrought iron cross-bars or ties. Each of these bridges contains 150 tons of grey iron, and was constructed at a cost of £4000.

Plan of Roadway

Fig. 8.—Craigellachie Bridge.

* Mr Pope, in his work upon Bridges, expresses his indignation in strong terms against many scientific gentlemen of that day for their seeming little faith in his design. For examples in iron bridges, we are much indebted to Mr Telford. A very fine iron bridge (see fig. 9), by that gentleman, spans a natural channel formed by the Spey in the Craigellachie rock. The span is 150 feet, and the arch is composed of four ribs, the rise being 20 feet. Each rib consists of two concentric arcs, forming panels, which are filled in with diagonal bars. The roadway is only 15 feet wide, and is formed of another arc of greater radius, to which is attached the iron railing; the spandrels are filled by diagonal ties, forming coarse trellis-work. The bridge rests upon stone abutments, with circular castellated towers and curved wing walls at each approach. This bridge is undoubtedly open to the objection, that it consists of two very dissimilar arches, which, owing to the variations of temperature, must be subject to very variable strains. The total cost of this bridge, including the blasting of rock on the west side of the river, was £8200.

Another bridge very similar, and of the same span, was erected by Telford over Dornoch Frith, called Bonar Bridge.

Several iron bridges, about the same date, were erected by Mr Rennie—one at Boston, over the Witham, has a span of 86 feet, with a rise of only 5 feet. Owing to the unequal cooling of the ribs in the foundry, or from some other cause, they were fractured in several places when loaded, but notwithstanding these fractures, the arch generally preserved its form.

We now come to the description of the two iron bridges erected over the Thames, in London, viz., Vauxhall Bridge and Southwark Bridge, the latter being in every respect a most remarkable structure, whether as regards its bold proportions or architectural effect.

The bridge at Vauxhall was designed originally by Mr James Walker; but in consequence of some disputes in regard to its construction, four engineers were ultimately engaged in its erection, namely, Mr Ralph Dodd, Sir James Barltlam, Mr Rennie, and Mr James Walker. It consists of nine arches, the span being only 78 feet, and the rise 29 feet, the piers are 12 feet thick; the roadway is 36 feet wide between the railings, including the footpaths of 6 feet each. Each arch is composed of ten cast-iron ribs 18½ inches deep, with a double flange 6 inches wide, the thickness of the middle web is 1 inch, and that of the flanges 2 inches. Each rib, with its spandrels, consist of three castings; there are, consequently, only two joints which divide the arc in three equal segments; the spandrels consist of light vertical panel work, the verticals in section forming a cross. The upper line of the ribs is straight, forming two inclines to suit the slope of the road; the ribs are united by diagonal strutting-frames, of which there are five rows in each arch. The ribs abut upon cast-iron plates, which are embedded in the pilaster of the piers and abutments. The roadway plates are of cast-iron, with flanges on the upper side, by which they are bolted together. Over each of the four faces of the middle piers, there is a cast-iron alcove; the railing is 4 feet 6 inches high, and composed of cast-iron verticle bars. The outline of the superstructure is a curve between the middle piers, with two straight inclines, tangents to it. The total length of the bridge is 809 feet. The first stone was laid on the 9th of May, 1811, and the bridge was opened to the public in July 1816. The iron-work was cast at Butterley, in Derbyshire, and the cost was £300,000 for the whole of the work included in the structure.

The Southwark Bridge (see fig. 10), designed by Mr Rennie, crosses the Thames between London and Blackfriars' bridges, and as an example of arch construction, stands confessedly unrivalled as regards its colossal proportions, its architectural effect, or the general simplicity and massive character of its details. The depth of the river being from 30 to 36 feet at high water, Mr Rennie decided on crossing it with three arches; the centre one having a span of no less than 240 feet, and the external arches 210 feet each; the rise of the centre arch is 24 feet, and of the external arches 21 feet. The piers and abutments are built of Scotch granite, and rest on platforms upon timber piles, protected by timber sheeting. The length of each pier, including the cutwater, is 78 feet, and the width 24 feet; the height from the cornices of the centre arch, to the surface of the water at high spring tide, is 42 feet; the total length of the bridge, with the abutments, is 800 feet, and the width of the roadway between the railing is 42 feet, which includes two footways of 7 feet each. Each arch consists of eight ribs, each rib being formed of thirteen vertical cast-iron segments or voussoirs; they do not abut immediately upon each other, but upon an intervening transverse plate, extending across the eight ribs at the junction of every voussoir, a most efficient bond being thus obtained between the individual ribs: there are fourteen such trans- verse plates on each arch, including the two at the abutments. The voussoirs are 20 feet long, and 6 feet deep at the crown, and gradually increase to 8 feet deep at the abutments; they are 3½ inches thick, and have double returns, 4 inches wide at their abutting edges, by which they are bolted to the transverse plate; fillets are cast upon this plate for the introduction of wedges to insure perfect contact throughout the whole depth of the voussoirs. The spandrels are cast independent of the voussoirs, and attached to them by means of a number of dental projections fitting into corresponding recesses in the voussoirs, both of which are dovetailed; their form is exceedingly simple and efficient; they consist of lozenge-shaped panels, formed by diagonal lines, and intersected at about the middle of the depth by a curved line much flatter than that of the arc of the voussoir.

Diagonal bracing-frames are introduced between each consecutive spandril, and the whole system is farther secured from lateral motion by means of four diagonal lines of bracing-bars intersecting each other and extending across every arch. The roadway is formed of cast-iron plates 4 feet wide, and alternately 22 and 11 feet long. These plates are strengthened by ribs, and are bolted together by flanges at their edges, they break joint with each other and rest alternately on six and four ribs, and thus contribute materially to the lateral strength of the structure. The parapets and cornice, as well as the balustrade, which is 4 feet 8 inches high, are all of cast-iron.

The iron-work was cast by Messrs Walker & Co., of Rotherham, and the total weight was about 5780 tons; each rib was carefully fitted together at the works before it was shipped for London. The total cost, including the approaches, connecting-avenues, &c., &c., amounted to about £800,000. The work was commenced on the 23rd of September 1814, and the bridge was opened in April 1819.

No provision having been made for expansion and contraction, considerable inconvenience arose from its effects, the paving stones of the footpaths having been broken and disturbed in adapting themselves to the change of form of the arches. The enormous weight of this structure contrasts rather disadvantageously with that of others we have described, but the importance of unusual durability in such circumstances was doubtless duly considered.

A short time after the completion of Southwark Bridge, two cast-iron arches were erected by Telford, viz., an elegant arch across the Severn at Tewkesbury (see fig. 11), of 170 feet span, with a rise of 17 feet only, consisting of six ribs about 3 feet 3 inches deep, the spandrels being filled in with light diagonal work; and an arch, of a span of 180 feet, over the Gloucester and Birmingham Canal at Galton, with a rise of 18 feet; in this case, the ribs consist of diagonal panel work.

It will be observed that all the iron bridges we have described have been arches, and the material used cast-iron. Wrought-iron has been but partially introduced, although its use had been suggested for such structures at an early period; it had, however, been used in the construction of suspension bridges, of which the Menai Bridge, by Telford, was one of the earliest and finest examples. It is evident that in many, if not in all of these structures, the arch has been treated as an arch of equilibrium, and that the two principles on which such construction should be based, were but imperfectly understood. The stability of an arch of masonry depends on its own weight, and the equilibrium of its individual parts, for maintaining its form; such also is the case to a great extent with cast-iron arches of colossal dimensions, such as the Southwark Bridge, where the weight of the rolling load bears only a small proportion to the weight of the structure itself; but the theory of equilibrium, as applied to arches of masonry, is evidently inapplicable when the rolling load bears so large a proportion to the weight of the structure, as is the case in most arches constructed of iron. Another important difference arises from the fact, that arches of masonry owe much of their stability to the friction and adhesion of the large bearing surfaces of the stones which form the voussoirs, whereas, the bearing surfaces of the segments of cast-iron arches are necessarily limited in dimension, and being in all cases attached by bolts, the arch is rigidly restrained from assuming a position of equilibrium and rest. In the Southwark Bridge, this difficulty was partly met by the introduction of the wedges we have described; and on the Pont d'Austerlitz, by ultimately closing the joints with spelter run in between the voussoirs. It is evident, on these grounds, that a cast-iron arch cannot be considered as a mere system of voussoirs assuming a position of equilibrium, but rather as a rigid structure, consisting of two inclined struts or beams, abutting against each other horizontally at the summit of the arch, and upon this assumption, we can arrive at the true direction and amount of strain, each semi-arch being treated as a rigid beam. Such considerations render the calculations of the strains in an iron arch extremely simple.

It will be observed, that in all the arches hitherto described, the thrust is resisted directly by the abutments, as in arches of masonry, and not by the tie-rods, as in the ordinary "bowstring girder," they are all therefore true arches, their claim to originality arising from the introduction of iron in lieu of masonry, and from the ingenious and skilful arrangement of detail displayed by the engineers.

We have now arrived at an epoch in civil engineering, which at once enlarged tenfold its sphere of action, and rendered impossible all reference to experience or precedent; and the arch and the beam, as well as every other established principle of construction, underwent, with miraculous rapidity, entire modification, and their application became wonderfully extended.

We allude, of course, to the introduction of railways, in which the application of iron takes an entirely new direction. The success that had attended the use of tramways in some collieries, led, in 1823, to the construction of the Stockton and Darlington line. The Liverpool and Manchester line, however, which was immediately afterwards commenced, and where the locomotive engine was first successfully applied, ranks as the true prototype of our present system.

Hitherto, bridges had been applied generally to high roads where inclined approaches were of small importance, and in determining the rise of his arch, the engineer selected any headway he thought proper, while every other consideration was similarly made subsidiary to the problem of constructing the bridge itself, and the completion of a single large bridge was an epoch in engineering history. On the introduction of railways, hundreds of roads, rivers, and valleys had at once to be spanned with level roads. Time was as important an element as economy or durability in the erection of these structures, while every conceivable difficulty arose from their limited headway, their bad foundations, their oblique directions, or their gigantic dimensions. Navigable waters, as well as crowded thoroughfares, had now to be crossed without interference with existing traffic, and the ponderous locomotive dashed over these new and hastily constructed works, instead of the quiet team. The arch was evidently at once inapplicable to the bulk of such requirements; new principles of construction became imperative, and the beam, with all its numerous modifications, at once superseded the iron arch as completely as the locomotive did the stage coach. The earliest simple iron beams of which we possess any account, are those used by Mr Telford in building a cotton-mill in Salford, in the year 1800 as recorded in "Tredgold's Essay on Cast Iron," by Hodgkinson, part ii. But the application of simple cast-iron beams to the construction of bridges originated with the late George Stephenson, who employed them on the Liverpool and Manchester Railway.

It will now be more convenient to discontinue our chronological history, and to avoid repetition, we shall, in the next chapter, give a classification of the various forms of girders now in use on railways, describe their general principles of construction, and proceed with our descriptions of some of the most remarkable examples. Under their respective heads, we here give, in a tabular form, some of the dimensions and other characteristics of the works hitherto described.

**Tabular List of the Bridges constructed as described in the First Chapter.**

| Name of Bridge | No. of Openings | Span | Rise | Total weight of Iron-work | Cost | Date of Completion | |----------------|----------------|------|------|--------------------------|------|-------------------| | Coalbrookdale | 1 | 100 ft. | 50 ft. | 378 tons | | 1779 | | Buildwas | 1 | 130 ft. | 40 ft. | 174 tons | | 1796 | | Sunderland Bridge | 1 | 230 ft. | 40 ft. | 250 tons | | 1796 | | Laason Bridge | 1 | 43 ft. | 10 ft. | 263 tons | | 1794 | | Saltash Bridge | 1 | 180 ft. | 10 ft. | 263 tons | | 1802 | | Pont du Louvre | 9 | 57 ft. | 10 ft. | 263 tons | | 1803 | | Pont d'Austerlitz | 5 | 106 ft. | 10 ft. | 263 tons | | 1803 | | St Denis | 1 | 39 ft. | 5 ft. | 263 tons | | 1808 | | Bristol Bridge | 1 | 100 ft. | 15 ft. | 150 tons | | | | Craigellachie Bridge | 1 | 150 ft. | 20 ft. | 8,200 tons | | | | Witham Bridge | 1 | 86 ft. | 5 ft. | 263 tons | | | | Vauxhall Bridge | 9 | 78 ft. | 29 ft. | 300,000 tons | | 1816 | | Southwark Bridge | 3[1] | 240 ft. | 24 ft. | 5780 tons | 800,000 | 1819 | | Tewkesbury Bridge | 1 | 170 ft. | 17 ft. | 263 tons | | | | Galton Bridge | 1 | 150 ft. | 13 ft. | 263 tons | | |

**CHAPTER II.**

We have, in the last chapter, traced the history of iron bridges of rigid form down to the re-introduction of the primitive straight beam or girder—apparently no doubt a retrograde step, when compared with the elaborate and elegant structures we have been considering, and on which so much scientific investigation and mechanical skill had been bestowed. We shall find, however, that the beam rapidly outgrew its original simple form and dimensions, and it is now scarcely to be recognised as the parent of those magnificent structures (far exceeding in dimensions the largest arches), which have become our most prominent monuments of engineering enterprise and skill. We cannot fail to be struck with the reverse order of progress which obtains in architectural history, where we find the beam or lintel characteristic of all the early temples, the arch and abutment characterising the progress of scientific construction.

The word beam, or girder, has thus entirely changed its original signification, and some notice of its present extended meaning is requisite. If we consider the various means employed for crossing space, we find, first, that the weight of the structure and its load is transferred to the bearing points on each side of the space to be crossed, always exerting there a vertical force or pressure corresponding with the weight; and secondly, that these vertical forces or pressures are invariably resolved at the centre of the span into direct horizontal forces.

Now, in order that vertical forces may be transformed into horizontal strains, and equilibrium maintained, some fulcrum or resistances must be interposed, and equivalent horizontal forces of an opposite character be exerted; and these fulcra and forces may either exist within the structure itself, where they are termed the "strains on a beam subjected to transverse pressure," or may be external, in the form of corresponding horizontal pressure upon the points of support, in which case the pressure is called "the thrust of the arch;" the oblique direction of the thrust of the arch is thus the resultant of the direct vertical weight of the mass itself, and the direct horizontal reaction of the thrust at the centre.

We have thus two distinct classes of bridges,—First, those in which the horizontal strains are counterbalanced by corresponding horizontal resistances at the abutments, the strain on them being consequently oblique, tending either to draw them together, or thrust them asunder. Suspension bridges and arches are evidently of this class, the simplest type of which would be two abutting struts as in the figure—

Secondly, those bridges in which the pressure upon the abutments is purely vertical, the horizontal strains being met by corresponding horizontal strains within the structure itself, the simplest type of such structures would be two abutting struts with a connecting tie-chain, as in

It is to every construction of this latter class that we propose to apply the term beam or girder.

All bridge structures will be thus included under these two types. The various forms of beam or girder now in use may, however, be again further subdivided, and we shall adopt the following classification:

First Type. 1st. Arches. 2d. Suspension bridges. 3d. Simple beams, including flanged girders, whether with plain or trellis work sides.

Second Type. 4th. Trussed girders. 5th. Bowstring girders. 6th. Tubular or hollow girders.

(1.) Iron Arches.

It would be tedious to devote much more space to a description of those structures, after the numerous examples cited, more especially as, in their application to railway practice, no important novelties were introduced.

The true theory of iron arches has been partially alluded to, and it has been shown that all such structures should be considered as consisting of two inclined struts—it is evident, however, that these struts being curved in the form of the arch, the load will not have an uniform effect upon them—but this adds little difficulty to the problem. The horizontal forces we have shown to be equal at the springing and centre of the arch; and the vertical pressure which is at each springing equal to half the weight of the centre structure, is, at any other point, equal to half the weight included between that point and a corresponding one on the other side of the centre.

The laws which govern the horizontal and vertical forces being thus known, it is only necessary to calculate their amount and their resultants to show the magnitude of the force in the direction of the curve at any point. The joints at each point should naturally be at right angles to the direction of these resultants, and the depths of the voussoirs should correspond with the amount of pressures.

The horizontal force, which is everywhere constant, may be found upon very simple considerations. It depends, first, upon the depth of the arch, or rise, as it is termed. Secondly, upon the horizontal distance of the centre of gravity of the half-loaded arch from its springing; and thirdly, upon the amount of load.

In considering the strains caused by the weight of the arch itself, exclusive of the load, it may be well to observe that, supposing the arch to be uniform in section, its weight per unit of horizontal dimension will not be uniform; and therefore, the horizontal distance of the centre of gravity of the half-arch from the abutment must be the element of length in the calculation, and this horizontal distance will vary, of course, according to the figure of the arch. Similar reasoning must be observed with regard to spandril arches, or those with a horizontal top chord. From the preceding remarks, it is evident that when the weight of the arch itself is considerable as compared with the load, it will be necessary, as we at first observed, to take the horizontal distance of the centre of gravity of the loaded half-arch, which will be at a point between that of the arch itself and that of the load, which, if uniform, will be at the quarter span.

The following simple proportion will therefore give at once the horizontal thrust:

As the depth of the arch is to the horizontal distance of the centre of gravity of one half of it from the springing, so is the weight of half the arch to the horizontal thrust throughout its length.

Let A B be such an arch, the points A and B being taken at the centre of the springing. Let the rise at the centre, measured from A B to the centre of the middle voussoir, be 10 feet, and let the whole weight of the structure and its load be 100 tons; the girder itself forming but an insignificant portion of that weight, the centre of gravity may be taken at the quarter span, or 40 feet horizontally from the point A.

Then as $10 : 40 = 50 : 200$ (The horizontal strain and the centre and abutments.

The same reasoning applies to all the description of beam that we have tabulated. In the "Bowstring," the horizontal thrust of the arch or bow is resisted by the tensile strength of the tie-rod or chain; and in the plain girder, by the tensile resistance of the bottom web. To determine the horizontal strains in all these various forms of beams a very simple rule is given in "Britannia and Conway Bridges," page 195, which is—

$$S = \frac{WL}{8d}$$

where S is the horizontal strain in the middle of the top or bottom member of the beam, W the total uniform load including that of the beam L the span, and d the depth (or in an arch, the rise). Applying this rule to the example just given, we shall have—

$$\frac{200}{100 + 150} = \frac{8 + 10}{8 + 10}$$

which agrees with the method before used.

We will call attention to another most useful and practical formula, explained also at page 196 of "Britannia and Conway Bridges," and which is of extremely general application, namely,—that in all arches, suspension bridges, tubes, flanged girders, or bowstring girders, and, in fact, every similar structure with top and bottom members, whenever the depth is $\frac{1}{2}$th of the span, and the load, including the weight of the structure, is equally distributed, the horizontal strain in the middle of both top and bottom members of the system is exactly equal to twice the weight of the system with its distributed load. Again referring to the example which is given in the proportions mentioned:

\[2 \times 100 = 200\] as before.

It must be remarked that, in solid beams, trellis beams, or any of those which have their top and bottom webs attached by intervening struts or continuous plates, the horizontal strains in such top and bottom webs are not, as in the tie of the "bowstring," everywhere equal; but they diminish as they approach the bearings, where they become zero. The rules we have given, when applied to these beams, only determine the horizontal strains in the middle.

Towards the end of this chapter, when describing beams of this class, we shall treat more fully of the theory of the various strains to which they are subjected. As regards the arch, from what we have shown, the reader will meet with no difficulty in finding the horizontal thrust, with the centre of gravity of the semi-arch, in any position; and, as we propose to confine ourselves to the principles of our subject, we shall leave to the reader all mathematical application, merely observing that a simple practical way of discussing the arch will be found to be its imaginary transformation into a beam, and the application of similar reasoning to its investigation.

The requirements of railway practice have by no means been favourable for the introduction of the iron arch the height required, from the circumstance that the roadway must be over the top, the practical difficulty of meeting the thrust, and the necessity of a perfect stability in its foundations, have all been drawbacks to its use, although some of our most elegant and efficient railway bridges are cast-iron arches.

The earliest examples we have of such an application of cast-iron arches are in a series of three bridges for crossing the London and Birmingham Railway over the Grand Junction Canal at Blisworth, Boxmoor, and Nash-mill.

That at Blisworth is of 50 feet clear span, and consists of six cast-iron arched ribs, having a rise of 8 feet; the four inner ones are arranged in two pairs, one under each line of rails; the two ribs composing each pair being 4 feet 11 inches from centre to centre, and a 6-feet space between each pair; the two single outer ribs are again 6 feet from those in pairs; making a total width from centre to centre of the outer ribs of 27 feet 10 inches. The depth of the ribs is 2 feet 3 inches at the springing, diminishing to 2 feet at the crown; their thickness is 2 inches, with a projecting flange 6 inches wide at the top and bottom; making a total sectional area of 51 square inches in each rib at the crown. They rest on cast-iron skew back plates, and these again on blocks of stone let into the brick piers. The ribs are each made in three equal segments bolted together, and are connected by a system of trussing which we shall hereafter describe. The haunches are filled in with three separate castings, one over each spandril, and one as a saddle over the crown. The pattern consists of bars of a cruciform section, crossing each other diagonally, and forming diamond-shaped panels, decreasing in size towards the crown, whose upper apices are connected together by a rib or top table, and their lower ones connected to the main rib by being keyed in between projections upon it. Upon the top tables just mentioned, and firmly bolted to them, are placed strong cast-iron plates 3 feet wide, \( \frac{3}{4} \)th of an inch thick, with flanges 4 inches deep all round, and diagonal flanges from corner to corner. These answer the double purpose of steadying and bracing together the spandrials, and also of carrying the ballast, which, however, is not used for bedding sleepers, the rails being carried in chairs resting on longitudinal balks of timber, and bolted down through them to the top table of the spandrials.

The peculiar feature of the bridge is in the system of trussing employed to connect the main ribs. At equal distances along the curved rib there are cast-iron struts furnished with skew ends, with bevelled edges, for the purpose of keying them in between projections cast on the main ribs; these struts are 12 inches deep, 2 inches thick, and all radiate towards a line joining the centres of curvature of all the arched ribs. The skew end of one strut is placed opposite that of the strut on the other side of the rib, so that they form a zig-zag line, the general direction of which is parallel to the abutments; between these struts are placed distance pieces of a cruciform section, with broad ends with bevelled edges, and keyed in between projections on the struts, in the same manner as the latter are fixed to the main ribs. The skew-back plates before mentioned extend the whole length of the abutment, and are of an irregular shape, so as both to fit the springing of the arch, and also to radiate in the same manner as the struts above mentioned, forming, indeed, the last of these struts on each side.

It will be observed that the whole of the bearing portion of this bridge is put together without any bolts (with the exception of those at the junctions in the main ribs, and those fastening the platform plates to the top tables of the spandrials), every joint being made with keys in the manner just described, so as to render motion in the joints almost impossible, and to assimilate the system, as it were, to one entire piece.

The bridge, indeed, though not perhaps remarkable for its great span, is one that justly deserves notice for the extreme care bestowed by the designer on the minutiae of all its parts, and the great rigidity given to it by the system of trussing so well adapted to the purpose.

The bridge over the canal at Boxmoor is of much the same description; the arrangement of the ribs is precisely the same as of those at Blisworth, but the span and rise are greater; the former is 66 feet, and the latter 11 feet 9 inches, the depth being 2 feet 9 inches at the springing, diminishing to 2 feet at the crown; the spandrials are similar to those at Blisworth, and also the top plates, except that they are lozenge-shaped instead of square. The trussing of the main ribs is different, the system of keying not being so entirely adhered to. It consists of cast-iron struts of a cruciform section keyed in between the main ribs in lines perpendicular to the abutments, and of cast-iron pipe struts, through the entire length of which wrought-iron tie-rods pass, and which extend from side to side of the bridge parallel to the abutments. These also are keyed in between the main ribs, but are not in any way connected with the struts described as being placed at right angles to the abutments.

The bridge at Nash-mill is precisely similar to the one at Boxmoor.

The outer ribs of these bridges are surmounted by light cast-iron railing which extends some distance each way past the arch to the top of the slopes on either side, and gives the bridge a neat and pleasing appearance.

There is also an elegant bridge on this principle, erected on the Midland Counties Railway over the River Trent, at its confluence with the Soar, near Sawley. (See fig. 15.) It consists of three segmental arches of 100 feet span, the radius being 130 feet. They are similar in principle to the arches of Vauxhall Bridge, but possess considerable architectural beauty, the spandrials of the ribs being formed with vertical panels in the Tudor style. Each arch consists of six ribs, strutted and tied by bracing-panels and rods; the ribs are double-flanged, and the rise is \( \frac{1}{5} \)th of the span; the depth of the rib is 3 feet at the springing, and 2 feet 6 inches at the crown; they are each in three castings, and the work is admirably executed. The platform is of timber; the width between the Balustrades being 27 feet. This bridge was completed by the Butterfly Company in 1839; its situation amidst fine scenery adds much to its successful artistic effect.

Numerous other examples might be given on all our Among the boldest structures of this class which have been contemplated, the arches designed to cross the Menai Straits, on the site of the present tubular bridge, are perhaps the most remarkable. These arches, two in number, were of great architectural beauty, the span being 350 feet, with a rise of 50 feet. They are engraved and described in *Britannia and Conway Bridges*, page 17, and plate 32. The process proposed for their erection was equally remarkable. It was necessary to dispense with all centering, which was to be effected by resisting the horizontal thrust of each successive voussoir by tie-rods passing over the pier, and attached to a corresponding voussoir on the opposite side. Each semi-arch would thus form a bracket tied back by an opposite semi-arch, and the horizontal thrust being thus entirely destroyed, the semi-arches would exert no pressure on each other when they met over the centre of the span, while the whole of the rods employed would only equal in their combined section the sectional area of a chain of sufficient strength to resist the horizontal thrust of the complete arch; the consideration of the strain on these individual rods affords an instructive illustration of the actual strains on the voussoirs of an arch.

Before leaving this part of the subject, we may allude to the introduction of deep iron arches in the construction of roofs, of which an interesting example exists at the Great Western Railway Station at Paddington, and in the roof of the transept of the Crystal Palace. The investigation of the strains in the arch in such cases is extremely difficult. The unequal distribution of the load, the great degree of curvature, and consequent great vertical depth from the springing, together with the attachment of the arch to its supports—all complicate the problem.

These curved ribs, however, are not true arches, since they may be considered as nearly free from thrust.

(2.) Suspension Bridges.

Under this head, we only intend, as before explained, to notice the attempts that have been made to render such structures sufficiently rigid for railway travelling. We may, however, observe, that similar reasoning to that which we have applied to the arch may be applied to suspension-bridges,—i.e., we may consider them as girders, by imagining the existence of a horizontal strut to resist the tension of the chains.

In comparing a suspension-bridge, or an arch with a girder of the same span, depth, and strength, it is evident that while the suspension-bridge will correspond with the lower member of the girder, the arch will correspond with the upper member, therefore there must be a theoretical advantage possessed by the suspension-bridge and arch in weight over the girder; but the arch is a structure which requires so many additions to preserve it from change of shape, that it cannot practically be taken as merely suffering compression. With the suspension-bridge the case is entirely different; the chain being in a state of equilibrium preserves its form, independent of all extraneous assistance, possessing unaided all the qualifications for resisting the strains of a horizontal load. The suspension-bridge is thus, undoubtedly, the lightest of bridge structures; and the only drawback to its application to railway purposes is its flexibility, which renders it unfit for the support of a rigid roadway. To overcome this difficulty, and combine the rigidity of the girder with the advantages of the chain, has been a problem which has naturally occupied the attention of engineers.

The construction of a suspension-chain for the erection of the tubes intended for the Britannia Bridge, for a long time occupied attention; but this was finally abandoned, owing to the difficulty arising from the expansion and contraction of the chain, which it was proved in a span of 460 feet, with versed sine of 40 feet, would amount to 4 inches. Such a degree of extension would increase the versed sine, and lower the centre portion of the tube as much as 9½ inches. Unless, therefore, the roadway be pliable enough to follow this descent and rise of the chain, it is evident that the chain would be at times either sustaining the whole weight or none at all. The trussing on any rigid roadway suspended from a chain must, therefore, be sufficiently pliable to yield to this motion of the chain, and to be uninjured by its constant action, consequently it must be subject to considerable deflection on the passage of a heavy load. The requisite bracing for securing even this partial rigidity would add, moreover, great weight and cost to the structure, so that, in fact, the economy of the chain would disappear; and, where circumstances will permit, it is far preferable to convert the chain into the lower member of a rigid beam, as in the Chepstow and Saltash bridges by Mr Brunel, which will be described under the head of Trussed Girders.

In the crossing of the River Niagara over the Niagara Falls, where scaffolding or the floating of any kind of structure to its place were equally impracticable, a suspension bridge was the only possible expedient; and the skilful and successful manner in which the difficulties of such a problem have been met deservedly characterize this work as one of the most remarkable that claims our attention.

The Niagara bridge which has been lately completed by Mr Roebling is indeed the only successful railway suspension bridge of large span. It crosses the Niagara River at a height of 245 feet above the water by a single span of 821 feet 4 inches, and forms the connecting link between the American States and Canada.

The superstructure may be best described as a hollow rectangular box, 18 feet deep and 24 feet wide, on the top of which the railway is laid, while the bottom, which is 25 feet wide, forms the roadway for public traffic—both these floors are constructed of timber beams; and each connecting side consists of a row of double posts or uprights of timber, each pair being 5 feet apart; between them wrought-iron diagonal bars are made to pass, extending each way to the fourth pair of posts at an angle of 45 degrees. The upper or railway floor is suspended from two wire cables at intervals of 5 feet, and the lower floor is suspended at similar intervals from two other wire cables which have a deflection of 10 feet more than the upper ones; these cables, four in number, are each 10 inches in diameter, and composed of seven strands, each containing 520 wires, making a total of 3640 wires. One strand forms the axis round which the other six are twisted; a section of the cable is shown in the accompanying figure. Sixty wires are equal to 1 square inch of solid section, therefore the total area of each cable is 60-4 square inches; or the total sectional area of iron supporting the structure is 241-6 square inches.

Each cable rests upon a separate saddle, there being two on the top of each of the four towers. The saddles are placed on ten cast-iron rollers, 5 inches diameter and 25½ inches long, which bear upon cast-iron plates 8 feet square and 2½ inches thick, strengthened by three parallel flanges which form two compartments for the reception of the saddles.

The ends of the cables are attached to cast-iron shoes, in each of which is inserted a wrought-iron pin which forms the connection with the anchor chains. These anchor chains are each imbedded in a solid shaft of masonry 7 feet by 3 feet, enlarged at the bottom to form a chamber 8 feet square cut in the rock. The shafts are sunk to a depth of 25 feet on the New York side, and 35 feet on the Canada side.

Each anchor chain is composed of nine links, the eight lower links being 7 feet long, and the ninth or uppermost 10 feet long. The lowest link consists of seven wrought-iron bars, 7 inches by 1¼ inches each, and amounting collectively to an area of 69 square inches. They are secured to a cast-iron anchor plate, by a pin 3½ inches diameter. From the fourth link the chain curves, and the section is gradually increased to an area of 93 square inches. There are two towers at each end of the bridge, based upon a mass of masonry 60 feet by 20 feet, which is pierced by an arch 19 feet wide, forming the entrance to the lower roadway. The towers are 60 feet high, 15 feet square at the base, and 8 feet square at the top.

Above the floors are 64 diagonal stays, extending from the saddles to the suspenders, amongst which they are equally distributed; they are formed of wire-rope 1¾ inches diameter. There are also 56 stays attached at their upper extremities to the soffit of the bridge, and at their other ends well anchored to the rocks below. The superstructure is thus tied down as well as suspended, and all undulations directly resisted.

The weight of the bridge is estimated as follows:

| Timber | 910-130 lbs. | |--------|-------------| | Wrought-Iron | 113-120 " | | Castings | 44-382 " | | Rails | 66-740 " | | Cables between towers | 534-400 " |

The total weight being 1668-722 lbs. = 745 tons.

The bridge was commenced in September 1852, and opened for traffic in March 1855. The total cost was £80,000.

It may scarcely be necessary to observe, that were a span of this magnitude required to cross a navigable channel, it would be impossible to adopt the system of stays we have described, and which are so essential in rendering the bridge sufficiently rigid for railway traffic. The engineer has here most successfully and judiciously availed himself of this and every other advantage which the peculiar site afforded, though, under ordinary circumstances, suspension bridges are not applicable to such traffic. Even in the bridge we are describing, it is found necessary to limit the speed of the trains to three miles an hour.

(3.) Simple Beams.

Under this head we shall include not only rectangular prismatic beams, which are but little used in bridge construction, but more especially that particular form of beam with top and bottom flanges, to which the term flanged girder is generally applied, the vertical web of such girders consisting either of open work or solid plate.

The investigation of the strength of a prismatic beam to resist the effect of a horizontal load, has engaged the attention of mathematicians from the time of Galileo, who considered that a beam loaded horizontally was only subjected to one strain, and consequently only exerted one resistance, viz., that of tension, or, in other words, a beam under pressure would turn on a fulcrum close to that pressure; but by the able experiments and investigations of our more modern philosophers this theory was shown to be fallacious, and we are greatly indebted to the mathematical treatment of the subject by Mariotte, Coulomb, Young, Barlow, Tredgold, and others, without which, and the more recent investigations by Mr Eaton Hodgkinson, we should never have had those magnificent triumphs of art and science which span the widest rivers, and which form the subject of our article. Yet the problem is by no means completely solved, even at the present day, for there still exists a great discrepancy with respect to the calculated and actual strength of a beam.

When, supported at each end, a beam is loaded horizontally, it is found by experiment that its upper surface suffers compression, and its lower surface suffers extension; such compression and extension evidently originating from the vertical support of the beam at each end. Each half of the beam may, therefore, be regarded as a lever in inducing these forces. Now, the beam is imagined to consist of a succession of indefinitely thin horizontal layers, and the compression and extension of these layers is found to be greatest near the upper and lower surfaces of the beam, and to diminish in some ratio towards the centre; it is, therefore, presumed that some layer must exist within the beam in which neither compression nor extension takes place. The position of this layer is called the neutral axis. Now, presuming such to be the case, the extension or com- pression of a layer must be as its distance from the neutral axis, and the force will be as the extension, if we admit the truth of Hook's principle, *Ut tensio sic vis*; also the force of a layer must be as its area, consequently the power of any layer to resist a horizontal load is as its force, its area, and the square of its distance from the neutral axis, and by summing up the moments of the indefinitely thin layers, we arrive at the well-known formula—

$$W = \frac{fbd^2C}{6l},$$

which also applies to a solid prismatic beam loaded as in the figure:


where $W$ is the load, $l$ is the horizontal length from the fulcrum, $d$ is the depth of the beam, $b$ its breadth, and $C$ is the constant derived from experiment. In the figure, however, the top is in tension and the bottom is suffering compression; in fact, the support in the middle may be considered as a weight acting upwards while the weights WW have the same effect as supports acting downwards.

It is remarkable that the importance of depth in an ordinary beam should so long have escaped notice. We find the floors of old houses invariably supported on massive square beams, with no regard to economy of material, or its proper disposal to resist strain. The use, however, of a more expensive material, such as cast iron, called attention to this important subject, and the solid beam merged slowly into the flanged girder; for it was an easy step, when the effect of depth in a beam was carefully considered, to perceive the value of the addition of a top and bottom flange, thus introducing the material where it was evidently acting to the greatest advantage.

The earliest flanged girders used in bridge construction were formed with their flanges of equal strength, and their vertical ribs were much more massive than requisite for the office which they had to perform, namely, that of simply uniting the flanges. The strength of such beams was calculated by peculiar formulae highly mathematical and complicated, insomuch as due account was taken of the strength of the middle web to resist horizontal strains. The mode of calculation generally adopted was to consider the section as a rectangle of the entire depth into the width of the flanges, and then to deduct the value of the side portions, the remainder being the strength of the flanged beam.

The rule deduced from the above reasoning was as follows:

$$W = \left(\frac{bd^2 - b_1d_1^2}{d}\right) C,$$

in which $W$ is the load to be sustained, $b$ the entire breadth, $d$ the entire depth, $b_1$ the breadth, exclusive of the thickness of the web, $d_1$ the depth between the flanges, $l$ the length, and $C$ the constant quantity, as before.

Upon the preceding mode of reasoning, it is assumed that when the outer layer is strained beyond its ultimate strength, the fracture of the entire section must ensue.

It seems reasonable to suppose that, with the above data, it would be only necessary to obtain by experiment the ultimate force of a unit of section to resist a direct strain (represented by $C$ in the foregoing rules), to enable us to calculate the strength of a prismatic beam. Yet, as we have before observed, such a calculation gives a result widely different to that of practice, the actual strength of the beam being invariably much greater than is determined by the calculation. Some assumption that has been made must therefore be erroneous, and the difficulty of the problem is in accounting for this anomaly. By some it has been assumed that the position of the neutral axis is not situated in the centre of gravity of the section of a beam, but is much nearer the compressed side. This, to a certain extent, is going back to the exploded Galilean theory, yet it is a singular fact that experiments agreed very nearly with this view. (See Experimental Researches on Cast Iron, by Mr Eaton Hodgkinson.) Subsequently, in a paper by Mr W. H. Barlow, read before the Royal Society, on a New Element of Strength in Beams subjected to transverse pressure, and called the Resistance of Flexure, an account of some valuable and interesting experiments was given, which were conducted with the greatest nicety, and which seem to prove that the axis of motion is in the centre of a uniform section; therefore the discrepancy before named has yet to be accounted for, and it seems evident that a great error is committed in considering the existence of a neutral axis as it is called, which suffers neither extension nor compression, the fact being that if it is inactive it is only in a horizontal direction, for there is no part of a beam sustaining a load that is not strained in some direction; and wherever a particle of which it is composed can be shown to be free from strain in one direction, in another direction that particle bears its full share of strain. It appears evident that the vertical reaction of the supports is not entirely met by the simple horizontal resistances of an indefinite number of independent layers. The layers should not be considered as independent like the leaves of a book, since they form altogether a solid mass; and we cannot imagine a top layer to be compressed say $\frac{1}{3}$ of its length, and the next below it $\frac{2}{3}$ without the top layer receiving considerable assistance from its less strained neighbour to which it is so intimately united, and which, in its turn, receives successively similar assistance from every other layer between itself and the neutral axis. The external layer is not therefore subject to the strain which is assumed on account of this assistance which it receives from subsequent layers, and fracture does not take place so soon as such a theory foretells.

The mode alluded to of dealing with the forces brought into action on a beam, when subject to transverse strain, must therefore lead to a misapprehension regarding the true directions of these forces; for instance, the neutral axis is supposed to be a point situated between the portion of the beam undergoing compression and that undergoing extension, and consequently neither subject to extension nor compression, it being regarded as absolutely inactive. Now, this would only be true if the forces called into play were truly horizontal, which in fact is what the present theory of beams assumes.

If, however, we consider the relative motions which take place amongst the particles comprising the upper portions of a beam when undergoing compression, and those undergoing extension in the lower portion, it will at once be perceived that an infinite number of diagonal lines, passing through the region near what is called the neutral axis, may be drawn exhibiting the direction of a variety of forces, both compressive and tensile, and which are not represented by the horizontal lines alone taken into account by the above theory. It is conceived that this mode of looking at the subject shows more clearly than any other that no actual neutral axis can exist; that is, no portion of a beam under strain can be in a quiescent or passive state. To illustrate this more clearly, let us refer to the figure which represents the centre portion of a beam, the vertical line being in fact the centre line.

Let us suppose the beam to be loaded at the top point b, then when deflection takes place, the points a and c will recede from each other, and at equal distances from the point d, but the other extremities of these diagonal lines will be fixed in the point b by equal forces, consequently there will be a direct strain down these lines of tension. Again, the diagonals de and df suffer compression, being shortened by the approach of the points e and f.

Innumerable other diagonal lines in all possible angles might be drawn from these points b and d, in which the forces of extension and compression might be traced crossing each other in all directions; therefore, as we have before observed, when a beam is subjected to such a strain as is caused by a horizontal load, and by which pressure naturally takes place, there is no part of such a beam in a quiescent state. Now, respecting the neutral axis referred even to horizontal strains, it is assumed to be free from such strains by the following geometrical reasoning,—viz., if a rectangular parallelogram be bent into a curve, the outer line being extended and the inner line shortened, there must be some line between the two which will neither be extended nor shortened; but it does not follow from this geometrical fact that the line is free from strain because it is unaltered in length. A beam is curved by the effect of strain throughout, and every line in it, excepting the vertical lines, is curved, including, of course, that called the neutral axis; and the fact of that line being curved shows evidently a molecular disturbance which is certainly produced by force, and therefore it is strained.

The other assumption in the above theory, which renders it dependent upon the kind of material of which the beam is composed,—viz., that the reaction of a compressed or extended fibre is proportionate to the amount of compression or extension,—has within moderate limits been proved to be correct as regards wrought iron and cast iron, the amount of the extension or compression caused by 1 ton in a bar 1 inch square, being respectively \( \frac{1}{5} \)th of its length, and \( \frac{1}{2} \)th of its length, or twice as great for the former as for the latter; similarly, if the pressure or tension is twice or three times as great, the alteration in length will also become twice or three times as great, but as we approach the ultimate strength of the iron this law entirely ceases, the compression or extension then increasing in a much higher ratio than the force applied. The assumption, ut tenet sic vis, on which this theory of a beam is founded, only therefore holds good within moderate limits; and this variation, combined with the erroneous assumption of the action of the fibres as being independent layers, before alluded to, gives rise to the anomalies which exist between theory and experiment, and renders the solution of the problem extremely difficult.

To Mr Eaton Hodgkinson the credit is more especially due of pointing out the proper relative proportions of the top and bottom flanges; but the section which these proportions rendered it necessary to adopt, caused the true valuation of the entire section to be so tedious, and even difficult, in the summation of the moments on each side of the neutral axis,—which in this case was not in the mid section,—that that gentleman thought it far better to consider the vertical rib as useless to resist horizontal strains, giving it only sufficient strength to resist lateral strains, to connect the top and bottom flanges, and transmit the strains between them. Therefore, without any complexity of formulae, the strength of the beam was considered to exist in its flanges, and to vary simply as their distance apart, or as the depth of the beam.

The investigations made by Mr Hodgkinson led directly or indirectly to a great part of the information we possess respecting the properties of wrought and cast iron. He found the ultimate resistance of cast iron to be as follows, viz., the extension 6½ tons per square inch, and in compression 40 tons per square inch. He determined, moreover, the variation in length within certain limits of a cast-iron bar 1 inch square subjected to strain to be about \( \frac{1}{5} \)th of its length per ton. Similarly with ordinary wrought iron, the ultimate resistance was determined to be 18½ tons per square inch in extension, and 16 tons in compression, and the elongation of an inch bar \( \frac{1}{5} \)th of its length per ton. He applied these practical results in determining the flexure and strength of girders and pillars, and corroborated the accuracy of his deductions by numberless experiments rivalling in practical value his earlier investigations.

If we neglect the vertical rib of a girder, a very simple view may be taken of its action in resisting a horizontal load, and the result will agree exactly with what we have given with respect to arched beams. In fact, the rule becomes universally applicable to beams generally, and may be briefly explained as follows:

Let ABCD be a girder, loaded at W, and supported on the points A and B. The forces are transmitted along the diagonal lines AW and BW. If we now construct the parallelogram of forces on the diagonals, we shall have \( Wg \) representing the vertical force, or half the weight transmitted to the bearings A and B, and pg will represent the horizontal strain at the centre of the beam, or, taking the whole depth, \( Wn = d \), and \( An = \) half the length, we shall have

\[ d : \frac{l}{2} :: \frac{W}{2} : \frac{Wl}{4d}, \]

which is the horizontal strain at the point W. Again, suppose we consider W to act as a fulcrum with the weight acting at A or B \( - \frac{W}{2} \); the moment of this force will be

\[ \frac{W}{2} \times \frac{l}{2} = \frac{Wl}{4}, \]

and the horizontal strain acting with the leverage \( d \) will be \( S \times d \); and by equating these \( Sd = \frac{Wl}{4} \),

or \( S = \frac{Wl}{4d} \), as before.

By a similar method of reasoning, the horizontal strain at any other point caused by a load in the middle can be determined.

Referring once more to the figure, we will endeavour to determine the strain at the point w.

Still considering the point W to act as a fulcrum, and the weight \( \frac{W}{2} \) to act upwards at each of the points A and B, each half beam will act as a cantilever or as a beam fixed at one end and loaded at the other.

The weight \( \frac{W}{2} \) will act with a leverage \( r = \text{distance} \times C \)

with an effect \( \frac{W}{2} \times r \), and the horizontal strain at \( w \) acts with a leverage \( d \), its moment being \( S \times d \); therefore

\[ Sd = \frac{rW}{2} \quad \text{and} \quad S = \frac{rW}{2d}, \]

which becomes, when \( r = \frac{l}{2} \), \( S = \frac{WI}{4d} \), as before.

The requisite strength of the top and bottom flanges of a girder to support any weight at the centre is thus at once determined.

If the weight, instead of being placed on the centre of the girder, be equally distributed over its length, it is evident we still have a load equal to \( \frac{W}{2} \) at each end of the girder as before, but the horizontal strain at the centre \( W \) is now only half what it was in the former case; it is, in fact, the same as though one-half of the entire distributed weight were accumulated at the centre, and this will be easily understood if we reflect, that although the weight at each end is still \( \frac{W}{2} \), each half of the beam no longer tends to fold itself around the point \( W \), at the centre of the beam as before, but around a series of points equally distributed along the beam as shown in the figure.

The weights \( w_1, w_2, w_3, \ldots \) may be considered as acting at their centre of gravity, half way between the points of bearing, and one-half only of their pressure acts directly at the centre in a vertical direction.

We may suppose the whole weight to exist in the form of two solid bars \( AC, CB \), laid upon the beam, one-half of the weight of which is evidently supported at \( A \) and \( B \), producing no transverse strain on the beam, while the other half alone acts directly at \( C \).

The beam at the centre therefore suffers from an equally distributed weight only half as much as though the same weight were accumulated at the centre, therefore we have for the horizontal strain in the centre, caused by a uniform load,

\[ S = \frac{IW}{8d}. \]

And to find the strain at any other point caused by a uniform load, we have the following proportion:—As the square of the half span is to the strain at the centre, so is the rectangle of the segments into which the given point divides the beam to the strain at that point. Or referring to the figure, we have, expressed algebraically,

\[ \left( \frac{1}{2} \right)^2 : S :: ab : \frac{4abS}{l^2} \]

the strain at the point \( O \).

For all these beams or girders a general rule pertains, which is

\[ W = \frac{adC}{l}, \]

in which \( W \) = the load, \( a \) the area of the section strained, \( l \) the length, and \( C \) the constant as before. With respect to this rule, however, it will be necessary to observe that the constant \( C \) will vary according to circumstances—that is, it will be different for beams loaded and placed in different positions.

The above rule equally applies to a solid section where \( a = bd \), and \( bd^2 = ad \); and to show the analogy between the preceding rules for the horizontal strains and the above, we shall have for a beam loaded in the middle

\[ S = \frac{WI}{4d}, \quad \text{and} \quad W = \frac{adC}{l}; \]

and therefore, in this instance \( C = \frac{4S}{a} \); but if the load be uniform \( C = \frac{8S}{a} \); this constant will, therefore, be different for all the various applications of the load and position of the beam.

We shall now consider those simple girders which have open work sides, including the trellis and Warren girders.

If the top and bottom flanges of an equally loaded girder are united by vertical bars, the elements of a beam are wanting in such a system, and flexure takes place, as in the figure. The diagonals \( CE, C_E, C_E, C_E, \ldots \) become elongated, while the diagonals \( AC, E_C, E_C, E_C, \ldots \) become shortened, this distortion being greatest at the ends of the beam, and nothing at the centre panel \( C_E \).

Assuming the curve to be a circle, the number of the panels to be \( n \), and \( T \) and \( T_1 \) to be tangents at \( C \) and \( C_1 \); it is evident, that since the chords are equal, the angles \( TCC_1, C_1CC_2, \ldots \) are all equal, and the angle \( DCT_1 \), which measures the distortion of the first rectangle, is \( n \) times as great as the angle \( TC_1C_2 \), which measures the distortion of the rectangle at the centre; consequently when \( n \) becomes infinite, as in a girder with plain sides, the angle in the centre = 0, or the strain on the centre diagonals is nothing, increasing regularly towards each extremity. This result must be considered rather as an illustration than a demonstration; but to illustrate this important subject more fully, let us first consider all the verticals and diagonals perfectly inelastic, confining our attention to one panel C₁E₁. It is evident that from the compression on the top flange, the two points C₁, C₂ must have approached each other, and the two points E₁, E₂ must become further removed from each other: and this change of form of the top and bottom flange is greatest at the centre of the beam and nothing at the ends; the ends of the beam CADB do not therefore remain parallel, but approach each other at the top, and recede at the bottom. Now it is of extreme importance to remark, that this distortion of the rectangle C₁E₁ from the compression and extension of its top and bottom lines does not necessarily involve any change in the length of the verticals or diagonals, or, in other words, it may take place without producing any strain in them.

If ABCD is the original rectangle, A₁, B₁, C₁, D₁ will represent the distorted rectangle, the side CD being compressed to C₁D₁, the side AB elongated to A₁B₁, the remaining sides A₁C₁B₁D₁ and the diagonals A₁D₁B₁C₁ remaining precisely of the same length as in the original rectangle, but the line of junction of the diagonals is raised from O to O₁, and the whole depth of the beam is diminished.

The girder may therefore, from mere change in its top and bottom flanges, assume the following form, even if the verticals and diagonals were perfectly inelastic.

If, however, these rectangles are filled in with solid plates, it is evident that these plates must be stretched and compressed, and consequently distorted through every atom of their composition, before the rectangle ABCD can be changed into the figure A₁B₁C₁D₁. Hence the remarkable stiffness and rigidity of girders with solid plates in their sides, and the small amount of deflection in such girders, as compared with those whose sides are formed of open or trellis-work; the difference resolving itself into this—that in the trellis girder deflection is only resisted by the elasticity of the top and bottom flanges, and in the girder with solid sides it is resisted, in addition to the above, by the elasticity of every particle of material of which the sides are composed.

Again, if we suppose the top and bottom flanges perfectly inelastic, and the verticals and diagonals alone capable of extension and compression, flexure takes place solely by the elongation of the diagonals CE₁, C₁E₂, &c., and by simultaneous compression of the diagonals A₁C₁, E₁C₂, &c., as in fig. 24.

In this case the distortion of the rectangles is greatest at the extremities of the girder, and nothing at the centre; the ends would now remain vertical, and, if plates were substituted for the diagonals, the change of figure of these plates would again involve compression, extension, and distortion of every atom of which they are composed.

We have then this important deduction, that as regards that portion of the entire flexure of a girder which arises from extension or compression of the flanges, the trellis-work, whatever may be its strength, does nothing towards its diminution, while the plate, on the contrary, does resist this flexure; and as the distortion is greatest at the centre of the girder, the centre panel in a trellis girder might be omitted; but it is by no means useless to insert a plate on the centre panel of a solid-sided girder, though such has generally been assumed to be the case.

Again, as regards that portion of the whole flexure of a girder which arises from elongation or compression of the verticals or diagonals, it is evident that such change of form is greatest at the extremities, and whether plates or trellis-work be used, the strength of the vertical rib should at those points be greatest, while at the centre it may be reduced to zero.

Now, in practice, the deflection of the girder arises from both the above-named causes combined, viz., from the elasticity of the flanges and that of the vertical rib; and to secure the least deflection possible, it is evidently necessary to strengthen the flanges at the centre, to avoid that which arises from their change of form; also to strengthen the vertical rib at the extremities, so as to avoid the deflection caused by its distortion. We therefore perceive that trellis-work can only be substituted for plates at a sacrifice of stiffness, and that plates are efficient in insuring stiffness, even in the centre panel of the beam.

If we consider the origin of the strain in a girder, we arrive also at the same result. Let A, B, C, D be the flanges of a girder with vertical bars between them. Half the weight of the girder \( \frac{W}{2} \) is supported by the vertical prop AC. Then the strain on AC₁ is to the vertical pressure at A, as the diagonal AC₁ is to the vertical AC; the strain on AC₁ varies therefore as the cosecant of the angle of inclination C₁AC.

Although in a girder with plain sides, the horizontal strain vanishes at the extremities, this is not the case with the trellis sides; for the strain on AE is evidently to the pressure at A, as AE is to EC₁, or varies as the tangent of the angle of inclination C₁AE. If this angle be 45°, then the tension on AE is exactly equal to the pressure at A.

The strut AC₁ may be replaced by the tension-rod CE, and with the exception that the strain will now be a tensile one, it will in every other respect be similar to the strain on A.

If the strut and tension-rod are both used, each may be assumed as resisting one-half of the strain that would come upon either singly.

If a thin plate is inserted, it may be considered as of no effect in resisting compression, but as replacing the tension-rod CE; and with respect to its requisite strength we may arrive at a very useful practical approximation as follows:— If \( C_1 \), \( E_1 \), and \( C_2 \), \( E_2 \) are the verticals as before, the portion of shaded plate, \( C_2 \), \( E_2 \) takes the place of the tension-rod between \( C_1 \) and \( E_1 \), and being subjected to the same strain, requires the same sectional area across the lines \( ab \) and \( cd \). Each fibre in the direction of the shading being also of the same length as the diagonal which they replace, it is evident that the quantity of material required per panel is approximately the same whether the diagonal or the plate be employed. We have alluded to the advantage on the side of the plate with respect to flexure. It is also evident, from the figure we have given of the distortion of the plate, that this tensile strain is not the only efficient resistance it offers to the fracture of the beam, so that the strength arrived at above is certainly on the safe side.

Having determined the requisite strength of the diagonals and verticals of the extreme panel, and of the centre panel, which, in fact, need only be of sufficient strength to act as an independent beam in supporting the rolling load which comes upon it, the strain on the intermediate panels will be as their distance from the extremity. This may be illustrated as follows:


If an equally loaded girder be cut in two at the centre, it will be perfectly restored by the insertion of a strut at \( C \) and a chain at \( E \). The tendency of either half to descend at \( E \) is precisely counterbalanced by the similar tendency of the other half, and no diagonal is necessary to maintain equilibrium. If a weight \( W \) be placed at the centre of one half, this equilibrium is destroyed, the point \( E \) now has a tendency to descend vertically, with a pressure equivalent to \( \frac{W}{2} \), and a strut at \( AC \), or a tension-bar \( BE \) equivalent to this strain must now be inserted to restore stability. Hence the centre panel requires diagonals proportionate to the weight of the rolling load.

Now, if the beam, instead of being cut through at the centre, be cut through at any point \( CE \), it is evident that a strut and a chain will no longer restore the stability of the system, the portion of the beam \( EA \), supposing its weight to be \( W_1 \), acts with a downward pressure \( \frac{W}{2} \) at the point \( E \), which exceeds the downward pressure \( \frac{W}{2} \) of the other portion. We therefore have a true measure of the vertical force at every point of the vertical rib, which is always equal to half the difference of the weights sustained on the two sections of the beam into which the point divides it, and at once determines the requisite dimensions of the diagonals, which are consequently in exact proportion to their distance from the extremity; so that if a beam be supposed 10 feet long, and to have 10 panels, with 1 ton placed upon each panel, the vertical strains commencing from the extremity will be as follows:

\[ \begin{align*} a &= \frac{10 - 0}{2} = 5 \\ b &= \frac{9 - 1}{2} = 4 \\ c &= \frac{8 - 2}{2} = 3 \\ d &= \frac{7 - 3}{2} = 2 \\ e &= \frac{6 - 4}{2} = 1 \\ f &= \frac{5 - 5}{2} = 0 \end{align*} \]

In addition to these strains, we have evidently to add the strain produced by the rolling load.

The strain on the verticals throughout is easily determined when the strain on the diagonal is known, the vertical becoming a strut when the diagonal is a tie-bar, and a tie-bar when the vertical is a strut. When a plate is used, the vertical evidently becomes a strut; and when a diagonal strut as well as a diagonal tie-bar is employed, or when the plates become very thick, as in cast-iron girders, it has evidently no duty to perform, and may be omitted. We have therefore all the elements of a perfect beam in the following arrangements (1, 2, and 3, fig. 31), in which ties are represented by dotted lines, and struts by full lines.

In No. 3 of fig. 31, we have evidently two systems of struts and ties, and omitting either of them, we have the ordinary Warren and Kennard girder as in fig. 32, which is doubtless the simplest possible form of trellis girder that can be constructed, and of which a magnificent example exists in the Crumlin Viaduct in South Wales.

We shall, however, first describe the Newark Dyke Bridge, which, with the exception of some small and badly designed cast-iron girders, was the first, and is still, as regards span, the largest bridge constructed on this principle. Newark Dyke Bridge carries the Great Northern line over a branch of the Trent near Newark, and was erected under the direction of Mr Joseph Cubitt.

This bridge (see fig. 34) consists of four independent girders, viz. two for each line of railway. The roadway is beneath the girders. The top flange of each girder consists of a series of cast-iron pipes butting end to end; the lower flange consists of wrought-iron links, and the flanges are connected by diagonals forming a series of equilateral triangles, and these diagonals are alternately struts and ties. The ties are formed of wrought-iron, and the struts of cast-iron; the length of each side of these triangles is 18 feet 6 inches; over the abutments the diameter is 13½ inches, and the thickness 1½ inch. They are turned and fitted into each other at the joints, where they are connected by eight bolts and nuts.

The diagonal struts and ties are connected with the top flange at the centre of every alternate casting by means of joint pins 5¼ inches diameter, passing transversely through the cast-iron pipes which are bored to receive them.

The bottom flange consists of wrought-iron links 18 feet 6 inches long, rolled in one piece of the uniform depth of 9 inches. They vary in number and thickness, increasing in total sectional area from the abutments to the centre, where there are fourteen links 2½ inches thick. Their ends are swelled laterally to receive the joint pins by which they are connected with the diagonals.

The diagonal links are of the same form and dimensions as those of the bottom flange, as regards length and width, but are increased in thickness from the centre towards the abutments.

The top tube, which is 259 feet long, is formed of 29 lengths of cast-iron pipe; at the centre of the span, their diameter is 1 foot 6 inches, and the thickness of metal 2½ inches.

The diagonal struts are of cast-iron, the general section is that of a cross like the sign \(+\), two sides of which are gradually increased in width from the lower to the upper end, in which is formed a jaw embracing the top tube and the diagonal links on each side of it. The joint pins pass through the two sides of this jaw (see fig. 35), through the two links and the tube, and then extend across the road to the opposite girder, and thus form part of a system of bracing between each pair of girders and over the roadway. At the bottom of the strut there is a circular end piece (see fig. 35), 13½ inches diameter and 5 inches thick, through which is inserted the joint pin similar to that at the top, fastening together the bottom ties and diagonal struts and links, and extending in like manner across the whole width of the bridge.

The horizontal bracing by which the top and bottom of each pair of girders are connected together, and their lateral stability is insured, consists of hollow cast-iron pipes extending across the road, and through which the joint pins also pass.

These struts are cross-braced by diagonal wrought-iron bars 2½ inches wide, and connected with them by bolts.

The platform consists of a flooring of Mersey fir 8 inches thick, resting on the links of the bottom tie. These links are suspended at the middle of their length by a pair of 14th inch rods secured to the top tube at the junction of the diagonals.

On the bearings at each end of the girders are cast-iron triangular frames, strengthened by a perpendicular rib from the apex to the base. They are braced transversely by arched ribs of cast-iron above, and by girders with bracket pieces below, to prevent any rocking motion. At the top of these frames are placed blocks of gun metal 10 inches square, upon which the ends of the top tubes rest. The whole weight of the bridge is thus supported on these frames.

The total length between the supports is 259 feet, and the depth from centre to centre of the joint pins is 16 feet. The clear span between the abutments is 240 feet 6 inches.

The total weight of iron is 244 tons 10 cwt., of which 106 tons 5 cwt. is wrought iron, and 138 tons 5 cwt. cast iron, to which must be added 50 tons for the platform, making the total weight of each bridge 294 tons 10 cwt. The cost, exclusive of the masonry of the abutments, and of the permanent rails, but inclusive of the staging for fixing and the expense of testing, was £11,008.

The Crumlin Viaduct.

As a further example of these girders, we have no engineering monument in this country more remarkable for lightness and novelty of construction than the viaduct, 150 feet span, which crosses the valley of Crumlin in South Wales, at an altitude of nearly 200 feet.

The lofty piers on which it is supported are equally novel, and in perfect keeping with the girders; they are composed of groups of cast-iron columns only 12 inches in diameter, cross-braced, with wrought-iron ties; and the slender and elegant appearance of this gigantic system of skilfully combined struts and ties can scarcely be imagined without seeing it. The merit of this design is due to Mr T. W. Kennard, by whom the viaduct was also erected.

The gradations are evident between the simple form of girders last described, through trellis work of greater and greater closeness until we arrive at the plate in which the trellis bars may be considered infinite in number. We shall not stop to investigate the actual deflections of such girders, which we have already seen is less as the trellis bars are more numerous, nor the effects of counter bracing on initial strain. These investigations, as well as other applications of these principles, such as in the construction of roofs, do not come within our present scope.

The great importance of the subject, however, cannot be too strongly urged upon the attention of the engineer. The evident defects which characterize most of our trellis bridges arise entirely from the want of any generally established principles as to their construction, and the too common error of believing that any number of mere tension bars, however arranged or thickly interlaced, can form an efficient vertical rib to a girder, or in any way modify the central strain on the top and bottom flanges which depends solely on the depth of the girder, and is perfectly independent of the system which connects them.

As the best examples we can introduce of well designed trellis girders we shall select the Boyne Bridge, and some simple trellis girders of peculiarly light construction, erected by Mr Edwin Clark on the Victor Emmanuel Railway in Piedmont.

In the first timber bridges constructed in America on Boyne the trellis principle, the trellis work, and, in fact, the section of all the parts throughout, was nearly uniform. As all the trellis rods in one direction act as ties, and in the opposite direction as struts, there is thus a tendency in such beams to buckle or twist unless the trellis work is proportioned for such strain. It is evident, also, from what we have stated, that as regards trellis work the distortion will be greatest near the extremities of the beams. In fact, this has been experienced in some of our first iron trellis bridges, including the Boyne Bridge, which is the largest, and one of the earliest of these structures, though great skill has been evinced in proportioning the various parts to the strains to which they are subjected.

This magnificent bridge or viaduct was erected by Sir John M'Neil on the line of the Dublin and Belfast Railway over the River Boyne, near the town of Drogheda.

It consists of three spans, viz., of a centre span of 264 feet, and two side spans, each of 138 feet 8 inches. The height above high-water spring tides to the soffit of the bridge is 90 feet.

The bridge is approached at each extremity by a series of arches 61 feet span.

The roadway is supported by two wrought-iron girders, braced together at intervals over the top. Each span is not isolated, but the whole are united into one continuous beam throughout the bridge, and every advantage is taken of the extra stiffness and strength thus obtained.

When several consecutive girders are thus united so as to form one single continuous girder, the pressure and the strains, from the weight of the bridge itself or its load, are entirely modified. Not only is the absolute deflection at the centre of the span decreased, but there are points of contrary flexure in its length, the portion over the piers presenting a concave surface at the under side, and the centre portion between the piers having a convex under surface.

The horizontal strains by this arrangement are now due only to the effect of the load on the spans between the points of contrary flexure, and the beam is virtually shortened. This shortening amounts in a perfectly continuous beam of equal spans to about two-thirds of the entire span.

In the bridge we are describing each girder has double sides formed of wrought-iron bars crossing each other at right angles, and at an angle of 45° with the horizon. The diagonal distance across the meshes, or distance from centre to centre of joints, is 7 feet 5 inches, and the total depth is 22 feet 6 inches.

The pairs of diagonal bars which sustain compression are inside, and connected together by lattice work; the tension bars being placed on the outside, and rivetted to the compression bars wherever they cross.

At the centre of the middle span the bars are 10½ inches wide by ¾ inch thick, and decrease towards the piers, where they are 4½ inches wide by ¾ inch thick.

The cross section of each beam is thus rectangular; the distance between the sides being 2 feet 3 inches, and the total width of the top and bottom flanges three feet.

The trellis sides are not attached directly to the top and bottom tables but to a vertical continuous plate, ¾ inch thick and 17½ inches broad, which is reckoned as part of these tables. The trellis work is thus only 20 feet 10 inches in depth, there being 2 feet 10 inches of side plate, which The bridge was opened for traffic on the 5th April 1855.

The bridge constructed to carry the line of the Victor Emmanuel over the River Isere, at a point between the villages of Montmeillen and Aiguebelle, in Savoy, is on the lattice principle, in which the diagonals forming the web are placed at an angle of 45° with the horizon.

The railway has but a single line of way, and is supported by two girders, which are continuous throughout, forming a total length of 558 feet.

The bridge has four uniform openings of 130 feet 4½ inches, and is supported by stone abutments at each end, and by three piers of solid masonry in the river.

The foundations of the piers are on cast-iron cylinders, three to each pier, 6 feet 6 inches in diameter.

The thickness of the masonry is 8 feet 10½ inches.

The level of rails is about 18 feet above the bed of the river; but the water in floods occasionally rises fully to this height, and, in the spring of 1856, it carried away a similar bridge a few miles lower down, and deposited the entire structure about 20 yards from its site in a perfect state.

The piers and abutments form an angle of 45° with the direction of the stream.

The depth of the girders is uniform throughout, being \( \frac{1}{12} \) of the span, or 11 feet 9 inches in extreme.

The usual width of the squares formed by the vertical gussets is 11 feet 3 inches, therefore there are twelve of these bays over each opening.

Over each pier, and at the abutments, large cast-iron frames are placed on each side of the girder to support the top flange and provide for the additional strain on the web caused by continuity; and beneath these are sets of cast-iron bed plates, between which rollers are inserted to allow of free expansion.

The diagonals of the web that have to withstand principally the force of tension are simply bars of which the width is reduced as the strains become less. For compression the struts are formed of T iron riveted back to back, and likewise graduated in size.

The gussets or vertical struts are made up of small angle irons and plates, which latter are stiffened on their edges by light T iron.

The diagonals and centre plates of the gussets fit between, and are rivetted to the deep vertical plates and angle irons of the upper and lower flanges.

The section of the top of the girder is of the form of a T, while that of the bottom flange is an inverted T.

The horizontal parts are formed of one row of plates, with their covers and packing pieces, and the vertical parts of two plates placed far enough apart to allow of the diagonal struts and ties fitting between and fastened to the horizontal chords by two heavy angle irons.

The roadway is supported by very light transverse open girders of wrought-iron rivetted beneath the lower flanges of the main girders with which they form right angles.

They are placed 3 feet 9 inches apart, and the weight is distributed over several of them by similar open wrought-iron girders fixed in a longitudinal direction under the lines of rail.

The flooring is simply of 2½ inches timber planking, laid parallel with the rails, and bolted to the transverse girders.

The sectional areas of all parts of the girders vary according to the calculated strains when their continuity as affected only by the weight of the permanent load is considered, but the strain under the testing load of a ton per foot run does not cause a strain of more than 3½ tons per square inch for compression, and 4½ tons for tension.

The greatest possible strain that can be brought upon the diagonals is much less.

The tendency of the upper part of the girder to lateral motion is constrained by a few light wrought-iron arches fixed immediately above the gussets, which being fastened to the roadway girders, the whole form strong rings or frames at such points as effectually preserve the rectangular shape of the trough. For this purpose it was necessary to use great care in fixing the main girders so that the gussets of each pier should be in a direct line.

The weight of iron work in each pair of main girders for each span was 50 tons, and of its proportion of roadway 21 tons. The total weight of iron in the whole structure is 322 tons wrought and 15 tons cast-iron.

On the same line of railway there are also two other bridges, similar in every respect, but of only a single span each.

The girders being therefore independent, the weight is slightly increased; that of the pair of girders to each bridge being 72 tons, and of the whole bridge with roadway 96 tons.

We will now give an example of a bridge with flanged girders, which also belongs to the class "Simple Girders," the Yssel. The bridge we are about to describe crosses the River Yssel, and carries the extension line of the Dutch Rhenish Railway.

The River Yssel, during the melting and breaking up of the ice, becomes a wide and deep stream, and very dangerous. The project of a bridge to carry the railway over it was at first considered by the Dutch engineers as being so very difficult of execution, and so impossible to make secure, that, when definite plans were submitted, an enormous increase of strength in the design was required to withstand the action of the ice, which they considered would become jammed together into one mass across the whole of the river, and thus, being acted upon by the whole force of the stream, would carry away the entire structure unless extraordinary strength and solidity were used. To meet these views the girders were made on the close boiler-plate principle, and the bridge was supported by cast-iron cylinders (two to each pier) each 15 feet diameter, except the centre cylinder, which carries the roller path of a swivel bridge, and is 28 feet diameter.

The bridge has six openings of which the two centre ones are each 50 feet in the clear, and are crossed by continuous wrought-iron girders, which revolve on the 28 feet cylinder, forming the swivel bridge with two openings. The other openings are each 164 feet in the clear.

The bridge carries a double line of railway, which it supports by three parallel girders, the centre ones being double the strength of the others. These principal girders are made perfectly continuous for the two spans on each side of the centre, and therefore their height is uniform between the centres of these spans. Since there are but two cylinders used for each pier, the centre girder is supported on "sandwich" girders, consisting of wrought-iron plates placed vertically, with wood bolted between them. By this arrangement the clear span is increased to 172 feet.

The upper flange of each main centre girder is formed of cast-iron, the extreme halves exactly corresponding, so that, if brought together, they would form an arch; but, as they are constructed, appear like an arch cut asunder, spaced apart by a parallel beam. The tensile strain in the top, induced by continuity, is entirely borne by wrought-iron plates attached to the web above the cast-iron, which extend throughout the parallel part, but vary in section proportionately to the strain; and therefore over the centre piers there is no strain whatever on the cast-iron, which, in consequence, is much reduced in size.

The lower flange of the centre girder is made up of four rows of plates, and four rows of angle iron. The web is made double, forming a box between which the cast-iron of the top flange is inserted. The side girders are simply of the ordinary I shape, but a single web plate being used, the castings of the top flange are attached outside the web, and are surmounted at the central part by a wrought-iron plate. The bottom flange is also formed of four rows of plates, of less width and thickness than those in the centre. These main girders are fixed in the centre, and the ends resting on rollers can readily expand. The roadway throughout the bridge is of 12-inch balks, placed 12 inches apart, suspended to the lower flanges of the girders. The swivel bridge turns on a central steel pivot and twenty rollers, each 2 feet 8 inches diameter, fixed to radial arms, and retained in place by a strong wrought-iron girder ring. They are of cast-iron, turned, and revolve between the planed conical surfaces of the upper and lower tram-plates. The adjustment for the bearing at the ends of the girders is by wedges moved by machinery.

The weight of wrought-iron used in the whole superstructure was 851 tons, and of cast-iron 336 tons, and the weight of wrought-iron in the lower ends of the cylinders was 64 tons, and of cast-iron 508 tons. Much of the above weight was unnecessary, and was merely added to overcome the scruples of the Dutch government engineers. Other examples of similar construction may be adduced, in which the bottom flange and web of the girders are formed of solid boiler plates, and the top flange of a combination of wrought and cast-iron, the latter being generally in excess. Amongst those remarkable for their large span may be mentioned the bridge that carries the line of the Manchester South Junction and Altrincham Railway over the River Mersey in the town of Warrington, which is of 180 feet clear span; and another, but larger structure, connecting the same line of railway with the Birkenhead, Lancashire, and Cheshire Junction Railway at Walton, near Warrington, which is parallel to the brick viaduct on the London and North-Western Railway, and also crosses the Mersey and Irwell Canal. The centre span of this bridge is 172 feet, and the others are of the respective spans of 63, 60, and 37 feet, with a bridge constructed to open of the clear span of 58 feet. The larger girders bear on 9 feet cylinders, and the smaller on cylinders of 6 feet diameter.

(4.) Trussed Girders.

It must be remembered that the only material employed in the construction of girders on their first introduction was cast-iron.

Its uncertainty and weakness when exposed to tensile strain, as in the lower flange of a girder, soon attracted the notice of engineers. Little benefit was obtained by increase of thickness; for the treacherous character of the material increased rapidly with the mass in which it is cast, and simple girders were thus limited in their dimension to very moderate spans. The difficulty of uniting cast-iron rendered impracticable the attempt to build up such girders of separate castings, and nothing remained but to attempt to strengthen the lower flange by the addition of wrought-iron tension rods. The first difficulty that presented itself was to secure a due degree of tension on the rods so employed, as the mere attachment of them to the lower member of the girder without initial strain, though it might prevent the destruction of the girder in case of the fracture of the lower flange, would evidently do but little towards preventing such fracture. It was with this object that the rods employed were attached at each extremity of the girder to its upper flange, and at the centre only brought down below the bottom flanges, and were then brought into tension. It is evident that by tightening the screws by which these rods were suspended any amount of initial strain could thus be put upon the wrought-iron ties, causing a corresponding counter strain in the girder itself; and while the rolling load was sufficient to cause a deflection equal to the counter strain thus given, no strain could come upon the lower flange of the girder itself. Provided the upper flange were sufficient to resist the thrust to which it is subject, it is evident that such girders are far less liable to accident than simple castings, and are capable of application to much larger spans.

The determination of the strength of such girders is, however, a difficult task. A serious accident, moreover, which occurred from the failure of a girder of this description at the Dee Bridge, near Chester, has entirely put an end to their employment. In these bridges, the cast-iron girder formed the whole depth of the truss, the tension rods passing beneath its lower flange; it therefore possessed considerable strength as an independent girder, without counting on any assistance from the truss. Such girders are, in fact, compound girders formed by combining the truss with the simple girder, the upper flange doing duty as a compression bar in both systems, and being thus subjected to two independent strains.

It is evident, therefore, that if the upper flange is simply proportioned to its duty as the top flange of the simple girder, it will be of insufficient strength for its additional duties as part of the truss. It has been argued, that from the perfect union of the top flange with the vertical rib, a considerable portion of the whole girder might be taken as forming part of the truss. It is, however, evidently impossible by calculation to say how far such assistance may be relied on; and a still greater objection exists in the fact that such girders consist of two systems, the ultimate deflections of which are utterly different,—the girder, for instance, may be broken before the truss attains half its ultimate deflection, or has done half its duty. The objection to this girder is common to all girders in which two independent systems are attempted to be blended; and, as a general principle, all such arrangements should be avoided. It has always been usual, in order to obtain additional strength in the attachment of the three castings of which such girders are composed, to increase their depth where they are united as well as at each extremity of the girder, and the tie-rods, instead of being in a line with the upper flange of the girder, are attached to the upper portion of this increased depth. It is certain that in such an arrangement we have no right, in calculating the strength of the truss, to count on the additional depth so obtained, and it has been contended that girders so trussed may in some cases have been actually weakened. This appears, however, to be scarcely possible, when we reflect that in all such girders as are usually constructed the tightening of the ties increases the camber of the girder. It is useless to say more on the subject of this form of girder, as since the adoption of wrought-iron for girders they have been entirely superseded; they were designed when no other means existed of obtaining iron girders of great span; and the melancholy accident which occurred at Chester is the only existing instance of their failure, while the evidence given on that occasion renders it highly probable that even in that case the fracture was occasioned by the train running off the line. In the trussed girders over the River Arno, in Italy, strained tie-rods were introduced beneath the lower flange from end to end of the girder, and the experiments made on this girder were highly satisfactory.

One example of this system exists in the bridge carrying the road from Banbury to Lutterworth over the London and Birmingham Railway (fig. 37). Span 64 feet.

There are six ribs which have a double curved form, or rather that of a parabolic spindle; the lower curve being formed by a wrought-iron tie-bar, and the upper one by a cast-iron arch; the space between the two is filled by cast-iron ornamental panels, which are made to act as struts, and for the purpose of keeping the bottom tie-bar in the proper line of curvature and strain. The tie is in two parts, with eyes at each end, and pins passing through them, which take the ultimate strain. A very ingenious arrangement is applied at the centre of the rib for the purpose of adjusting the degree of tension upon the tie-beam. It consists of a vertical tubular strut, attached to the top arch by a screw, which is also used to force it down upon the tie. On the top of the arch there is a kind of plate-band cast with it, which forms the level for the cast-iron road plates. The structure is exceedingly tasteful in appearance; and the nice arrangement in the parts makes it something more of a mechanical contrivance than is generally understood by an iron bridge.

As an example of these bridges, we shall describe one of the earliest and largest, viz., the trussed girder bridge of 63 feet span, erected by Mr Bidder, for carrying the Blackwall Railway across the Minories in London. This structure, as well as all the bridges on the line, was not originally designed to carry locomotives, the line being worked by a rope. It is fortunate that in this, as in most of our early railway works, sufficient excess of strength was given to allow of the greatly increased weight of our present railway traffic as compared with what was then anticipated. The bridge consists of 6 girders, viz., two outside girders and 2 girders beneath each line of rails. Each The girder is formed of three separate castings with upper and lower flanges. The upper flange is 8 inches wide, and the bottom 2 feet wide. The general depth is 3 feet, but the depth is increased at each extremity of each casting to 4 feet 6 inches. The joints by which they are bolted end to end is thus 4 feet 6 inches deep. The joint is further strengthened by wrought-iron clips beneath the bottom flange.

The trussing rods, by which the tensile strain on the lower flange is relieved, and by which an initial camber was given to the girders, consist of wrought-iron bar 5 inches wide and 1 inch thick, placed in pairs on each side of the girders. These links are attached to the top flanges of the girders at their extremities, and descend diagonally to the bottom flanges at each joint, where they are connected with each other, and with the girder by a pin passing through the latter. They are tightened up by means of keys at each end of the girder.

The bridge has subsequently been widened by moving one of the outside girders so as to admit of the introduction of another line of rails, the girders being strengthened by the addition of top pieces which are firmly bolted to the upper flange, making the total depth of altered girders 5 feet.

After the accident which occurred at Chester, similar additions were made to many then existing girders, an additional top flange being inserted in a direct line between the attachment of the tie-rods.

The above girders resemble in principle another ordinary form of trussed girder used for travelling cranes and other purposes where light girders of considerable span are required, timber being frequently used for compression, and wrought-iron tie-bars for tension. The depth in such cases is generally obtained by the insertion of one or more light cast-iron standards or struts between the tie-bar and the timber. The principles on which the strength of such girders is calculated are perfectly identical with those given at page 592, insomuch as the centre of the timber strut may be taken as a fulcrum, and the strain on the rods depend upon the distance from this point. The stiffness of such girders is of course greatly increased by the addition of diagonals between the standards, and in this form, when made of durable materials, they become excellent girders for all the purposes where moderate spans only are required.

We have, however, two examples of bridges of enormous dimensions constructed on the same type, viz., the Chepstow and Saltash Bridges, both of which have been erected by Mr I. K. Brunel, and both of them equally remarkable for their gigantic proportions, and the engineering difficulties which had to be overcome, not only in the superstructure, but in their foundations.

The Chepstow Bridge (fig. 39) carries the South Wales Railway over the River Wye at a height of 46 feet above high water. The remarkable rise of tide which characterizes the Bristol Channel is well known, and at Chepstow is no less than 41 feet; at high springs the elevation of the bridge above low water is thus nearly double. In this bridge two kinds of girder are employed,—one half of the bridge consists of ordinary wrought-iron girders of 100 feet span, resting on cast-iron columns; in the other half, the roadway is carried over the river by the trussed girders, which have a span of no less than 305 feet. This portion of the bridge is in fact a rigid suspension bridge, the tension of the chains being resisted not as an ordinary bridge by attachment to the ground at each end, but by a horizontal cylindrical wrought-iron column, or strut, 9 feet in diameter and 8ths inch thick, which reposes on the towers at each end. of the bridge. Instead of the ordinary catenary, the chain consists of three straight links only. The rigid form of the chain is preserved, and the flexure of the horizontal column is prevented by their mutual attachment by diagonal and vertical bracing, the girders which carry the roadway are suspended from the chain at two points only, viz., at each end of the centre link. These girders are thus divided into three nearly equal spans, and are supported at each end by cast-iron.

The tower at the extremity of the bridge rests upon the precipitous rock which bounds the river, but at the other end upon a pier consisting of six cast-iron columns, which pass through 50 feet of mud down to the rock beneath. The mode of sinking these cylinders was novel. They were placed in their position on the site of the pier, which is dry at low water, and the mud was excavated till they began to sink with their own weight, when fresh lengths were added on the top as the previous lengths sank down. They were thus ultimately bedded on the rock, and filled up with concrete. The cylinders are carried up to a height of 190 feet, and are connected at the top by the cast-iron framing and tower which carries the tubes.

The weights of the various parts are as follows:

| Description | Weight (Tons) | |--------------------------------------------------|--------------| | 298 feet run of tube and butt plates | 127½ | | Hoop of ditto over piers | 7¼ | | Side plates, bottom ditto, for attachment of main chains | 15 | | Side plates for attachment of diagonal chains | 2¼ | | Stiffening braces, 26 feet apart | 4½ | | Rivet heads and snaps | 4½ | | **Total weight of one tube** | **161¼** |

| Description | Weight (Tons) | |--------------------------------------------------|--------------| | Main chains, eyes, pins, &c. | 105 | | Diagonal chains, ditto | 23 | | Vertical trusses | 184 | | Saddles, collars, &c., at points of suspension | 22 | | Main roadway girders, transverse floor girders, &c.| 130 | | **Total weight of iron in one roadway** | **460** |

The tubes or struts are of uniform section throughout, and are formed of sixteen equal plates ¾ inch thick, and two side plates ½ inch thick. The plates are all 10 feet long, lapped together at sides, and butt-jointed at the ends with double butt plates, and riveted together with two rows of rivets.

(5.) Bowstring Girders.

The difficulty of obtaining abutments capable of resisting the thrust of large arches, more especially when the use of iron in their construction allowed of great diminution of their rise or versed sine, led naturally to the addition of a chain or tie to resist their thrust. Again, the outline of ordinary girders with parallel top and bottom flanges, when of any magnitude, is by no means agreeable. Depth being the only visible element of strength, the eye does not fail to perceive that such an outline implies equal horizontal strains throughout, where we feel that the strains at the ends of a beam merge entirely into a vertical direction, and that depth is there useless. In large girders the depth at the centre was therefore alone increased, and the top took an arched form, the curve being generally a parabola; the analogy of such a girder with the bowstring arch is at once apparent. It is, however, of great importance to understand thoroughly the difference between these two systems.

The strain at the centre of each of them is, as we have before seen, perfectly identical. The weight supported at the ends is also in both cases equal to half the weight of the girder; but the manner in which these similar vertical forces are converted into similar horizontal forces between their origin at the points of bearing and their mutual equilibrium at the centre of each girder, is in each case entirely different, and the horizontal strain in the top and bottom flanges varies accordingly.

In an uniformly loaded ordinary girder, with parallel top and bottom flanges as we have already shown, the horizontal strain varies at any point of the flanges as the rectangle of the segments into which the point divides the span. We must, therefore, in order to insure equal strains throughout, either diminish the section of the flanges in that proportion as we recede from the centre, or we must diminish the depth of the girder in a similar proportion, preserving uniformity of section in the flanges. In either case the strain per square inch remains constant throughout the whole length of the flanges; but, as their actual section at corresponding points is entirely different, it is evident that the actual strains to which they are subjected are also entirely different. Now, as these strains arise entirely from the action of the vertical rib which connects the flanges, it is evident that the duties of the vertical rib are entirely different in the two cases.

In the parallel-sided girder (fig. 40), in which the upper and lower members taper off to 0 at the extremities, the plate A requires sufficient stiffness and strength, both vertically and diagonally, to support half the weight of the girder, and will be the thickest side plate in the girder; but in the parabolic-shaped girder (fig. 41), where the flanges maintain their full section to the end, the plate A requires no such additional strength, and may be reduced to nothing. In fact, if we now omit the vertical rib altogether, fig. 41 is a bowstring girder. These girders, therefore, have little analogy except as regards the strain at their centres, or, which alone is identical on both; the difference between the two girders is evident.

In fig. 40 the horizontal strain in the flanges is at a maximum at the centre, and decreases to 0 at each end, and the section varies in a similar proportion.

In fig. 41 the horizontal strain on the flanges is constant throughout their length, and the section is also uniform throughout.

In fig. 40 the vertical strains from the point of support are thrown gradually into the flanges by the action of the ver- They are nothing at the centre, and at a maximum at the ends where they are entirely resisted by the vertical rib.

In fig. 41 the vertical strain is thrown directly into the flanges without the intervention of any vertical rib.

Now, a bowstring girder is such an arch, with horizontal ties to resist its thrust in lieu of abutments, and as regards supporting its own weight, if the arch be one of equilibrium, and the ties are suspended from the arch to preserve their horizontal position, no diagonal bracing or vertical rib of any kind is necessary. We may, therefore, at once apply all the principles applicable to the ordinary arch in investigating the strains of such a girder.

It is evident, however, in the same manner as with the arch, that such a structure in equilibrium would not support any unequal load. The best means of giving rigidity to this skeleton arch is a problem on which much ingenuity has been expended. There are three methods by which the rigidity has been obtained,—1st. By giving sufficient rigidity to the arch itself; or the ties themselves, or both, to ensure the requisite stiffness as in the High-level Bridge at Newcastle; 2d. By cross-bracing between the arch and the ties as in the Monkland Canal Bridge; and, 3d. By an independent system of trussing or framing, combined with the arch and ties as in the proposed girders for the Mayence Bridge, and in most American timber bridges. By any of these additions, or rather by a combination of them all, it is probable that this system of girder is capable of very great extension. An example of each of these bridges is here given.

The earliest railway bridge on the bowstring principle is that over the Regent's Canal, near Chalk Farm, on the London and Birmingham Railway (see fig. 42).

It is composed of three main ribs of cast-iron open panel-work, whose outline is parallel but which includes an arc extending to its extremities of length and depth, and intersecting the vertical bars which form the panels. The span is 50 feet and the height of the ribs 10 feet. The section of each rib is in the form of a hollow rectangle 2 feet 11 inches wide, and the space between its sides is filled with diagonal bracing-frames 5 feet 10 inches apart.

The railway is carried by cast-iron girders of the fish-bellied shape, 28 feet between bearings and 1 foot 10 inches deep in the middle; they are suspended from the bracing-frames in the main ribs by wrought-iron suspension rods 2½ inches diameter; there being sockets in the bracing-frames to receive their upper ends, and in the ends of the cross-girders to receive their lower ends.

The centre main rib performs double duty; and its bracing frames have double sockets, and carry two suspension rods. In addition to the ribs themselves in resisting the strain of the load there are longitudinal tie-bars under each rib, there being four under each outside rib in a horizontal row, and eight under the centre rib in two horizontal rows. These tie-rods are secured to the bearing ends of the main ribs, and are in three lengths, each united by sockets, gibs, and keys. Upon the cross girders are oak sleepers for the rails; and the entire space between the rails is filled in with cast-iron plates perforated in the form of trellis-work. The outsides of the outer ribs are ornamented with cast-iron mouldings and fret-work. This bridge is of very bold design, and certainly a novelty as regards construction.

The finest example of the kind of structure we are alluding to is undoubtedly the High-level bridge at Newcastle-on-Tyne. This bridge (fig. 43), which crosses the River Tyne, unites the towns of Gateshead and Newcastle. The Tyne runs through a deep valley or ravine, on each side of which these towns are built. The old bridge crosses the river at the bottom of the valley, and the want of a bridge at the high-level was long severely felt by the in- habitants for the accommodation of their local traffic. So far back as the commencement of the present century, various plans were projected to meet this requirement; but the great cost of the undertaking rendered it at that time impossible to carry any of them into effect, and they were reluctantly abandoned.

The traffic between the two towns, however, rapidly and constantly increased, especially when Gateshead became the terminus of the Southern Railways, which were thus entirely isolated from the lines north of the Tyne; some other means of communication between the termini at Gateshead and Newcastle became now, in fact, indispensable, the cost of conveyance of passengers and merchandise by coaches and omnibuses across the old bridge reaching the enormous amount of £1,000 per week.

In a work so indispensable for both interests the local authorities gladly co-operated with the railways, and it was at length determined to construct the present bridge, and to carry both a public road and the railway across it.

There are two platforms, the upper platform carries three lines of railway, while the lower forms the common public road.

The breadth of the river at this spot, at high water, is 515 feet, but the whole distance between the Gateshead station and the central terminus on the Newcastle side is about 4000 feet. The approaches of the railway (as will be seen by the engraving) are curves in contrary directions, but those of the public road below are straight.

The bridge is in six spans, each of 125 feet, and the superstructure is supported on stone piers and abutments, at a height to the solit, above high water, of 83 feet.

The foundations consist of piles, the spaces between which are filled up with concrete. Many of the piles are 40 feet long, and all are driven through the hard sand and gravel, forming the bed of the river, until they reach the solid rock.

Many difficulties occurred in driving the piles which considerably retarded the progress of the work, and, among others, the peculiar effect of ebb and flow during this operation is worthy of note. At flood-tide the sand became so hard as almost totally to resist the utmost efforts of driving, while at ebb the sand was quite loose, and allowed of doing so with facility. It was therefore found necessary to abandon the driving on many occasions during high water.

The difference between high and low water is 11 feet 6 inches.

Another difficulty arose from the quicksands beneath the foundations. Although the piles were driven to the rock bottom, the water forced its way up, baffling the attempts to fill in between them; this, however, was remedied by using a concrete made of broken stone and Roman cement, which was continually thrown in until the bottom was found to be secure.

The piles were driven by Nasmyth's Steam Pile-driver, Nasmyth's this being one of the first cases in which this valuable pile-engine was used, a foundation was thus obtained, which certainly would not have been possible by the ordinary means.

The ram weighed a ton and a-half and had a fall of 2 feet 9 inches. It was worked incessantly, night and day, driving at the rate of sixty or seventy strokes per minute. In several instances the pile heads burst into flame, and burnt fiercely under this rapid action of the ram.

In setting out the spans previous to the driving of the Gauge permanent piles, guage piles were driven with the ordinary pile-engine as deep as they would go. When the steam pile-driver was introduced, an experiment was tried with it upon one of them, which was driven to a farther depth of 15 feet.

One of the foundation piles was tested with a load of 150 tons which was allowed to remain several days, and upon piles, its removal no settlement whatever had taken place. The piles are 4 feet from centre to centre, and the utmost that can come upon one of them is 70 tons, supposing none of the weight to be carried by the intervening space of planking and concrete.

The foundations of the abutments are upon a bed of strong clay, under-lying the sand and gravel at a depth of 18 feet; no piles are used; the abutments are of stone similar to the piers. The roadway is carried for a considerable distance upon each side of the bridge upon masonry arches of very durable construction. Designs for the proposed entrances to the bridge, and in keeping with the architectural features of the structure were designed by the engineer, but have not yet, for financial reasons, been carried into effect.

The cofferdams for the piers (none being used for the abutments) were formed in double tiers, filled in with puddle. The piles were not drawn, but were cut off level with the bed of the river by a circular saw; the lower part remaining to protect the foundations as well as to avoid disturbing the bottom by their extraction.

The foundation course lies about 2 feet below low water. The stone is of a hard and durable quality, and came from the neighbourhood, viz., a portion from Heddon-on-the-Wall, and the remainder from Corbridge.

The dimensions of the piers are as follows:

- Area of foundations: 784 ft² by 22 ft wide. - Base, including cutwater to about 5 feet above high water: 67 ft by 19 ft. - Footing or springing of piers on cutwater: 48 ft by 10 ft. - Shaft of piers up to level of superstructure: 45 ft by 14 ft. - Lengthened by an arched opening of 11 ft 10 inches.

The most novel features, however, in this structure are the bowstring arches over the centre portion of 900 feet. This space is crossed by six similar spans. The four centre spans cross the river, the remaining two are on the slopes on either side.

Each bay is crossed by four main arched ribs with horizontal tie-bars to resist the thrust. The upper roadway rests upon the arches, the lower is suspended from them by wrought-iron suspension rods.

Each arch is cast in five segments, strongly bolted together, and when entire is 125 feet in span, with a rise of 17 feet 6 inches from the centre of the tension bars, and of 18 feet 1½ inches from the upper surface of the bed plates. The depth of the arch at the crown is 3 feet 6 inches, and at the haunches 3 feet 9 inches. The section is that of a double-flanged girder, the flanges being 12 inches wide, and 2 inches thick on the outer arches, and 3 inches thick in the internal arches, which have a greater weight to support. The vertical ribs are of the same thickness as the flanges.

The sectional area of the external rib at the crown is 133 inches, and of the internal ribs 189 inches; the combined area of section at the crown in the four arches being 644, and at the haunches 706 square inches.

The ties consist of flat wrought-iron bars, 7 inches by 1 inch of best scrap iron, with eyes of 3½ inches diameter, bored out of the solid, and pins turned and fitted closely.

Each external rib is tied by four of these bars, and each internal rib by eight. The sectional area of each external tie is 28 inches, and of each internal tie 56 inches, giving a total area of 168 square inches.

These bars were all tested to 9 tons on the square inch.

The four ribs are disposed in pairs (see cross section); the two internal ribs being 20 feet 4 inches apart in the clear for carriage road, and the space between the internal and external ribs is 6 feet 2 inches, forming a footpath on each side.

The arched ribs are braced between the footpath space by cast-iron vertical and longitudinal bracing-frames, with 2-inch tie-bolts passing through them, which extend as near as possible to the haunches, allowing only a sufficient headway for the passengers.

The spandrels between the arches and the beams which carry the railway, are filled with cast-iron vertical square pillars, also braced across with diagonal bracing frames, similar to those between the ribs. On the tops of these spandrel pillars are trough-shaped longitudinal girders, which extend along the bridge as continuous beams.

The ribs have square bosses cast on them, corresponding with the spandrel pillars, and forming horizontal tables for them to stand on; these were all truly bored and faced.

The spandril pillars are continued downwards, concealing the suspension bolts, and add great stiffness to the superstructure.

On the longitudinal top-girders are the cross-girders, also of a trough form, cast in one length, extending across the whole four ribs. The cross-girders have pockets or shoes cast on them, to receive the ends of the longitudinal timber-bearers upon which the road-planking is spiked.

All butting joints were planed on their upper surfaces to receive the bearing ends of the ribs, which were also planed.

On the abutments all the bearings are allowed to expand and contract, but on the next bearings they are fixed down firmly with four 1½-inch bolts and nuts; on the next pier again they are free, and so on alternately fixed and free.

There are no rollers, but the bearings have sliding surfaces provided for the purpose.

The motion caused by expansion and contraction, ascertained by observation, was, during a variation of temperature of 32 degrees, 92 inch in the whole six spans—153 inch per span.

Each arch was temporarily erected at the contractor's works, and tested before removal, and all the other detailed parts received a separate test previously to this final trial.

The planking is 3 inches thick, jointed and tongued with hoop iron, and laid in two courses crossing each other at right angles. The upper course is caulked and pitched with as much care as would be bestowed on a ship's bottom.

The lower roadway is formed of cross planking in a similar manner, and is paved with wooden blocks 4½ inches cube of a new construction. The blocks are cut at the upper surface, in form of a shoulder (fig. 44), so that when they are all in close contact they show so many grooves, which extend to half their depth (fig. 45), the channels being an inch in width.

The surface on which the blocks were laid was covered with pitch; the blocks were dipped in hot pitch and laid, and the grooves filled with broken stone and gravel; a layer of pitch, and finally of sand was spread over all. Six years' wear has made but little impression upon this paving.

The quantity of masonry was—

In the abutments and approaches, 360,222 cubic feet ashlar; 2434 yards rubble.

In the five river piers, 321,387 cubic feet ashlar; 877 yards rubble; 1712 yards concrete.

Total, 681,609 cubic feet ashlar; 4311 yards rubble; 1712 yards concrete.

The total weight of cast-iron was 4728½ tons.

The total weight of wrought-iron was 321½ tons.

Total 5050 tons.

The cost of the bridge was—

Masonry, including coffer-dams £119,000

Iron work, including road, railway, &c. 114,000

Temporary bridge 10,000

£243,000

The first heavy portion of the superstructure was cast in February 1847, and the bridge was opened by the Queen in September 1849.

As an example of one of the simplest forms of bowstring Monkland girders which have been constructed, we shall describe the Canal large wrought-iron girders designed by Mr Edwin Clark Bridge for replacing a timber bridge over the Monkland Canal on the Caledonian Railway. The laminated timber arches of the old bridge were always much distorted, and as in many similar bridges which were constructed on our early railways, were beginning rapidly to decay, from the play of the timbers and the infiltration of water into the joints and between the planks of which the arches were composed. Such arches are undesirable for exposed structures in a damp climate, though peculiarly adapted for roofs where the arches are sheltered from the weather, as in many stations on the Caledonian line, and more especially on the magnificent roof of the Great Northern Railway station at King's Cross. The railway passed over these arches, which, from their dangerous state, were shored up, and it was of the utmost importance to replace them by some other structure without interfering with the traffic over more than one line at a time. Wrought-iron bowstring girders were therefore used on account of their lightness and the facilities they gave for adding but little weight to the old bridge during their construction upon it. The arch or top member of these girders is partly wrought, and partly cast-iron, and this construction has been found advantageous and economical. A similar combination has been adopted by Mr Clark in the bridge at Arnhem over the Yssel, on the Walton Viaduct, the large girder-bridge at Warrington, &c.

The lower flange or tie consists of wrought-iron plates as in the bottom of an ordinary flanged wrought-iron girder. The stiffness of the tie is secured by a deep vertical rib which forms a portion of its effective section, and which renders it sufficiently rigid not only for supporting the roadway, but also for assisting materially in preserving the arch from vertical change of form. This is farther secured by a similar vertical rib which forms a portion of the effective section of the arch. The vertical and diagonal bracing which connects the arch and the tie are attached conveniently to these vertical ribs. The arch consists of three ribs, the centre being nearly double the strength of the external ribs. The total length of the girders is 148 feet, and the depth is about \( \frac{3}{4} \) th of the length, or 15 feet.

The duty of the diagonal bracing is simply to preserve the form of the girder under a rolling load, and in order to insure a sufficient connection between the arch and the tie at each extremity, the panels at the extremities are filled in with close plates. The transverse timbers which carry the roadway, and which are simply bolted up beneath the lower flange, are 8 inches deep, and are placed close side by side, running across the whole bridge. Such a roadway is renewed with extreme facility, and occupies the least possible depth. The arches are braced together over the roadway.

The whole weight of these girders for the double line is only 128 tons, and their substitution for the old bridge did not occasion the least interruption to the traffic. The sectional area of the top flanges is 120 square inches, viz., 60 square inches of wrought-iron, and 115 square inches of cast-iron. The sectional area of the lower flange or tie is 150 square inches.

As examples of the third method of securing vertical rigidity in such girders we may refer to most of the large timber viaducts erected in America, where the arch is combined in numberless ways with horizontal trussed or trellis girders with great ingenuity and simplicity of detail. Wrought-iron arches of a somewhat similar description have been designed by Mr Clark for the bridge at Mayence on the Rhine.

In this case the arches have a versed line of about one-tenth of their length, and are connected over several spans as well as rendered rigid by a continuous wrought-iron trellis girder of half the depth of the arch which is in section a wrought-iron rectangular box.

This roadway girder is extremely light, its duty being only to distribute the weight of the rolling load. It offers great facilities for the erection of the arches, and secures them from lateral motion by partially filling in the spandrels over the piers. It also breaks the extremely undulating outline which independent arches would present. It is moreover evident that great benefit is derived from the continuity thus given to the various spans. It is believed that, in cases where the roadway can be placed on the top of this continuous girder to allow of cross-bracing beneath it, this system is peculiarly applicable to spans of great magnitude.

There is a peculiar bow-string bridge erected on the Gloucester and Birmingham Railway, by Captain Morsom (fig. 46), which is worthy of notice.

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**Fig. 45.—Cheltenham Bridge.**

The roadway is carried by two main ribs of 45 feet span, each of which is in form of an arch with an arched chord; the top member is 9 inches deep by 3 inches in thickness, and the lower member or chord is one foot 6 inches deep, and is of the double-flanged form. The whole rise of the top member of the rib from the springing is 7 feet 3 inches, and that of the lower member or chord is 1 foot 6 inches; the space between them at the centre being 4 feet 3 inches.

Each rib is in five castings; and the top and bottom members are tied together by vertical bolts, 2 feet 5\(\frac{3}{4}\) inches apart.

The roadway plates are carried on a series of cast-iron beams on the skew, one end resting on the walls, the other in shoes, provided on the lower member of the ribs. Three of the cross beams bear at both ends on the walls; the beams themselves are curved with a camber of 1 foot 6 inches, and are tied together with bolts. The whole platform is carried by roadway plates of cast-iron, on which the road materials are placed.

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(6.) Tubular Girders.

In the ordinary plate girder we seem to have arrived at the limits of simplicity of construction. A top and bottom flange with a vertical rib, all composed of plates, and united by angle irons, form a beam, the calculation of the strength of which is within the reach of every one. As soon, however, as we exceed very moderate limits, difficulties arise entirely beyond the reach of theoretical research, which are in fact quite independent of the action of such a beam as a girder, but arise from the distortion of form to which such a system, supported only on two distant and limited bearing surfaces, must as a whole be liable, more especially as expansion and contraction prevent any rigid attachment even on these contracted supports.

It is in meeting such difficulties in large structures that the skill and judgment of the engineer is really taxed when he exceeds the limits of actual experience. In the design of the Britannia Bridge, it was the mere arrangement of materials to resist the transverse strain which composed the difficulty of the problem. It was rather the practical construction of any such structure at all; the difficulty of obtaining the materials required, or of adopting such as were obtainable to such new purposes, and of devising a beam not merely of sufficient strength for its ultimate use as a bridge, but of sufficient independent rigidity for retaining its form not only when in place, but during its erection on its temporary scaffold, its floatation on unstable pontoons, and the ultimate raising of it into its place suspended isolated from four simple chairs.

The tubular girder alone seems really adapted to such varied strains; and it is difficult to conceive even now, with all our subsequent experience, any other means than those adopted of solving the great problem which thus inaugurated a new epoch in the history of bridge construction, and led directly not only to our present theoretical knowledge of the principles of beams, but to all those numberless elegant and ingenious practical combinations of wrought-iron in bridge construction, which it has been our province to describe.

These remarks on a subject now becoming old, are necessary, because great misapprehension exists as to the real objects in view in the construction of the first tubular bridges, and which, though probably the only constructions possible under such circumstances, and unsurpassed in point of simplicity of construction or economy by any other kind of beam, are yet not necessarily of universal application. The chief examples existing at present of tubular bridges of any magnitude are the following:

1. The Conway Bridge. 2. The Britannia Bridge. 3. The Brotherton Bridge over the River Aire. 4. The bridge over the Nile. 5. The Victoria Bridge over the St Lawrence.

In all these cases, a tubular bridge simply means a pair of girders, with their top and bottom flanges of sufficient horizontal extension to meet and combine, forming a rectangular tube or trough; through the interior of which, or upon the top of which, the roadway may be laid.

It may here be stated, that the result of experiments made with that object had directly proved that which might at first sight appear problematical, viz., that with such extensive horizontal development of the top and bottom flanges, the whole of their sectional area acts effectually in resisting extension or compression throughout the entire width; and, in fact, in cases of absolute fracture, where such beams have been broken in the experiments, the tearing asunder of the bottom plates actually commenced at about the middle of the tube, and not at the outside edges. The whole of the principles applicable to simple girders are thus directly applicable to these bridges, and full details of these principles and of their application, with a minute description of the construction and erection of the Britannia and Conway Bridges, together with an account of the extensive series of experiments made in connection with these works, and of the practical deductions thus arrived at, will be found in Mr Edwin Clark's work on this subject, to which the reader has before been frequently referred. We shall here merely give a few details, and draw attention to some of the circumstances which must be taken into account in considering these works as simple beams.

The Britannia Bridge, which carries the Chester and Holyhead Railway over the Menai Straits (see figs. 47 and 48), consists of two independent continuous wrought-iron tubular beams 1511 feet in length, and weighing 4680 tons each, independent of the cast-iron frames inserted at their bearings on the towers. They are 15 feet wide, and vary in depth from 23 feet at the ends to 30 feet at the centre. They rest on two abutments and three towers of masonry, at a height of 100 feet above high water. The roadway is laid along the bottom, viz., one line of rails in each tube. The centre or Britannia tower, which is altogether 230 feet high, is built on a rock in the middle of the Straits. The bridge has thus four spans, viz., two spans of 460 feet over the water, and two spans of 230 feet over the land. On each side the weight of a single span of 470 feet is 1587 tons, and of a span of 242 feet 630 tons. These tubes repose solidly on the centre tower, but repose on roller beds on the land towers and abutments. Now, these gigantic dimensions are by no means the only remarkable features in this work. The opponents of the Holyhead Road had imposed conditions on the Chester and Holyhead Railway, which were thought insurmountable with respect to this bridge. The navigation was not to be interrupted—no scaffolding could thus be used—and the clear height of 100 feet was to be retained throughout; arches being objected to unless the springing and not the centre was at this elevation. The tides set through this portion of the Strait with a velocity of 9 miles per hour, and the quiet water at each turn of the tide lasts but for a very short space of time. The tubes were designed to meet all these requirements; they were so constructed at a considerable distance from their permanent site on the shores of the Straits, they were floated upon pontoons upon these rapid tides to the base of the towers, and they were then drawn up by hydraulic presses to their required height. They were here united through the towers by the insertion of shorter lengths, and ultimately brought into the conditions of continuous beams as regards strain, by the means employed for their junction. It is evident such structures would be designed specially for such varied circumstances, for example,

As soon as they were completed on temporary platforms, these platforms were removed, and they became isolated beams; the ends were accordingly strengthened with cast and wrought-iron framing for this special object, and had they always remained there the sides might have been throughout considerably lighter than they are; they now weigh nearly 40 per cent. of the whole weight. But in the next operation, that of floating, the tubes were liable to be supported at any point of their length, besides being subjected to chances of considerable distortion, and to disasters which on more than one occasion did actually threaten their entire destruction. The stiffening frames and gussets, which in an ordinary girder would have only been necessary at the ends, became therefore necessary throughout the whole length, and even the top and bottom were considerably modified, as it is evident that while overhanging the pontoons on each end to the extent of 70 feet, that the top, instead of being in compression was thrown into extension; the weight of the tubes was consequently much increased by these arrangements. Again, they had to be raised by being suspended freely from four chains. Provision for this suspension from such limited attachment had also to be made of a totally opposite character from that made for their vertical support when on their bed; and, ultimately, when raised to their place, they remained no longer independent beams, but were converted into continuous beams,—parts before in tension being now thrown into compression, and vice versa; while the ends which were before subject to no horizontal strain were now exposed to greater strain than even the centre of the span. And, last of all, during the act of raising one of these enormous masses, the press from which it was suspended burst, and one end of the beam fell through a space of no less than 9 inches on to a loose uneven heap of planks beneath it, bulging in the bottom plates, breaking all the castings, distorting seriously the sides and stiffening frames; while the broken press itself, which descended from a height of about 100 feet above, broke through the top plates and completed the crippling of the whole section of support. It may surely be doubted whether anything but a tube could have stood such unexampled violence; and in proportioning the parts of a structure destined for such usage, the mere consideration of the strain to which as an ordinary beam it would be subjected, formed but a part of the problem; no direct comparison can therefore be made between the weight of this bridge and an ordinary beam. If this were the case with the large spans, it is still more so with the small spans of 230 feet, which as simple beams would weigh only 230 tons each, whereas their actual weight is 650 tons. But it must be borne in mind that as regards the bridge itself these small spans were not required at all, and that they were merely designed and used as counterpoises for the large tubes, for the important purpose of converting them into continuous beams by their overhanging weight. By examining their detail, it will be found they are designed solely for this special purpose, their use as beams being made entirely subsidiary.

Some misapprehension exists on the object and importance of the cells of which the top and bottom of these tubes is composed. These cells are rectangular, there being eight of them in the top and six of them in the bottom, and they run throughout the bridge. With respect to their importance, it must be observed that the whole section of the top of the Britannia tube at the centre is 64825 square inches, and of the bottom 58543 square inches, and that the tube is 15 feet wide; the thickness of a single plate to ensure this section would therefore have been 27 inches for the top, and 23 inches for the bottom; and had such a plate been procurable, nothing better could have been desired, and the cells would be unnecessary. Such a thing, however, is evidently impossible, and the engineer in this, as in numberless other details, had to adopt what he could obtain; now the arrangement of the plates in cells is almost the only conceivable arrangement possible for obtaining the required section, allowing access, at the same time, to every part for construction and future maintenance. This alone led to their use in the bottom of the tube, where their form was totally unimportant. With respect to the top, however, it was of great importance, since thick plates could not be had, to ascertain the best form of cell for resistance to compression that could be devised with thin plates. A series of valuable experiments by Mr Eaton Hodgkinson led to the use of the rectangular cells as actually used, not because such form presented any peculiar advantage over any other form, as some have imagined, but because these experiments demonstrated that cells of that magnitude and thickness were independent of form, and are crushed only by the actual crushing of the iron itself; under these cir- circumstances, the square cells were used as the best practical method of obtaining the sectional area required.

Similar misapprehension also exists as to the considerations which led to the rectangular form of the tubes themselves. Now, the result of direct experiments made with round, oval, and rectangular tubes—there being precisely the same section and weight in all three, and, consequently, different depths—was, that the circular tube was the weakest, and the oval tube the strongest, the rectangular form being intermediate. The oval tube was, indeed, first studied with a view to its use. Its form, however, was not favourable, neither for its actual construction, nor for its connection with the suspension chains, which were originally intended to be used in the erection; and practical considerations, in this case, also compelled the use of the rectangular tube. It must also be remarked, that the result of experiments made on round, oval, and rectangular wrought-iron tubes, when reduced to the same depth and compared, was in favour of the rectangular form, although within ordinary limits the form was not proved to be a matter of very great importance. It may be added, that this bridge has now been in use six years, that the deflection has been carefully tested, from time to time, with the utmost precision, and that not the slightest perceptible increase has taken place during that period. The care with which the painting has been attended, and the protection afforded by the roof, have also entirely preserved it from the slightest damage by oxidation; and it is difficult to conceive that even the lapse of centuries can in any way affect such a structure, or to doubt that it will remain one of the most durable, as it certainly is one of the most remarkable monuments of the enterprise of the present century.

The Conway Bridge is in most respects similar to the Britannia Bridge, the local peculiarities of site being nearly similar. It consists of only two tubes of 400 feet span, placed side by side, and weighing each 1180 tons. The same provisions, as regards its strength, were made for floating it to its place and raising it; but, as the span is single, and it is not a continuous beam, the general arrangement of the plates is entirely different on the two bridges.

In the Brotherton Bridge, on the York and North Midland, the span of which is nearly identical with the small spans of the Britannia Bridge, viz., 225 feet, it was possible to compose the top of a single plate, and no cells whatever have been used, either on the top or bottom.

The depth of these tubes is 20 feet, and their width between the side plates is 11 feet.

Each tube rests on two sets of rollers at one extremity, the other being bolted down to the pier. The rollers are placed at the extreme outsides of the tube, immediately under the sides, extending inwards only 2 feet 3 inches.

The roller-plates are bedded on creosoted timber. The bearings are stiffened by cast-iron standards, three on each side, firmly bolted to the side plates. Each standard is about 40 inches in sectional area at the bottom, and tapers to about 30 inches section at the top.

The rails are laid on longitudinal timbers, which are supported at intervals of 5 feet by angle iron brackets, riveted to the cross beams or keelsons.

The weight of one tube is as follows:

| Between the bearings the wrought-iron | 198 Tons | |--------------------------------------|----------| | On the bearings the wrought-iron | 13 | | Cast-iron on the bearings | 14½ | | Cast-iron in rollers and plates | 9½ |

Total weight ........................................... 235

The rigidity of the tube exemplifies remarkably the advantage of solid or close sides in diminishing deflection. A circumstance also occurred in the construction of this bridge which illustrates one of the great advantages peculiar to tubes, viz., their independent strength.

The form given to these tubes is not rectangular, but was slightly pyramidal; their width at the bottom was 11 feet 10 inches, and at the top only 11 feet. Now, after the opening of the bridge, the width at the level of the carriage windows was objected to by the government inspectors, although they had previously sanctioned the width on the first tube erected. It became necessary, therefore, to widen the tubes, and this was done in a very interesting manner, viz., by literally opening the top of the tube down a centre line throughout its length, and inserting in the opening a longitudinal plate, 10 inches broad, from end to end. In this manner the sides were evidently moved farther apart to a less extent at the level of the carriages. By these means, and the removal of a portion of the projecting rib of the T irons, the whole was sufficiently widened. No other kind of beam could evidently have retained its form during so extraordinary an operation.

The principal feature in the Egyptian Railway Bridges is, that the road is carried upon the top of the tubes, and not in their interior.

There are two tubular viaducts upon the Egyptian Railway. The larger one crosses the Damietta branch of the Nile near Benha, and the smaller one crosses the Karrineen Canal at Berket-el-Saba (Lake of the Lion). These viaducts unite two old roads, formerly connected by a ferry, and each is contiguous to a vice-regal palace.

In the larger viaduct there are ten spans or openings, the two centre ones comprising one of the largest swing-bridges that has been attempted.

The total length of the swing-beam is 157 feet; it is balanced at the middle of its length on a large central pier. When open to the navigation a clear water-way is left on either side of the central pier of 60 feet. Each half of the beam sustains its own weight as a cantilever, 66 feet long.

The eight remaining spans are 80 feet in the clear, arranged four on each side of the centre portion; and the total length of the viaduct between the abutments is 865 feet.

The piers consist of wrought-iron cylinders, 7 feet in diameter below the level of low Nile, and 5 feet diameter above that level. They were sunk by a pneumatic process to a depth of 33 feet below the bed of the river, through soil of a peculiarly shifting character, and are filled in with concrete.

There are six of these cylinders in the central pier which supports the swing-bridge; and the adjacent piers on either side of the centre have each four cylinders; each of the remaining piers has two cylinders only. The tops of the cylinders are covered by cast-iron circular plates which rest entirely upon the concrete, special care being taken to prevent any contact with the cylinders. On these circular plates rest the upper cast-iron plates which connect the piers, and form a seating for the bearing-plates of the beams.

The beams or tubes are 6 feet 6 inches deep, and 6 feet 6 inches wide at the bottom, tapering to 6 feet wide at top, and they rest at their ends on rollers working between planed surfaces to admit of the motion caused by expansion and contraction.

The tubes carry a single line of way on their tops, the rails being laid on longitudinal sleepers, and there is also a roadway 4 feet wide on either side, supported by wrought-iron brackets bolted to the sides of the tube.

These roadways are of corrugated iron, resting on the brackets, and stiffened by strips of bar-iron placed transversely on the top.

The six cylinders for the central pier are also provided with cast-iron circular plates, as before described, and surmounted by a framework of cast-iron, uniting the tops of the cylinders, and serving as the lower tramway for the rolling machinery.

The revolving machinery consists of a turn-table containing eighteen accurately turned conical rollers, their angle being determined to the greatest nicety, and corresponding with the angular surfaces of the tram-plates between which they revolve.

The diameter of this turn-table is 19 feet from centre to centre of the rollers.

The whole of the rollers, together with the wrought-iron circular frame to which they are attached, form an independent system, usually termed the "live-ring," held in its position by the central pivot. The frame of the "live-ring" is connected with the rollers by radial spindles with gun-metal gudgeons at the periphery and centre. And, to prevent any difference in angular speed between the rollers and centre portion, a very excellent arrangement is adopted, which consists in a diagonal strap passing over the central wheel, and extending to the outer periphery. This strap is keyed up to any adjustment in which it firmly keeps the radial spindles.

The swing-tube is firmly attached to the upper tram-plate by a system of cast-iron bracket-work and strong bolts and nuts; forming, in fact, as is most essential at this point, an exceedingly rigid attachment. The centre pivot is of forged iron, 9 inches diameter, and turned accurately to fit its bearings. To insure a firm fixing for this pivot, it is made to pass through the entire depth of the lower tram-plate into a socket provided for the purpose, in which position it is firmly keyed. The bridge is turned with facility by a capstan worked by two men, with gearing communicating with the large rack surrounding the lower tram-plate.

To prevent accident to the swing-bridge when open, "Fenders" are placed up and down stream, similar in construction to the piers of the bridge. At the bearing ends of the swing-bridge arrangements are made for locking it in its position. These consist of fixed inclined planes attached to the under surface of the bearing ends of the tube and corresponding wedges which slide on the piers, which are made to recede and advance by means of a screw turned by gear-work.

In the Birket-el-Saba Viaduct the swing portion forms spans on each side of 43 feet, and the fixed portion consists of two spans of 70 feet each. In other respects the viaducts are precisely similar.

Both were commenced in May 1853, and completed for traffic in October 1855.

It may be here remarked that the duties of a swing-bridge in resisting strain are greatest when the bridge is open and only sustaining its own weight. This will be readily understood by considering the entire swing-bridge in the condition of a beam supported in the middle, and loaded at the ends, the action being, in fact, similar to that illustrated by fig. 17. The centre point in the figure may represent the centre pier of the swing-bridge; therefore it is evident that the depth must be proportioned to the entire length of the swing-beam and not to the mere span on each side of the centres; and by the simplest reasoning it will be found that the load of a railway train, if amounting even to one ton per foot, will not produce so much strain on the bridge when it is resting at its ends on the outer piers, as the beam will produce itself when open and overhanging; for in the former position, the load, though probably double the weight per foot run of the beam itself, is acting on a span supported on each end, equal to less than half the length of the beam, whereas in the latter case the weight of the entire beam is acting at a span which gives it more than twice that leverage.

There are numerous other iron railway bridges of which we could give examples, but those already given comprehend the whole of the principles of construction adopted. The engraving (Plate II.) of some of the most important bridges will give perhaps a more comprehensive idea of their magnitudes.