O F JOHN A. ROEBLING, CIVIL ENGINEER, TO THE PRiTSIDENNTS AND DiN.LECTOBiS OF T1El NIAGARA FALLS SUSPENS:.ONT ANiD NTIAGAIR'A.F iLLS INTERNTATINl'AL BRIIDGE COMPANIES, ON THE CONDITION OF THE AUGUST 1, 1860. TRENTON, N. J.: MURPHY & BECIITEL, PRINTERS, OPPOSITE THE CITY HALL. 1860. TO THE PRESIDENTS AND DIRECT ORS OF TI-IHE NIAGARA FALLS SUSPENSION, AND NIAGARA FALLS INTERNATIONAL BRIDGE COMPANIES. GENTLEMEN: After an absence of two years, I have again visited the Niagara Railway Suspension BBridge, and have during a stay of three dlays, on the 18th, 19th and 20th of July, made a thorough examination of the work. I now present to you the following report: The Nialaral Bridge was openled for railway traffic on, the 18th of March, 1855; the lower floor for common travel was completed and in use the year previous. The number of trains and trips of single enginea, which at the present time pass over the Bridge in twenty-four hours, averages about forty-five. This great traffic accounts for the rapid wear of the rails, many of which require renewal. After a thorough examination of all parts of the work, I am unable to report any change. The camber of the floors and the deflection of the cables, as you well know, depend upon the temperature of the atmosphere. The relative level of the floors is the samne as it was in 1855. In order to be better enabled to judge whether tle stiffness of the superstructure has been impaired by a five years' traffic, I placed a leveling instrum-ent between tihe towers on the NTew York side, and observed the process of gradual deflection caused by five trains. 4 A train, conmposed of the engine " Essex," and tender, of 35 tons weight, drCawing 10 empty cars, produced a deflection in the centre of 0,4G2 feet A small engine, drawinig 2 loaded passenger cars, I baggage car and I loaded cattle car, 0,540 "" Another light engine with 5 loaded passenger cars and 1 bag gage car, 0,520 1" The engine " Essex" and tender alone, 0,315' The same engine returning with 8 loaded cattile cars, each holding 17 to 18 cattle of the largest size, 0,789 A short., but heavy train, such as the last., -when in the cenrter of the Bridge between the stays, produces the greatest deflection, comparatively. A longTer train, loaded at the same rate and extending over the limits of the stays, deflects the work but little mlore. In proportion, as the enclds of the floor are weighed down, the center is kept "up. Bry comparing the above observations wfith those of 1855, we discover no essential difference. The glreat experimlental train, which covered the whole Bridge wvith loaded cars, propelled by two engines, produced a deflection of ten inches. A similar train passed over now will do the same. The extreme rise and fall of the floor, owing to the contraction and expansion of the cables, amounts to nmore than two feet. But the cables being at liberty to contract and expand, this process can never affect their strengthll. In my report of 1855 I stated the aggregate ultimate strength of' the 4 suspension cables at 12,000 tons. Permanent wreight, supported by cables, 1,000 " Tension resulting, 1,810' Proportion of permanent tension to strength, 1:6.63 Tension produced by a train of 250 tons, 452' Aggregate tension, 2,262 Proportion of workingtension to strength, 1: 5.30 5 This liberal allowance of strength and freedom fronm -vibration will insure the durability of the cables. The question has been repeatedly askled, why trains are not allowed to pass over this Bridge at a higher rate of' speed than five nmiles an hour? This limitation is looked upon as a sigln of tacitly acknowledged wTeakness, and has been frequently referred to as a strong argum-lent against Suspension Bridges for railwTay purposes. This matter I discussed in my report of 1855, but I will explain again and more fully. The first great object of this limitation of speed is scafely. Although it may look somewhat timid in this fast going age, to see frieight trains move at the rate of five miles per hour, and passenger trains at even a less rate, yet Nwhen it is considered that this slow speed insures absoluCte sacfcfy, no matter what accident may happen to a train-the traveling conmmunity ought to be satisfied iwith this cautious tarrancgement. What would be gained b y a highler speed? Nothing whatever. The bridge forms a link between two termini, and there is always time to make connections. Passengers will prefer to cross at a slow rate in order to enjoy the splendid scenery during the passage. The track is so constructed as to form a trough of three feet depth between the girders, into which a car or locomnotive will instantly drop, the -moment it breaks down or leaves the track-provided there is no great bhleadc way. Should such an accident happen to a train, the broken down car', engine or track will act as a powerfiul brake, and will check its motion. Alhen planning the work, absolute safety mwas miade the first condition, and the track has been constructed accordingly. I would also remlark in this connection, that any further addition of fender-pieces to the track, as an additional mzeamns of safety, as has been proposed of late, would only prove an unnecessary incumbrance. A greater speed than five mniles per hour for passenger trains should never be permitted for the reasons stated. But should a much heavier freight business have to be accommodated in the future, the speed of freight trains may be increased wnithout injury to the work. All that will be tecessary is, to keep the track in perfect order, and to maiilntain a continuous bearing at the rail joints to prevent concussions. I will further state here, that by an additional expenditure of $20,000, the stiffness of the bridge may l)e so far increased as to admit of the highest practicable speed of freight trains, without producilg the slightest injurious effect upon the structure. I make this statement deliberately for the information of those professional opponents to Suspension Railway Bridges, who lhave rnade it tllheir business to cast doubts upon the permanency of this work. I also expect to demonstrate this when resuming the works on the Kentucky River Bridge, on the Lexington and Danville Railroad, which, wThell completed, will form a single span of 1224 feet from center to center of towers, over a chasm of 300 feet deep. The woodwork of the Niagara Bridge, being kept well painted and otherwise well protected, will last forty years and more. The old wooden St. C~lair Bridge, at Pittsburgh, Penn., which I removed to mnake roon for a new Suspension Bridge, recently completed, has stood exactly forty years. All its principal timbers of pine and oak, on removal, were found good and sound. A portion of this material, after being well tarred, has gone into the new suspension floor, and will no doubt render good service for another forty years. My views of the durability of the cables have undergone no change since 1855; they have only been strengthened by additional experience. This being a subject of great importance and of general interest, I embrace this opportunity to express myself more fully, and thus perhaps to contribute towards a better understanding of the nature of iron. The fact is well known, that wrought iron under cer 7 tain conditions will undergo certain radical changes, And so will all kinds of matter. The mlaterial untiverise is not by any means constituted upon the principle of' immntabiitty. Material existence is but a theatre of change, of breaking down, of reduction and of reeoastruction of the elements of matter. The Egyptian pyramids are even now undergoing a slow process of disintegration. The dry air of that region, slow in action, is still sure to do its appointed work. And as all hunan fabrics being but material constructions, will have to succumb to the same inexorable law, we can not expect that the Niagara Bridge will form an exception. Two kinds of changes are klnown, which will affect the strength of iron and other metals. The one is wrougAht by the chemical process of oxidation, and can be guarded against effectually, and is so guarded in the Niagara Bridge. All iron and wire within reach are kept well painted, and thus preserved against rust. The anchor chains and their connections with the cables, inside of the anchor masonry and in the rock below, after three coats of paint, are protected by the cemuent grout, w-hich forms a solid envelope, excluding air and moisture. [But aside from the mechanical protection thus afobrded, ]I depend principally, as was explained in my report of 1855, upon the well known chemical action of calcareous cements in contact with iron. Oxygen has a greater aflinity for lime than for iron. So long, therefore, as the cement will combine with oxygen, or in other words, has not become completely crystalized, which is a very slow process inside of heavy masonry, the iron will be protected. The cement, not exposed to the air, when setting slowly, has a tendency rather to expand than to contract; but suppose there should be cracks around the anchor bars, large enough to admit air and moisture. Water will then find its way through those cracks, but on reaching the iron, will be more or less impregnated with cement and thus add another protecting coato The chem 8 ical principle, which I have explained here, I apply daily in my factory for the preservation of wire against dampness. I have also carried on direct experiments for a numlber of years, iwhich have convinced me of the preserving property of calcareous cements in damp situations. On examining recently the anchor bars of the Monongahela Suspension Bridge at Pittsburgh, built sixteen years ago, I found theml perfectly preserved, as far as the cement, in which they are embedded, was removed. To satisfy yourself on this subject, I shall propose in a few years more, to remove the anchor blocks and to examine the upper links of the anchor cha ns of the Niagara Bridge. It should be remembered, that good cement grout, when not disturbed by any mechanical action or by a current of water, will set perfectly solid, and will become as hard as sand stone in course of time, and without shrinking. The anchor chains of the Niagara ]Bridge are, in my opinion, effectually guarded against oxidation. But ironi under certain conditions will undergo another change, which is not so well understood, and is indeed as yet a partial mystery. And this fact has been seized upon as an invincible argument against iron bridges generally, and against the Niagara Bridge especially. I refer to the supposed and popularly so-called granulation of fibrous wrought iron. Although this subject has engaged my attention for a series of years, and I have taken pains to obtain correct information, I yet hesitate to express any decided opinions, that would cover the whole field of investigation. The question at large I consider open yet. This much only I believe to be settled, that good iron will undergo no change in course of time, unless it is acted on by great heat, or is under the influence of strong continuous vibrations under tension. As an exception to this last proposition, may be cited the case of old anchors and chains, wlich, after being 9 exposed on the ground or in the ground, a great lenlgtli of time, had become considerably rusted and reduced in strength. Aside from rusting, magnetic influences were supposed to have been at iwork in destroying the strength of these irons. But it should be remarked, that none of these cases have been sufficienitly well examined to warrant sound conclusions. It is true, that the earth forms a great magnet, whose magnetism is nmaintained by the sun; and that the magnetic colndition of all metals is more or less depending upon the great parent magnet. A steel maagnet, that has lost its power or tension, when buried in the earth, will be restored bv its tmagnetic currents. But how far the cohesion and elasticity of wrought iron may be affcted by these currents, we are yet ignorant of. When a bar of iron is drawn apart by a tensile strain, the fractured ends are magnetically excited, and will attract iron filings, at the saime time that they become heated. Both phenomena, magnetism as well as heat, will always accompainy the forcible rupture of iron, as can be readily ascertained by experilment. The same phenomena are also exhibited when iron is halmmered cold, the heat in this case being more apparent than the m agn etis m. The cohesionl andl elasticity of wrought iron, although. different properties, appear to be closely related. In speaking of elasticity, I nmean the natural elasticity, and not what is produced by the forced process of tempering. And here may be pointed out a marked, physical difference between steel and iron. While the hardening or tempering of steel can be carried to almost any degree, that of the latter can not. Whatever destroys or impairs the elasticity of iron or steel, Awill also affect its cohesion. And this fact has also a signi-ficant magnetic bearing. Tempered or hardened steel possesses more tensile strencgtlh than soft steel. Now when tenmpered steel looses its hardness by annealing, it assimilates nearer to soft iron in its relation to magnetism. Red-hot iron is not attracted by a magnet, while a steel 10 m-agnet entirely looses its magnetic properties on beingc heated red hot. Another remarkable fact is, that artificial as well as natural magnets, when ovterloacded, become weakened. And so does the cohesion and elasticity of an iron or steel bar become weakened by overloading. The limit of elasticity, or of the recetperatiny force, as it might be termed, of iron and steel is generally stated at one-third of their ultimate strength. I am of the opinion, that this is much over-estimated for soft pulcddled irons, and zuede'r-estimated for good hammered charcoal irons, and still more for steel. The force which holds together the molecules of iron, is termed cohesion. IHeat will expand iron, and when applied intensely and continuously, will melt it, and will thus destroy all cohesion, and at the same time all elasticity and all magnetic tension. It follows then that heat of a certain degree is opposed to cohesion and elasticity. And this explains why large masses of wrought iron, when being forged, and thus subjected for a considerable length of time to an annealing process, will, in the centre, become greatly reduced in cohesion and elasticity. The previously existing fibre in the faggots will change into a coarse crystaline texture, because the iron being in a pasty and nearly molten state, and the mechanical effect of hammlering being confined to the surface, and not penetrating to the centre, the formation of large crystals will be left undisturbed. Broken car-axles sometimes appear to have undergone a similar change. The fact is, that they generally exhibit a crystaline fi'acture. But I suspect, that malny new axles, although manufactured out of fibrous roughbar, mwill, when finished and broken before they are used, also exhibit a crystaline fracture. In my own practice I have witnessed the fact that an experienced manufacturer, anxious to satisfy me, did nlot succeed in manufacturing round bolt of four to five inches diameter out of good fibrous roughbar, without produciug a crystaline texture in the centre. The oftener he piled the iron, the worse the result. On the other hand, I 11 never heard of a failure when the bolt was forged entire under the hammer out of good and well worlked, and thoroughly hammered charcoal blooms, their rough ends cut otf: The most fibrous bar iron may be broken so as to lresent a granular and somewhat crystaline fracture, and this iwTithout undergoing any molecular change in the texture. Take a fibrous bar, say ten feet long, but the longer the better, nip it in the centre all around with a cold chisel, then poise the bar upon the short edge of a large anvil, aind a short piece of iron, placed eight or nine inches from the edge on the face of the anvil, then strike a few heavy blows upon the nip, so that each blow will cause the bar to rebound, and to vibrate intensely, and the result will be a granular and somewhat crystaline fracture. Now take up the two halves, and nip them again all aroulnd, about one or two inches off' the fractured ends, break them off by easy blows over the roernd edge of the anvil, and the fibre will appear again. This experiment proves that a break, caused by sudden jars and intense vibration, may show a granular and even crystaline fracture, without having changed the molecular arrangement of the iron. All fibres are composed of mineral crystals, drawn out and elongated or flattened; and the fracture may be produced so as to exhibit in the same bar, and within the same inch of bar, either more fibre or more crystal. But a coarse crystaline bar will under no circumstances exhibit fibre; nor will a well worked out fibre exhibit coarse crystals. My own view of this matter is, that a molecular change, or so called granulation or Crystalization, in consequence of vibration or tension, or both combined, has in no instance been satisfactorily proved or demonstrated by experiments. I further insist that crystalization in iron or any other metal can never take place in a cold state. To form crystals at all, the metal must be in a highly heated or nearly a molten state. 12 On the other hand, I am witnessing the fact daily, that -vibration and tension combined will greatly affect the strength of iren without changing its fibrous texture. The cohesion and elasticity of wire and wire rope will be rapidly destroyed by great tension and vibration combined. Whether I shall be able to account for it or not, there stadCs the fact. But what is true of iron wire, applies with equal force, and when all circumstances and conditions are dluly proportioned, with even greater force, to larger masses. The extensive opportunities which my pursuits offer, to make experiments and observations on wire and wire rope, authorize a positive expression on this subject. A great deal of fancy speculation has been indulged in of late years on this question of granulation and crystalization, but generally by men whose opinion can have no weight. Now, while the fact remains that iron and steel will lose their strength by vibration and tension, it is proper to state, also, in this connection, that this loss of strength bears a due proportion to the extent and duration of the vibration and tension. WVire ropes may lose their strength by three months service, with]otl exhibiting much vwear; anid they may also last ten years, running all the time, anrd be greatly worn, before their strength is so far reduced as to be unfit to do duty. I will state here, that there are now ropes of my manufacture on the inclines of the Morris Canal, which have run nine years. This great durability is owing to comparative absence of vibration, in consequence of slow speed and good maclhinery, although a high tension is maintained. The greater the elasticity and cohesion of the iron or steel, the better it will support vibration and tension, always provided, that the extent of this vibration and the amount of tension are kept within safe limits. Vitness as examples the durability of watch-springs, piano wire, sofa and wagon springs, etc., etc., etc. Wrought-iron, that has become brittle, as for instance 13 chain, car axles, wire or wire rope, on being annealed, will have its softness andl apparently also its strength restored. As far as softness is concerned, this is correct; but in regard to strength, when applied to wire or wire rope or to fine chains, it is a mistake. Soft annealed wire possesses only half the strength which hard wire has, and is without ally elasticity. But wire rope without elasticity is worthless; very little work will nmake it brittle again and vworse than before. It is different with heavy chains and with car-axles. Made of indifferent material, crystaline or brittle when new, they will be greatly improved by an alnnealing process at the very beginnin cg; alnd if this process is repeated from timne to time, their lifetimlie may be prolonged. I maintain that a good car-axle, made of good material, and finished at the proper heat, by hammrering or rollinlg, is stiffer and stronger than the same axle, when again subjected to ann ealing without hammering or rollinlg. Annealing restores softness, but at the same time reduces cohesion and elasticity. To restore the iroll of a brittle car-axle fully, can only be done by a full heat, with halmmering or roililng, which of course will reduce its diameter. The opinion prevails, that a well drawn out fibre is the only sure sign of tensil strenoth. This however is true only whlen applied to ordfiazrty' qualities of bar or rail iron. The fact is different with good charcoal irons and with steel. The greatest cohesion is accompanied by a fine close-grained uniform appearance of texture, whlich, under a magnlifying glass, exhibits fibre. The color is a silvery lustre free from dark specks. The finer and more closegrained the texture, the nearer the iron approaches to steel. Those who are familiar -with good Swedish or Norway irons, will support these statements. These facts alone should be sufficient to disprove the erroneous lnotion that good iron and steel, which should always be granular, will become so only by vibration, and will thereby lose their strength. But it is importalnt to keep in mind tlle distinction between a fine uniforin granular fracture, and 14 a coarse crystaline friacture. Where coarse crystalization appears, there is a want of contact and compactness, consequently of cohesion and strength generally. Wire cables, car-axles, piston-rods, connecting-rods, and all such pieces of machinery, which are exposed to great tension as well as torsion. and vibration, should be manufactured of iron which not only possesses great cohesion, but also a high degree of hardness and elasticity. The best car-axles now in use, are those made of soft steel by KIrupf, in Germany. This steel is manufactured from the spathic ore or natural steel ore, of the celebrated mines at Muessen in Siegen, Prussia. A correct report on these axles was given to me by one of the Prussian Commissioners of Railways, in whose district Krupf's works are located. They are safe in cold weather and seldom known to break. This proves that soft steel with more of a granular texture than fibre, possesses a much greater elasticity and strength than the best fibrous iron; and it also furnishes another strong proof against the granulation theory, so much credited in this country. It may be objected, that steel is a different metal fiom iron. But all irons and steels are only so many different alloys of the same metal. There is no essential difference between the two. WThat constitutes the true chemical and physical difference between the two varieties, is not so clear. The old idea, that steel owes its distinguishing properties to a greater per centage of carbon alone, is no longer maintained. There are not two metallurgists who agree as to the proper per centage of carbon that good steel ought to contain. The ablest chemists who have analyzed iron and steel, from Karsten and Berzelius down to the present day, have not been able to give us a correct analysis of these two metals. Mr. Mushet, jun., has recently shown that the excellence of steel is depending' upon the presence of Titanium, a substance formerly overlooked. But so long as the chemistry of iron and of steel is still without a sure basis, we must fall back upon well discerned empyrical facts. 15 The capacity of irons to resist vibration and tension differs much in different qualities, and still greater is this difference when the irons are exposed to a very cold tenperature. The tubular bridge at Montreal will not last as long as one in Great Britain of the same dimensions, imaterial and workmanship, and rendering the same service; and still less than the tubes over the Nile in Egypt. One hard winter in Canada will be as trying to the structure as ten years are in Great Britain. In order to examine the fitness of various qualities of iron for the manufacture of wire rope, I undertook, during the hard winter of 1856, at my establishment at Trenton, a series of experiments, when the thermometer was five to ten degrees below zero. The samples for testing, about one foot long, were reduced in the center to exactly three-quarters of an ilnch square, and their ends left larger, were welded to heavy eyes, mlaking in all a bar of three feet long. Thus prepared, they were thrown outside of the mill, covered wTith snow aind ice, and left exposed for several days and nights. Early in the morning, before the air grew warmer, a salmple, enclosed in ice, would be put into the testing machine, and at once subjected to a strain of 26,000 pounds, the bar being suspended in a vertical position, left free all around. A stout millhandcl, armed with a billet of one and a half inch in diameter and two feet long, then struck the sample horizontally a number of blows, hitting the reduced section as hard as he could. The blows were counted and continued until rupture took place. Care was taken to maintain a tension of twenty-six thousand pounds during this test, by screwing up the lever, while the sample kept stretching Other means for producing vibration were attempted, but none proved so effective as the hitting with an iron bolt. I would remark here, that most of these irons would support from seventy to eighty thousand pounds per square inch; and that good samples of threequarters of an inch square, would support a strain of twenty-six thousand pounds for a whole week, with no 16 visible stretching', provided all vibration and jarring wvas avoided. But the least jar would produce a permanent elongation. Without going into the details of these interesting and instructive experiments, I will only state that the number of blows which the different samples resisted, when encased in ice, ranged from three to one hundred and twenty. Inferior qualities of a crystaline texture would break at the third or fourth blow. Good samples of refined puddled bar resisted very well, and went up to sixty blows, while the better qualities of hamlmered charcoal irons, supported up to one hundred and twenty blows, stretching and drawingl all the tinme. Indeed, it seemed a w iredrawing process on a rough scale. On the tension being reduced to twenty thousand pounds, some good samples resisted the almost incredible number of three hundred blows, before breaking. Such qualities of iron may be depended upon for the construction of wire cables and car-axles. They will be safe at the North Pole, while inferior qualities may answer verv well in warmer latitudes. Well observed facts of the durability of irons, when exposed to tension and vibration, are of more value than speeulative opinions. I will here record a few more facts, experienced by myself. In 1844 I removed the old timber aqueduct over the Allegheny river at Pittsburgh, the heaviest work of that description in the United States, consisting of' seven spans of one hundred and fifty feet reach. It had stood fourteenl years. All tlhe suspension bars taken out of the old trusses and arches, and originally made of good puddled iron, on being tested and worked up into bolts for the new wire suspension aqueduct, proved of good quality, as good as such irons generally are. During the great fire at Pittsburgh in 1845, the old Monongahela bridge, of eight. spans, a heavy Burr structure, burned down. I contracted to put up a suspension bridge, and accepted all the old materials, which were not 17 consumed, including about thirty tons of hammlered charcoal iron of excellent quality. This iron, after a severe usage for over thirty years, was found so good that I had it all drawn into wire. Every bar was good for sixty thousand pounds per square inch, as strong and tough as it ever could have been before going into the bricldge. The old structure was loose and limber, producingl considerable vibration on all vertical bars. On excavating for the southern ainchorage between the old wing-walls of the old Monongahela bridge, a number of round bars of one and a quarter inches diameter, about forty feet long, good puddled fibrous iron, was taken up. They had served as tie bars) to keep the retaining walls fi-iom spreading. Screwed up tight, they had been under Around about twenty-five years, emibedded in clay. The outside rust, firmly combined with clay and sand, appeared to have formled a protective coat. At any rate the strength of the iron had not suffered at all from oxidation, its quality was as,good as any puddled bar ianLaufactulred at thle present day. Last year, while removing tlhe old St. Clair Street Bridge over the Alleghany River at Pittsburgh, to make roon for a new Suspension Bridge, since completed, I examined the old iron witlh considerable interest and care. All this iron had been manufactured about forty-one years ago, and had been the result of the first attempts at puddling ever made west of the Alleghany Mountains. The manufacturer, who is still living, informed mne that in those days puddling was not well understood, and that, althouglh the stock was good cold blast charcoal pig, the iron turned out of a highly crystaline texture. It proved so on its fracture, but of a good color, the texture was uniform and not coarse. On being heated and drawn lown to half its size, it made a strong fibrous iron; all it wanted was work. There was not one fibrous bar in the whole lot of suspension bars; they were all alike crystaline and brittle in texture. This iron had, from the manufacturer's own testimony, undergone no change; it was as crystaline on the last day as it was onl the first. But there was another quality of iron in the same structure. The straps and bolts which connected the chords with the posts and braces, had been manufactured of a good quality of hammlered charcoal iron, and a most capital iron it proved, after forty years' service. I will also draw attention to those interesting experiments, made recently by I/Mr. Albert Fink, on a number of suspension bars, taken out of his bridges on the Baltimore and Ohio Railroadcl, for the purpose of testing their strength after seven years' service. These tests exhibited a rate of strength, which is only possessed by good iron, and led Mr. Fink to the conclusion that seven years' wear had not affected the bars. All irons form alloys of pure iron, mixed with carbon and other impurities. A certain amount of impurities in the shape of good cinder appears to be necessary to impart strength and cohesion to this metal, and also to mlake it lmalleable, and to give it welding properties. The purer the iron is, the higher the heat at which it will weld. Compare for instance good Swedish iron with common puddled bar. While the latter will weld at a low heat, the former requires a much higher heat. Compare their fracture and color. The good Swedish bar will exhibit either a fine granular appearance or fibre, accompanied by a silvery lustre, showing comparative purity; the puddled bar will be of a dark color, with a graphit lustre, and will show a coarse texture or loose fibre. During the process of puddling, as well as of blooming, the melted pig-iron is mixed with cinder, and this mixture, which will adhere by cohesion, prevents the:orlmation of large crystals, which is the tendency of pure iron in a mnolten state. Now by working (bringing- to natture, as the puddler calls it,) this mixing and crystalization is promoted. The subsequent squeezing and rolling of the pauddled ball, or the hammering and shingling of the bloom, will have the effect of condensing, larminating, reducing and drawing out these crystals, at the 19 same time, removing and squeezing out the superabundant cinder from between the metallic crystals. Thus the drawn out fibre is composed of an aggregate of pure iron threads and leaves, enveloped in cinder. Pure iron as well as very impure iron is weak; the maximum strength and toughness is obtained by a certain mixture of pure iron with carbon and cinder, thoroughly worked and incorporated. When the fibrous and laminar aggregation becomes so dense as to be fit for the manufacture of steel, then are by this very process sufficient impurities expelled, and the greatest degree of cohesion is obtained, Hence strong steel can only be made of strong iron, no matter what chemicals may be administered during the process. Keeping the above process before our mind, we may now understand why even the best fibrous wrought-iron, iwhen exposed to long continued vibration under tension, or to tortion, bending or twisting, must inevitably become brittle, because the ironz threads and lamincae beconme loosenzed in their cinder enrvelooes. But the cohesion between the iron and its cinder once destroyed, and its strength is gone. Now whether cohesion is the result of magnetic attraction (according to Fraday,) or otherwise, this process appears to be purely mechanical. But let the explanation, which is here offered, be correct or not, the fact remains that fibrous iron and all kinds of iron and steel, will be'rendered brittle by vibration and tension, or by bending and twisting, with/out undergoing any mysterious change in its molecular arrangement. It is only within the last one hundred years that wrought-iron has become a necessity on public and private works. Large structures, entirely composed of iron, are of a still more recent date. Long experience on a large scale is therefore wanting. But as far as it goes, the opinion is fully sustained, that good iron, not overtaxed by tension and vibration, and otherwise preserved, will prove one of the most durable building materials at our disposal. The Menai Chain Suspension 1Bridge hlas now stood about thirty-six years, and is still considered a safe work, although it has, for the want of stiffness, on several occasions, suffered severely from gales. The old Wire Suspension lBridge, at Friburgl, in Svitzerland, has been in use about twenty-seven years, but it does not possess enough of strength and stiffness to guarantee its safety much longer in its present state. It should be rememlnbered that there are many suspension bridges in this country, as iwell as in Europe, built without any regard to stifiless, and are therefore constantly subjected to vibration, which must greatly limit their durability. T]lhe cables of the Niagara ]Bridge, on the other hand, are free from vibration, consequently will last as lon g cas the nature of good wrought-iron will permit, when subjected to a moderate tension, not exceeding one-fifth (f its ultimate strength. This durability I anm- unwiilling to estimate at less than several hundred years. Iron has emnphatically become the mazterial of the acfc. IUpon its proper use the future comlfort and physicaRl advancemlent of the human race Till principally depend. It w;ll yet be the harbinger of peace, as already it has given us the means of locomotion and of intelligent intercourse. The subject of this paper is therefore of great importancel and is entitled to a truthful consideration. I will close this report by repeating once imore, that the cables of the Niagara Bridge are mzade of a superior quality of imaterial; that they possess an abundance or strengthl; that they are free fronm vibration; that they are well preserved and taken care off; and consequently that they may safely be trusted for a lonog series of years. Respectfully submitted, by Your obedient servant, JOHN A. ROEBLING, Civil Engineer, Trenton, N. J., Aug. 1, 1860.