The credit of having invented the arch is almost universally assigned to the ancient Romans, though the period of its introduction and the date of its first application to bridge building are unknown. That some centuries before the Christian era, the timber bridges of Rome had not been superseded by those of more permanent construction is implied in the legend of the defence of the gate by Horatius Cocles—a tale which has stirred the heart of many a schoolboy, and is known to everybody by Macaulay’s spirited verses, in which

Some of the arched bridges built by the Romans remain in use to this day to attest the skill of their architects. The Ponte Molo at Rome, for example, was erected 100 B.C.; and at various places in Italy and Spain many of the ancient arches still exist, as at Narni, where an arch of 150 ft. span yet remains entire. Until the close of the last century the stone or brick arch was the only mode of constructing substantial and permanent bridges. And in the present century many fine bridges have been built with stone arches. The London and Waterloo Bridges across the Thames are well-known instances, each having several arches of wide span, attaining in the respective cases 152 ft. and 120 ft. The widest arch in England, and one probably unsurpassed anywhere in its magnificent stride of 200 ft., is the bridge across the Dee at Chester, built by Harrisson in 1820. At the end of last century cast iron began to be used for the construction of bridges, a notable example being the bridge over the Wear at Sunderland, of which the span is 240 ft. But with the subsequent introduction of wrought iron into bridge building a new era commenced, and some of the great results obtained by the use of this material will be described in the present article. In order that the reader may understand how the properties of wrought iron have been taken advantage of in the construction of bridges, a few words of explanation will be necessary regarding the strains to which the materials of such structures are exposed.

Such strains may be first mentioned as act most directly on the materials of any structure or machine, and these are two in number, namely, extension and compression. When a rope is used to suspend a weight, the force exerted by the latter tends to stretch the rope, and if the weight be made sufficiently great, the rope will break by being pulled asunder. The weight which just suffices to do this is the measure of the tenacity of the rope. Again, when a brick supports a weight laid upon it, the force tends to compress the parts of the brick or to push them closer together, and if the force were great enough, the brick would yield to it by being crushed. Now, a brick offers so great a resistance to a crushing pressure, that a single ordinary red brick may be capable of supporting a weight of 18 tons, or 40,320 lbs.—that is, about 1,000 lbs. on each square inch of its surface. Thus the bricks at the base of a tall factory chimney are in no danger of being crushed by the superincumbent weight, although that is often very great. The tenacity of the brick, however, presents the greatest possible contrast to its strength in resisting pressure, for it would give way to a pull of only a few pounds. Cast iron resembles a brick to a certain extent in opposing great resistance to being crushed compared to that which it offers to being pulled asunder, while wrought iron far excels the cast metal in tenacity, but is inferior to it in resistance to compression.

The following table expresses the forces in tons which must be applied for each square inch in the section of the metals, in order that they may be torn apart or crushed:

Besides the direct strains which tend to simply elongate or compress the materials of a structure or of a machine, there are modes of applying forces which give rise to transverse strains, tending to twist or wrench the pieces or to bend them, or rupture them by causing one part of a solid to slide away from the rest. Strains of this kind no doubt come into play in certain subordinate parts of bridges of any kind; but if we divide bridges according to the nature of the strains to which the essential parts of the structure are subject, we may place in a class where the materials are exposed to crushing forces only, all bridges formed with stone and brick arches; and in a second class, where the material is subjected to extension only, we can range all suspension bridges; while the third class is made up of bridges in which the material has to resist both compression and extension. This last includes all the various forms of girder bridges, whether trussed, lattice, or tubular. The only remark that need be here made on arched bridges is, that when cast iron was applied to the construction of bridges, the chief strength of the material lying in its resistance to pressure, the principle of construction adopted was mainly the same as that which governs the formation of the arch; but as cast iron has also some tenacity, this permitted certain modifications in the adjustment of the equilibrium, which are quite out of the question in structures of brick and stone.

The general principle of the construction of girder bridges is easily explained by considering a simple case, which is almost within everybody’s experience. Let us suppose we have a plank supported as in Fig. 139. The plank will by its own weight sink down in the centre, becoming curved in the manner shown; or if the curvature be not sufficiently obvious, it may always be increased by placing weights on the centre, as at g. If the length of the plank had been accurately measured when it was extended flat upon the ground, it would have been found that the upper or concave surface, a b, had become shorter, and the lower or convex surface, c d, longer when the plank is supported only at the ends—a result sufficiently obvious from the figure it assumes. It is plain, then, that the parts of the wood near the upper surface are squeezed together, while near the lower surface the wood is stretched out. Thus, the portions in the vicinity of the upper and lower surfaces are in opposite conditions of strain; for in the one the tenacity of the material comes into play, and in the other its power of resisting compression. There is an intermediate layer of wood, however, which, being neither extended or compressed, receives no strain. The position of this is indicated by the line e f, called the neutral line. If the plank, instead of being laid flat, is put upon its edge, as in Fig. 140, the deflection caused by its weight will hardly be perceptible, and it will in this position support a weight which in its former one would have broken it down. There is in this case a neutral line, e f, as before; but as the part which is most compressed or extended is now situated at a greater distance from the neutral line, the resistance of the material acts, as it were, at a greater leverage. Again the portions near the neutral line are under no strain; they do not, therefore, add to the strength, although they increase the weight to be supported, and they may, for that reason, be removed with advantage, leaving only sufficient wood to connect the upper and lower portions rigidly together. The form of cast iron beams, Fig. 141, which were used for many purposes, depends upon these principles. The sectional area of the lower flange, which is subjected to tension, is six times that of the upper one, which has to resist compression, because the strength of cast iron to resist pressure is about six times greater than its power of resisting a pull. If the upper flange were made thicker, the girder would be weaker, because the increased weight would simply add to the tension of the lower one, where, therefore, the girder would be more ready to give way than before. If we suppose the vertical web divided into separate vertical portions, and disposed as at Fig. 142, the strength of the girder, and the principle on which that strength depends, will be in no way changed, and we at once obtain the box girder, which on a large scale, and arranged so that the roadway passes through it, forms the tubular bridge. It is only necessary that the upper part should have strength enough to resist the compressing force, and the lower the extending force, to which the girder may be subject; and wrought iron, properly arranged, is found to have the requisite strength in both ways, without undue weight. The various forms of trussed girders, the trellis and the lattice girders, now so much used for railway bridges, all depend upon the same general principles, as does also the Warren girder, in which the iron bars are joined so as to form a series of triangles, as in Fig. 143.

Girders have been made of wrought iron up to 500 ft. in length, but the cost of such very long girders is so great, that for spans of this width other modes of construction are usually adopted.

GIRDER BRIDGES.

The Britannia Bridge, which carries the Chester and Holyhead Railway across the Menai Straits, is perhaps the most celebrated example of an iron bridge on the girder principle. It was designed by Stephenson, but the late Sir W. Fairbairn contributed largely by his knowledge of iron to the success of the undertaking, if he did not, in fact, propose the actual form of the tubes. Stephenson fixed upon a site about a mile south of Telford’s great suspension bridge, because there occurred at this point a rock in the centre of the stream, well adapted for the foundation of a tower. This rock, which rises 10 ft. above the low-water level, is covered at high water to about the same depth. On this is built the central tower of the bridge, 460 ft. from the shore on either side, where rises another tower, and at a distance from each of these of 230 ft. is a continuous embankment of stone, 176 ft. long. The towers and abutments are built with slightly sloping sides, the base of the central or Britannia tower being 62 ft. by 52 ft., the width at the level where the tubes pass through it, a height of 102 ft., being reduced by the tapering form to 55 ft. The total height of the central tower is 230 ft. from its rock foundation. The parapet walls of the abutments are terminated with pedestals, the summits of which are decorated by huge lions, looking landwards. As each line of rails has a separate tube, there are four tubes 460 ft. long for the central spans, and four 230 ft. long for the shorter spans at each end of the bridge. Each line of rails, in fact, traverses a continuous tube 1,513 ft. in length, supported at intervals by the towers and abutments. The four longer tubes were built up on the shore, and were floated on pontoons to their positions between the towers, and raised to the required elevation by powerful hydraulic machinery. The external height of each tube at the central tower is 30 ft., but the bottom line forms a parabolic curve, and the other extremities of the tubes are reduced to a height of 22¾ ft. The width outside is 14 ft. 8 in. Fig. 144 shows the construction of the tube, and it will be observed that the top and bottom are cellular, each of the top cells, or tubes, being 1 ft. 9 in. wide, and each of the bottom ones 2 ft. 4 in. The vertical framing of the tube consists essentially of bars of ⟙-iron, which are bent at the top and bottom, and run along the top and bottom cells for about 2 ft. The covering of the tubes is formed of plates of wrought iron, rivetted to ⟙- and ∟-shaped ribs. The thickness of the plates is varied in different parts from ½ in. to ¾ in. The plates vary also in their length and width in the different parts of the tubes, some being 6 ft. by 1¾ ft., and others 12 ft. by 2 ft. 4 in. The joints are not made by overlapping the plates, but are all what are termed butt joints, that is, the plates meet edge to edge, and along the juncture a bar of ⟙-iron is rivetted on each side, thus: . The cells are also formed of iron plates, bolted together by ∟-shaped iron bars at the angles. The rails rest on longitudinal timber sleepers, which are well secured by angle-iron to the ⟙-ribs of the framing forming the lower cells. More than two millions of rivets were used in the work, and all the holes for them, of which there are seven millions, were punched by special machinery. The rivets being inserted while red hot, and hammered up, the contraction which took place as they cooled drew all the plates and ribs very firmly together. In the construction of the tubes no less than 83 miles of angle-iron were employed, and the number of separate bars and plates is said to be about 186,000. The expansion and contraction which take place in all materials by change of temperature had also to be provided for in the mode of supporting the tubes themselves. This was accomplished by causing the tubes, where they pass through the towers, to rest upon a series of rollers, 6 in. in diameter, and these were arranged in sets of twenty-two, one set being required for each side of each tube, so that in all thirty-two sets were needed. There are other ingenious arrangements for the same purpose at the ends of the tubes resting on the abutments, which are supported on balls of gun-metal, 6 in. in diameter, so that they may be free to move in any manner which the contractions and expansions of the huge tubes may require. Each of the tubes, from end to end of the bridge, contains 5,250 tons of iron. The mode in which these ponderous masses were raised into their elevated position is described in the article on “Hydraulic Power,” as it furnishes a very striking illustration of the utility and convenience of that contrivance. The foundation-stone of the central tower was laid in May, 1846, and the bridge was opened in October, 1850. The tubes have some very curious acoustic properties: for example, the sound of a pistol-shot is repeated about half a dozen times by the echoes, and the tubular cells, which extend from one end of the bridge to the other, were used by the workmen engaged in the erection as speaking-tubes. It is said that a conversation may thus be carried on with a person at the other end of the bridge, a distance of a quarter of a mile. The rigidity of the great tubes is truly wonderful. A very heavy train, or the strongest gale, produces deflections in the centre, vertical and horizontal respectively, of less than one inch. But when ten or a dozen men are placed so that they can press against the sides of the tube, they are able, by timing their efforts so as to agree with the period of oscillation proper to the tube, to cause it to swing through a distance of 1¼ in.—an illustration of facts of great importance in mechanics, showing that even the most strongly built iron structure has its own proper period of oscillation as much as the most slender stretched wire, and that comparatively small impulses can, by being isochronous with the period of oscillation, accumulate, as it were, and produce powerful effects. Bridges are often tried by causing soldiers to march over them, and such regulated movements form the severest test of the freedom of the structures from dangerous oscillation. The main tubes of the Britannia Bridge make sixty-seven vibrations per minute. The expansion and contraction occurring each day show a range of from ½ in. to 3 in. The total cost of the structure was £601,865.

A stupendous tubular bridge has also been built over the St. Lawrence at Montreal, and the special difficulties which attended its construction render it perhaps unsurpassed as a specimen of engineering skill. The magnitude of the undertaking may be judged of from the following dimensions: Total length of the Victoria Bridge, Montreal, 9,144 ft., or 1¾ miles; length of tubes, 6,592 ft., or 1¼ miles: weight of iron in the tubes, 9,044 tons; area of the surface of the ironwork, 32 acres; number of piers, 24, with 25 spans between the piers, each from 242 ft. to 247 ft. wide.

Another singular modification of the girder principle occurs in the bridge built by Brunel across a tidal river at Saltash, Fig. 145. Here only a single line of rails is carried over the stream, which is, however, 900 ft. wide, and is crossed by two spans of about 434 ft. wide. A pier is erected in the very centre of the stream, in spite of the obstacles presented by the depth of the water, here 70 ft., and by the fact that below this lay a stratum of mud 20 ft. in depth before a sound foundation could be reached. This work was accomplished by sinking a huge wrought iron cylinder, 37 ft. in diameter and 100 ft. in height, over the spot where the foundation was to be laid. The cylinder descended by its own weight through the mud, and when the water had been pumped out from its interior, the workmen proceeded to clear away the mud and gravel, till the rock beneath was reached. On this was then built, within the cylinder, a solid pillar of granite up to the high-water level, and on it were placed four columns of iron 100 ft. high, each weighing 150 tons. The two wide spans are crossed by girders of the kind known as “bow-string” girders, each having a curved elliptical tube, the ends of which are connected by a series of iron rods, forming a catenary curve like that of a suspension bridge. To these chains, and also to the curved tubes, the platform bearing the rails is suspended by vertical suspension bars, and the whole is connected by struts and ties so nicely adjusted as to distribute the strains produced by the load with the most beautiful precision. When the bridge was tested, a train formed wholly of locomotives, placed upon the entire length of the span, produced a deflection in the centre of 7 in. only. This bridge has sometimes been called a suspension bridge because of the flexible chords which connect the ends of the bows; but this circumstance does not in reality bring the bridge as a whole under the suspension principle. The section of the bow-shaped tube is an ellipse, of which the horizontal diameter is 16 ft. 10 in. and the vertical diameter 12 ft., and the rise in the centre about 30 ft. Beside the two fine spans which overleap the river, the bridge is prolonged on each side by a number of piers, on which rest ordinary girders, making its total length 2,240 ft., or nearly half a mile; 2,700 tons of iron were used in the construction. As in the case of the Britannia Bridge, the tubes were floated to the piers, and then raised by hydraulic pressure to their position 150 ft. above the level of the water. The bridge was opened by the late Prince Consort in 1860, and has received the name of the Albert Bridge.

SUSPENSION BRIDGES.

The general principle of the suspension bridge is exemplified in a chain hanging between two fixed points on the same level. If two chains were placed parallel to each other, a roadway for a bridge might be formed by laying planks across the chains, but there would necessarily be a steep descent to the centre and a steep ascent on the other side. And it would be quite impossible by any amount of force to stretch the chains into a straight line, for their weight would always produce a considerable deflection. Indeed, even a short piece of thin cord cannot be stretched horizontally into a perfectly straight line. It was, therefore, a happy thought which occurred to some one, to hang a roadway from the chains, so that it might be quite level, although they preserved the necessary curve. In designing such bridges, the engineer considers the platform or roadway as itself constituting part of the chain, and adjusts the loads in such a manner that the whole shall be in equilibrium, so that if the platform were cut into sections, the level of the road would not be impaired.

Public attention was first strongly drawn to suspension bridges by the engineer Telford, who, in 1818, undertook to throw such a bridge across the Menai Straits, and the work was actually commenced in the following year. The Menai Straits Suspension Bridge has been so often described, that it will be unnecessary to enter here into a lengthy account of it, especially as space must be reserved for some description of other bridges of greater spans. The total length of this bridge is 1,710 ft. The piers are built of grey Anglesea marble, and rise 153 ft. above the high-water line. The distance between their centres is 579 ft. 10½ in., and the centres of the main chains which depend from them are 43 ft. below the line joining the points of suspension. The roadway is 102 ft. above the high-water level, and it has a breadth of 28 ft., divided into two carriage-ways separated by a foot-track. The chains are formed of flat wrought iron bars, 9 ft. long, 3¼ in. broad, and 1 in. thick. In the main chains, of which there are sixteen, no fewer than eighty such bars are found at any point of the cross section, for each link is formed of five bars. These bars are joined by cross-bolts 3 in. in diameter. The main chains are connected by eight transverse stays formed of cast iron tubes, through which pass wrought iron bolts, and there are also diagonal ties joining the ends of the transverse stays. The time occupied in the construction was 6½ years, and the cost was £120,000. This bridge has always been regarded with interest for being the first example of a bridge on the suspension principle carried out on the large scale, and also for its great utility to the public, who, instead of the hazardous passage over an often stormy strait, have now the advantage of a safe and level roadway.

The Clifton Suspension Bridge over the Avon, near Bristol, is noted for having a wider span than any other bridge in Great Britain, and it is remarkable also for the great height of its roadway. The distance between the centres of the piers—that is, the distance of the points between which the chains are suspended—is more than 702 ft. Part of the ironwork for this bridge was supplied from the materials of a suspension bridge which formerly crossed the Thames at London, and was removed to make room for the structure which now carries the railway over the river to the Charing Cross terminus. Five hundred additional tons of ironwork were used in the construction of the Clifton Bridge, which is not only much longer than the old Hungerford Bridge, but has its platform of more than double the width, viz., 31 ft. wide, instead of 14 ft. A view of this bridge is given in Fig. 146, where its platform is seen stretching from one precipitous bank of the rocky Avon to the other, and the river placidly flowing more than 200 ft. below the roadway. The picturesque surroundings of this elegant structure greatly enhance its appearance, and the view looking south from the centre of the bridge itself is greatly admired, although the position may be at first a little trying to a spectator with weak nerves. The work is also of great public convenience, as it affords the inhabitants of the elevated grounds about Clifton a direct communication between Gloucestershire and Somersetshire, thus avoiding the circuitous route through Bristol, which was required before the completion of the bridge.

The use of iron wire instead of wrought bars has enabled engineers to far exceed the spans of the bridges already described. The table on page 199 shows that iron wire has a tenacity nearly one-third greater than that of iron bars, and this property has been taken advantage of in the suspension bridge which M. Chaley has thrown over the valley at Fribourg, in Switzerland. This bridge has a span of no less than 880 ft., and is constructed entirely of iron wires scarcely more than ⅒ in. in diameter. The main suspension cables, of which there are two on each side, are formed of 1,056 threads of wire, and have a circular section of 5½ in. diameter. The length of each cable is 1,228 ft., and at intervals of 2 ft. the wires are firmly bound together, so as to preserve its circular form. But as the cable approaches the piers, the wires are separated, and the two cables on each side unite by the spreading out of the wires into one flat band of parallel wire, which passes over the rollers at the top of the piers, and is again divided into eight smaller cables, which are securely moored to the ground. Each of the mooring cables is 4 in. in diameter, and is composed of 528 wires. In order to obtain a secure attachment for the mooring cables, shafts were sunk in the solid rock 52 ft. deep, and the ingenious mode in which, by means of inverted arches, an anchorage in the solid rock is formed for the cables, will be understood by a reference to Fig. 147. The cables pass downwards through an opening made in each of the middle stones, and are secured at the bottom by stirrup-irons and keys. The suspension piers are built of blocks of stone, very carefully shaped and put together with cramps and ties, so as to constitute most substantial structures. These piers are embellished with columns and entablatures, forming Doric porticoes, enclosing the entrances to the bridge, which are archways 43 ft. high and 19 ft. wide. The roadway is 21 ft. wide, and is supported on transverse beams, 5 ft. apart, upon which is laid longitudinal planking covered by transverse planking. The roadway beams are suspended to the main cables by vertical wire cables, 1 in. in diameter. The length of these suspension cables of course varies according to their position, the shortest being ½ ft. and the longest 54 ft. in length. Each suspension cable is secured by the doubling back of the wires over a kind of stirrup, through which passes a plate of iron, supported by the two suspension cables, the latter being close together, and, indeed, only separated by the thickness of the suspension cables, which hang between them. The roadway has a slight rise towards the centre, its middle point being from 20 to 40 in. above the level of the ends, according to the temperature.

To test the stability of the bridge, fifteen heavy pieces of artillery, accompanied by fifty horses and 300 people, were made to traverse it at various speeds, and the results were entirely satisfactory. Indeed, a few years afterwards the people of Fribourg had another wire bridge thrown over the gorge of Gotteron, at about a mile from the former. This, though not so long (640 ft.), spans the chasm at a great height, and in this respect is probably not surpassed by any bridge in the world—certainly not by any the length of which can compare with its own. The height of the roadway above the valley is 317 ft., or about the same as that of the golden gallery of St. Paul’s Cathedral above the street. The structure is very light, and the sensation experienced when, looking vertically downwards through the spaces between the flooring boards, you see the people below diminished to the apparent size of flies, and actually feel yourself suspended in mid-air, is very peculiar, as the writer can testify.

The Americans have, however, outspanned all the rest of the world in their wire suspension bridges. They have thrown a suspension bridge of 800 ft. span over the Niagara at a height of 260 ft. above the water, to carry not only a roadway for ordinary traffic, but a railway. Suspension bridges are not well adapted for the latter purpose, but there seemed no other solution of the problem possible under the circumstances. The bridge, however, combines to a certain extent the girder with the suspension principle. The girder which hangs from the main cables (for they are made of wire), carries the railway, and below this is the suspended roadway for passengers and ordinary carriages. The engineer of this work was Roebling, who also designed many other suspension bridges in America.

The spans of any European bridges are far exceeded by that of the wire suspension bridge which crosses the Ohio River at Cincinnati, with a stride of more than 1,000 ft.; and this is, in its turn, surpassed by another bridge which has been thrown over the Niagara. This bridge, which must not be confounded with the one mentioned above, or with the Clifton Bridge in England already described, merits a detailed description from the audacity of its span, which is nearly a quarter of a mile, and entitles it to the distinction of being the longest bridge in the world of one span.

The new suspension bridge at the Niagara Falls, called the Clifton Bridge, of which a view is given in Fig. 147a, is intended for the use of passengers and carriages visiting the Falls, and it is also the means of more direct communication between several small towns near the banks of the river. The bridge is situated a short distance below the Falls, crossing the river at right angles to its course at a point where the rocks which form the banks are about 1,200 ft. apart. The distance between the centres of the towers is 1,268 ft. 4 in., and the bridge has by far the longest single span of any bridge in the world, the distance between the points of suspension being more than twice that of the Menai Bridge, and more than six times the span of the widest stone bridge in England. This remarkable suspension bridge was constructed by Mr. Samuel Keefer, and was opened for traffic on the 1st of January, 1869, the actual time employed in the work having been only twelve months. The cables and suspenders are made of wire, which was drawn in England at Warrington and Manchester, and the wires for the main cables were made of such a length, that each wire passed from end to end of the cable without weld or splice. The length of each of the two main cables is 1,888 ft., and of this length 1,286 ft. usually hangs between the suspending towers, the centre being about 90 ft. below the level of the points of suspension. This last distance, however, varies considerably with the temperature, for in winter the contraction produced by the cold brings up the centre to 89 ft. below the level line, while in summer it maybe 3 ft. lower. The centre of the bridge is about 190 ft. above the water in summer, and 193 ft. in winter. The cables are each formed of seven wire ropes, and each rope consists of seven strands, each strand containing nineteen No. 9 Birmingham gauge wires of the diameter of 0·155 in. The cables of this bridge do not hang in vertical planes, since in the centre they are only 12 ft. apart; while at the towers, where they pass over the suspension rollers, they are 42 ft. apart. The end of the platform which rests on the right bank is 5 ft. higher than the other, and if a straight line were drawn from one end to the other, the centre of the roadway would be in winter 7 ft. above it, and in summer 4 ft. From each point of suspension twelve wire ropes, called “stays,” pass directly to certain points of the platform. The stays are not attached to the cables, but pass over rollers on the tops of the towers, and are anchored in the rock, independently of the cables. The longest stays are tangential to the curve formed by the main cables, and they are fixed to the platform at a point about half-way to the centre. Other stays proceed from the platform at intervals of 25 ft., between the longest and the end of the bridge. The thickness of the stays is varied according to the strain they have to bear, and they form not only a great additional support to the platform, but they also serve to stiffen the bridge and lessen the horizontal oscillations to which the platform would be liable from the shifting loads it has to bear. There are also stays which transversely connect the two cables. The wire ropes by which the platform is suspended to the main cables are ⅝ths of an inch in diameter, and have such a strength that the material would only yield to a strain of 10 tons. These suspenders are placed 5 ft. apart and are 480 in number, the lengths, of course, being different according to the position. To each pair of suspenders is attached a transverse beam, 13½ ft. long, 10 in. deep, and 2½ in. wide. Upon these beams—which are, of course, 5 ft. apart from centre to centre—rests the flooring, formed of two layers of pine planking 1½ in. thick; and the roadway thus formed constitutes a single track 10 ft. in width. Along each side of the platform is a truss the whole length of the bridge, formed of an upper and a lower beam, 6½ ft. apart, united by ties and diagonal pieces. The lower chord of the truss is 2 ft. below the road, and on it rolled iron bars are bolted continuously from one end of the bridge to the other. The last arrangement contributes greatly to stiffen the platform, vertically and horizontally. In the central part of the bridge the flooring-boards are bolted up to the cables, and there are studs formed of 2 in. iron tubes, so that the platform cannot be lifted vertically without raising the cables also; and as thus 81 tons of the weight of the cables vertically rest upon the platform, great steadiness is secured, inasmuch as the central part of the cables must partake of any movement of the platform, and their weight greatly increases the inertia to be overcome. In order still further to prevent oscillations as much as possible, a number of “guys” are attached to the bridge. These are wire ropes of the same thickness as the suspenders, and they connect the platform with various points of the bank—some going horizontally to the summit of the cliffs, others vertically, but the majority obliquely. There are twenty-eight guys on the side of the bridge next the falls, and twenty-six on the other side. The thickness of the wire rope of which they are made being little more than ½ in., they are scarcely visible, or rather appear like spider lines. About 400 ft. of the length of the bridge in the centre is without either guys or stays except two small steel ropes, which, tightly strained from cliff to cliff, cross each other nearly at right angles at the centre of the bridge. The suspension towers are pyramidal in form and are built of white pine, the timbers being a foot square in section and very solidly put together, so that they are capable of bearing forty times the load which can ever be put upon them. The towers are surmounted by strong frames of cast iron, to which are fixed the rollers carrying the cables and stays to their anchorage. The weight of the bridge itself, together with the greatest load it can be required to bear, amounts to 363 tons. Its cost was £22,000, and it was constructed without a single accident of any kind.

The foam of the great falls is carried by the stream beneath the bridge, and in sunshine the spectator who places himself on the centre of its platform sees in the spray driven by the wind, not a mere fragment of a rainbow, or a semicircular arc, but the complete circle, half of which appears beneath his feet. The gorge of the Niagara is very liable to furious blasts of winds, for by its conformation it seems to gather the aërial currents into a focus, so that a gentle breeze passing over the surrounding country is here converted into a strong gale, sweeping down with great force between the precipitous banks of the river. Indeed, one would suppose that the cavern from which Æolus allows the winds to rush out, must be situated near Niagara Falls. The bridge is not disturbed by ordinary winds, although during its construction, before the stays and guys were fixed, it was subject to considerable displacement from this cause. The peculiar arrangement of the cables, by which they hang, not vertically, but widening out from the centre of the bridge, giving what has been termed the “cradle” form, has proved of the highest advantage, so that, with the aid of the guys and stays, and the plan of attaching the central part of the roadway to the cables, the bridge is believed to be capable of withstanding without damage a gale having the force of 30 lbs. per square foot, although its total pressure on the structure might then amount to more than 100 tons. The stability of the structure was severely tested soon after its erection by a furious gale from the south-west, by which the guys were severely strained; in fact, many of them gave way. In one case an enormous block of stone, 32 tons in weight, to which one of the guys was moored, was dragged up and moved 10 ft. nearer the bridge. This and some lateral distortion of the platform, which was easily remedied, was all the damage sustained by the bridge. By an increase of the strength of the guys, &c., and the addition of the two diagonal steel wire ropes mentioned above, the bridge was soon made stronger than before. Some years ago, when the Menai suspension bridge was exposed to a storm of like severity, that structure suffered great damage, the platform having been broken and some of it swept away. In the great gale which swept down upon the Niagara bridge, although the force of the wind was so great that passengers and carriages could not make headway, the vertical oscillations of the bridge never exceeded 18 in., an amount which must be considered extremely satisfactory in a bridge of the kind, having a span of nearly a quarter of a mile.^[4]

CANTILEVER BRIDGES.

The great Forth Bridge, now (December, 1889) approaching completion, is the first bridge on the cantilever and central girder principle that has been erected in Great Britain, and it has also the distinction of being by far the widest spanned bridge in all the world. We are told by the engineers of the bridge that the cantilever and girder principle is by no means new, for it has been adopted hundreds of years ago by comparatively rude tribes in the construction of timber bridges, to which it readily lends itself. Such bridges are described as having been erected by the natives of Hindoostan, Canada, Thibet, etc., even at remote periods. The principle of the cantilever and girder construction was well illustrated by Mr. Baker, one of the engineers of the bridge, at a lecture given by him at the Royal Institution, by means of what he termed “a living model,” of which (Fig. 147b) shows the general arrangement. Two men, seated on chairs, extend their arms and hold in their hands sticks, of which the other ends butt against the chairs. The central girder is represented by a shorter stick, suspended at a and b. We have here the representation of two double cantilevers, the ropes at c and d, connected with the weights, representing the anchorages of the landward arms of the cantilevers. When a weight is placed on a b, which was done in the “living model,” by a third man seating himself thereon, a tensile strain comes into action in the ropes and in the men’s arms, while the sticks abutting on the chairs have to resist a compressing force, and the weight of the whole is borne by the legs of the chairs, also under compression. Now let the reader imagine the men’s heads to be 360 feet above the ground, and about a third of a mile apart, while the distance between a and b is 350 feet, and he will have a rough but sufficiently clear idea, not only of the principle upon which the Forth Bridge is constructed, but also of the magnitude of one of its spans. To complete the comparison, Mr. Baker further invited his hearers to suppose that the pull upon each arm of the men is equal to 10,000 tons, and that the legs of each chair press on the ground with the weight of more than 100,000 tons.

The Forth Bridge spans the estuary at Queensferry nine miles north-west from Edinburgh, and its purpose is to afford uninterrupted railway communication along the eastern side of Scotland. It will, in effect, shorten the railway journey between Edinburgh and Perth, or Aberdeen, by nearly two hours. Queensferry had long been established as a usual place for crossing the Forth, and readers of Scott’s “Antiquary” will remember that the first chapter describes how Monkbarns and Lovel, by some accidental delays to the coach, lost the tide, and had to wait, to sail “with the tide of ebb and the evening breeze,” finding themselves, in the meanwhile, pretty comfortable over a good dinner at the “Hawes Inn.” This inn still stands, its situation being close to the southern end of the great bridge. A design for the erection of a light suspension bridge at the same spot was published at the beginning of the present century, but although the spans were to be equal to those of the present bridge (17,000 feet), the different scale of the projects may be inferred from the total weight of iron to be used being estimated at 200 tons, while 50,000 tons will be required for the structure now approaching completion.

In 1873, an Act of Parliament was obtained authorizing the construction of a suspension bridge at Queensferry, to carry the railway over the estuary. The design comprised practically two bridges, each carrying a single line of rails, the bridges being braced together at intervals. The central towers were to have been 600 feet high, or about 100 feet loftier than any other erection then existing in the world. The designer was the late Sir Thomas Bouch, and preparations were made for carrying out the plans by the erection of workshops and the manufacture of bricks for the piers. But the project was knocked on the head by the terrible disaster at the Tay Bridge, in December, 1879, when several of the central piers were overturned by the force of the wind, with swift destruction to a passing train, which was precipitated into the water, and every one of about ninety persons in the train perished. Sir Thomas Bouch having been the designer of the Tay Bridge, public confidence in his plan was shaken to such an extent, that the four railway companies who were promoting the construction of the suspension bridge abandoned the project in favour of a design on the cantilever and central girder system, which was then brought forward by Mr. (now Sir John) Fowler and Mr. Baker. When the Bessemer process had made steel attainable at a cheap rate, these engineers recognized the advantages which cantilever bridges, made of that material, presented for the wide spans required for carrying railways across navigable rivers, and in 1865 they had designed such a bridge, with 1,000 feet spans for a viaduct, across the Severn, near the position of the present tunnel. It was not, however, until 1881 that the designs for the Forth Bridge were published in English and American engineering journals. These designs at once attracted attention, and scarcely a year had elapsed before a railway bridge was built for the Canadian and Pacific Railway, on the same principle, and this has been followed by others since. It is, however, absurd to allege that the engineers took their ideas from America, merely because these smaller undertakings have been completed before the great work that dwarfs them all was open for traffic. The construction of the Forth Bridge on its present design was commenced in January, 1883. Its site at Queensferry is at a point where the estuary narrows, and where, in the very middle of the channel, there is a small rocky island, called Inchgarvie, that furnishes a solid foundation for the great central pier. On each side of this island the channels are about one-third of a mile wide, and more than 200 feet deep, and through them the tide rushes with great velocity. The impossibility of building up any intermediate piers, under such circumstances, is sufficiently obvious—the currents must be crossed at one span, if a railway bridge had to be made. The formation of the piers for such a work presented many novel problems, and much of the work had to be commenced in deep water; that is, the ground of rock or hard clay had to be prepared, in some parts, as far as 90 feet below high water. Each pier stands on four caissons, which are great tubes or drums of iron and steel, filled up with concrete. Each weighed, when empty, about 400 tons, but when filled up with concrete, the weight would be about 3,000 tons. The diameter of each is 70 feet, and the deepest one is sunk 89 feet below the water, and it was with no little labour that some of them were put in their places. Each caisson has an outer and an inner tube, is 70 feet in diameter at the base, and 60 feet at the top. Seven feet from the bottom, an air-tight partition formed a chamber in the lower part of the caisson, about 70 feet in diameter, by 7 feet high, and shafts sufficiently large to admit the passage of men and tools led from the top. Air was forced into this chamber, when the caisson had been sunk, expelling the water, and then men descended through the shafts and locks, in which a high pressure of air was also maintained, and excavated the material at the bottom, until the caisson had, by its own weight, sunk to the depth required. The work in this air chamber was carried on by means of electric lights, and ten or twelve weeks were occupied in sinking each caisson. The pressure of the air in the working chamber was sometimes as high as 35 pounds per square inch, or sufficient to maintain the mercurial column in a barometer 72 inches high, instead of the ordinary 29 or 30 inches. It was found that the labour in the compressed air chamber could not be done by our home workmen, as they were quite unaccustomed to the high air pressures required to keep out the water; but arrangements were made for the assistance of a staff of French workmen, inured to the conditions by long working under water in the construction of the docks at Antwerp.