the rude wheels keyed on to the axles, as was generally the case in baggage-waggons, and without grease, the friction must have been so enormous that a slight addition to the lifting power required by a steep incline must have been of comparatively little consequence. Where pack-saddles are used this is even more apparent; the load a horse can carry on its back is so small in proportion to its tractive power, that the steepness of the road is of comparatively little consequence. The mode by which all these difficulties were overcome was so graphically described by Sir W. Armstrong, in his opening address at the meeting of the British Association at Newcastle, that it may as well be given in his own words :

When coal was first conveyed in this neighbourhood from the pit to the shipping place on the Tyne, the pack-horse, carrying a burden of 3 cwt, was the only mode of transport employ

ed. As soon as roads suitable for wheeled carriages were formed carts were introduced, and this first step in mechanical appliance to facilitate the transport had the effect of increasing the load which the horse was enabled to convey from 3 cwt. to 17 cwt. The next improvement consisted in laying wooden bars or rails for the wheels of the carts to run upon, and this was followed by the substitution of the four-wheeled waggon for the two-wheeled cart. By this further application of mechanical principles the original horseload of 3 cwt. was augmented to 42 cwt. The next step in the progress of railways was the attachment of slips of iron to the wooden rails. Then came the iron tramway, consisting of cast-iron bars of an angular section; in this arrangement the upright flange of the bar acted as a guide to keep the wheel on the track. The next advance was an important one, and consisted in transferring the guiding flange from the rail to the wheel; this improvement enabled cast-iron edge rails to be used. Finally, in 1820, after the lapse of about 200 years from the first employment of wooden bars, wrought iron rails, rolled in long lengths, and of suitable section, were made in this neighbourhood, and eventually superseded all other forms of railway. Thus, the railway system, like all large inventions, has risen to its present importance by an series of steps; and so gradual has been its progress that Europe finds itself committed to a gauge fortuitously determined by the distance between the wheels of the carts for which wooden rails were originally laid down. Last of all came the locomotive engine, that crowning achievement of mechanical science, which enables us to convey a load of 200 tons at a cost of fuel scarcely exceeding that of the corn and hay which the original packhorse consumed in conveying its load of 3 cwt. an equal distance.'

At the point at which we now stand our mechanical skill has become so great, that the civil engineers have been forced to seek out the lowest levels, to carry long viaducts across our valleys, to bore tunnels throngh

mountains, and to scheme out a whole new system of intercommunication, in order to prevent the necessity of lifting a train up an incline from neutralising the advantages derived from the conquest achieved over the frictional element. Notwithstanding all our ingenuity, we can never, of course, get entirely rid of this difficulty; but we have done wonders in this direction, and are daily accomplishing more.

In addition to the normal difficulties from friction and weight, the crossing of rivers formed a third, that long impeded transport by land. Fords are not always practicable, ferries are always inconvenient; but to make a permanent roadway across a running stream was a difficulty which in early stages of the their architectural skill, the Egyptians never science seemed nearly insuperable. With all seem to have attempted it, at least they never tried to bridge the Nile; and as they made their own canals, and these were dry more than half the year, they had it all their own way as to how they would cross them, and were probably content with planks, or at the utmost with flags of stone resting on upright supports.

The Greeks had few rivers that were not fordable, and never consequently gave their minds to the subject; but the Romans faced the problem boldly, and with that grandeur of conception which characterised most of their architectural undertakings. There are still standing arches built by them of more than 100 feet span, springing at

more than 100 feet from the bed of the river.

Their greatest undertaking of this sort was probably Trajan's Bridge, over the Danube; but the superstructure was only of wood, though the piers were of stone and 180 feet apart, as near as can now be ascertained.

In modern times the bridge over the Dee at Chester is the largest arch that has yet been attempted in stone. It is 200 feet span, with a rise of only 42 feet; and Brunel built a bridge of brick over the Thames at Maidenhead of two elliptical arches, each 128 feet span, with only 22 feet rise. Though these surpass all that has been done elsewhere in their respective materials, it is probable that these dimensions might be exceeded, if it were worth while; but it is scarcely probable it will be found worth while, as iron is every day more and more employed in the composition of such structures. Before, however, it entirely supersedes the more durable materials, it is fortunate for us that we possess such a beautiful building as London Bridge, perhaps the most perfect specimen of its class in the world. It is constructed wholly of granite, with a centre arch 152 feet span, and with a roadway slightly but gracefully curved. This is far more pleasing

far in the application of iron to bridges, they perceived that though the metal possessed the quality of resisting compression to ten times the extent of the materials they had usually been employing, it was even more re

Long before these great bridges were erected, it had occurred to engineers that iron might probably be employed in building bridges. As early as 1775 Mr. Pritchard built one at Colebrook Dale, 100 feet span, and in 1795 Thomas Wilson erected one at Sunderland, 237 feet clear span, with only 260 tons of metal, while the centre arch of Southwark Bridge, only 3 feet more in width, contains 1665 tons. Hitherto these two have not been surpassed by any arches of the same kind; but Telford proposed to replace old London Bridge with one of a single arch, 600 feet span, and afterwards begged to be allowed to span the Menai Strait with one of nearly the same extent. More recently Mr. Page proposed to cross the Thames just above the Tower with a single arch of 750 feet clear span, to carry two lines of rails and a roadway 24 feet wide, besides footways. Bold as the project may appear, still Mr. Page's experience and admitted knowledge of the subject are such that no one doubts its feasibility. From various causes none of these great schemes have been carried out, though there seems no reason to doubt that they might have been executed with success. As the resistance to pressure in cast iron is as nearly as may be ten times that of stone, there seems at first sight no reason why an arch of iron, 1000 feet span, should not be made as easily with the same weight of material as one of 100 feet of stone; and as blocks can be cast with more precision than they can be hewn, and fitted with flanges and other constructive expedients, even the most gigantic arches ought to be far easier to build in this material. The one element of uncertainty is the contraction and expansion of the metal from heat; but there seems little cause to fear it. When we first made railroads we allowed a quarter of an inch free space between each bar, and took every precaution for freedom of expansion and contraction till one man, bolder than the rest, proposed to butt them one against the other and join them with fish-plates. This has now been done, so that the rail from London to Aberdeen is one continuous unbroken bar; it neither expands nor contracts, but submits, and so probably would a bridge, provided the abutments were sufficiently firm, or if it did expand, it would probably be marked only by a slight elevation at the crown of the arches.

Before, however, engineers had proceeded

long in finding out how best to avail themselves of this peculiarity. By suspending the roadway from a chain hanging from the summits of two tall towers, they in the first place got wholly rid of the bugbear of expansion or contraction, and were also able to span a greater space with an infinitely smaller quantity of metal than was required for a bridge in compression. So great, indeed, was the economy of weight, that there seemed no practical limit to the extent of the span, while all other structures were liable to be broken by their own weight when extended beyond certain moderate dimensions.

Unfortunately these good qualities were accompanied by others which disappointed the sanguine hopes that were at one time entertained of this mode of construction. Its very lightness rendered it liable to undulation, always unpleasant and sometimes dangerous; and its weight was frequently not even sufficient to resist the action of the wind, which ruined at one time the chain pier at Brighton, and seriously damaged the bridge over the Menai Straits, as well as that at Montrose. Notwithstanding this, Telford's great work has answered its purpose perfectly for the last thirty-seven years, and now that it has been strengthened, may still span the Straits for the next three centuries; while, considering the time when it was erected, it is one of the boldest as well as one of the most graceful works of modern engineering skill.

On the Continent, where scientific knowledge is generally in advance of practical skill, they have carried this principle to excess, by using wire, which is iron in the most perfect form for tenacity. This has reduced the weight of the bridge so much relatively to the load, as to render the undulation excessive, and frequently to lead to the most frightful accidents. Still the bridge over the Sarine at Friburg has stood for thirty years, with very slight repairs, though its span is 870 feet, while that of the Menai Strait is only 570, and the bridge which recently crossed the Thames at Hungerford Market, which was our largest and typical example of the class in England, was only 6761.

The boldest and grandest application of this principle is the bridge constructed for railway traffic by Mr. Roebling, just below the Falls of Niagara. So rapid has been the progress of engineering science, that if any one had proposed twenty years ago to throw

a railway bridge over a chasm 800 feet wide | and 245 feet above such a foaming torrent as that of the Niagara, he would have been looked on as a madman. Yet this has now been accomplished, and by very simple means. The bridge cousists of a rectangular tube 20 feet deep by 26 feet wide, or rather two floors 18 feet apart-the upper carrying the railway, the lower the roadway for ordinary traffic. These are connected together by a series of wooden posts, braced together by diagonal iron tie-rods. By bracketing out from the rocks, the free length of the tube is reduced to 700 feet, and it is then suspended from towers 821 feet apart from centre to centre by four wire cables of 10 inches section, and each containing 3640 separate wires. These are further assisted by numerous braces radiating from the towers, and a multitude of ingenious minor contri

vances.

When a train weighing more than 300 tons passes over the bridge, the deflection is said to be only 10 inches; and certain it is that so far it has answered all the purposes for which it was intended, but nevertheless it seems too frail and fairy-like a structure for the rough usage of railway traffic; and trains are not allowed to move across it at a higher velocity than a man can walk. With great care and continuous repairs it may do its work for years to come, but it may any day deposit its load in the boiling flood beneath, and so again separate the provinces it has so boldly united. Indeed, taking it altogether, there can be little doubt that the tubular girder proposed by Robert Stepheuson for the same purpose would have been a better piece of engineering. It would have cost more in the first instance, for if the published

accounts are to be believed, the suspension bridge cost only 100l. per foot forward; but the durability of the tube would have been practically unlimited, its safety undoubted, and an occasional coat of paint all the repair it would have required.

Fortunately for the engineers it is their privilege to be allowed to think. They are not, like the architects, first forced to inquire whether or not a thing was done in the fifth century before, or the thirteenth century after Christ, before they are allowed to act, and the progress of improvement in iron bridge building has consequently been rapid. For certain purposes a cast-iron bridge, wholly in compression, was no doubt a very perfect thing, so also was a wrought-iron bridge wholly in tension; but it was easy to predict that the most perfect result would be attained by a structure which should combine these two properties, so as to take the greatest possible advantage of both.

The best method of effecting this was fully investigated, and practically settled, by the very complete and exhaustive set of experiments which were undertaken by Robert Stephenson and his associates before commencing his great work, the Britannia Bridge. The conclusions then arrived at were so sound and satisfactory that it is scarcely probable any extensive railway structures will in future be carried out on any other principle, though for local traffic simple compression or tension structures may still be used.

Although the principles then evolved are now thoroughly understood by every engi neer, they are so novel and so little appreciated by the general reader, that it may be worth while to try to explain what they are before proceeding further.

spreading as well as by the abutments. It will also have this further advantage, that, as the tie expands equally with the arch, and the structure is one homogeneous whole, with only a perpendicular bearing in its supports, you have a better bridge than before.

It is remarkable that the Italian architects in the Middle Ages tried this principle in all their Gothic structures; but an iron tie to a stone arch is both mechanically and artistically a mistake. The expansion and contraction of the metal is always working when the stone is at rest, and the flimsiness of the one compared with the mass of the other always produces an effect so disagreeable that the true Gothic architects on this side of the Alps never adopted it. They always applied a stone abutment to a stone arch, which was as essentially the proper and legitimate mode of construction as the iron tie to the iron arch is now seen to be.

This principle of construction, once seized, was used in fifty different forms. One of the most obvious was to frame the arch and the tie strongly together, as shown in the righthand side of the diagram, making what is called the bow-and-string bridge, and to run the roadway along the tie, in which form it has been extensively employed in railway structures. At the High Level Bridge at Newcastle the spandrils are filled up level (as on the left of the diagram), and the railway runs along the top, the roadway along the string. At Saltash and Chepstow, Brunel substituted a bent wrought-iron tube for the cast-iron arch, and tied the ends together by a chain drooping in the centre, and suspended his roadway from both. At Mavence, Dr. Pauli improved on this by substituting a wrought-iron T girder for the tube, and proportioning all the other parts more scientifically together, so as to produce what is theoretically perhaps the most perfect truss yet executed. The three spans of the German bridge are only 333 feet each, while the span of the two at Saltash is exactly 100 feet more; but the proportion of the parts is so perfect, that the principle admits of extension up to the limit at which a girder would tend to break itself by its own weight.

The defect of these bridges is that they are a little too clever. If the load were always evenly disposed over their whole surface, and at rest, no doubt every cubic inch of iron would always be doing all its work; but a railway train weighing 400 or 500 tons, and rushing at a speed of forty miles an hour, is a sad disturber of equilibriums; every part that ought to be in tension is at times thrown into compression, and every strut at times becomes a tie, so that engineers generally have agreed to adopt a plain straight girder

VOL. CXIV.

L-12

instead of those with these beautifully calculated curves. The same thing occurred with rails in the infancy of the system. Every mechanic saw, and every mathematician calculated, that a fish-bellied rail must be stronger than a straight one; but the practical result is, that all rails are now made with parallel sides, and there is not one of the other class in existence on any locomotive railway in Europe. It will probably be the same with bridges when the true conditions of the problem come to be more perfectly appreciated, except, perhaps, in structures of such magnitude that the weight of the load bears a very small proportion to the weight of the girder, and where the saving of every ton of iron becomes of importance lest the weight of the bridge should itself become a source of weakness.

Barring such exceptional cases, engineers are generally agreed in making the top and bottom flanges of their girder bridges practically parallel to one another, and when these are of wrought-fron, in putting the same quantity of metal into both. According to strict calculation, the proportions between the top and bottom ought to be as six to five; but as the lower or tension part depends wholly on its rivets, and the top or compres sion piece might almost be stuck together with glue, the same amount of metal is practically required for both, and the form in which it is disposed is mechanically immaterial. The cellular system has some convenience, but it does not seem to give any strength proportioned to the additional cost and difficulty of construction.

One of the most obvious ways of applying these principles is by means of what is called the Warren girder. This consists of a series of straight cast-iron tubes above, butting one against the other, and a chain of wrought-iron links below, and then connecting these two systems by struts and ties placed diagonally where wanted. Theoretically nothing can be more perfect than this arrangement: it is simple, but almost too simple; if one thing goes wrong all goes wrong; and more margin is wanted for the violent irregularities of railway traffic.

Perhaps, after all, there is nothing better than the simple tubular girder, which was evolved out of the first experiments, and used with such success in carrying the Hollyhead Railway across the Menai Straits. The first and most obvious proposal for this bridge was one of cast-iron in compression, which would have been the cheapest and most architectural mode of effecting the object, but the Admiralty interfered, and insisted that a clear headway of 100 feet above high water should be maintained throughout.

To meet this difficulty a tube suspended by chains was then suggested, nearly similar in principle to the one recently erected at Niagara; but as the investigation proceeded, it was found that the chains might be dispensed with, if a tube of sufficient rigidity could be constructed to carry any railway train across the greatest opening, which here was 460 fect clear. So complete were the investigations, and so careful the execution of the whole work, that subsequent experience has added little to the knowledge then attained; and, besides being the first, it is, considering the difficulties of the execution, one of the most perfect works of its class. In extent, and in some respects for cleverness of execution, even this bridge is surpassed by that across the St. Lawrence at Montreal, which, though only a single tube, is 6592 feet long, but the centre span is only 330 feet, and the remaining 24 openings average 242 feet. The great engineering difficulty was the erection of such a structure on so rapid a river, frozen at times, and at the breaking up of the ice bringing down great bergs which threaten to overwhelm everything. All these difficulties have been successfully surmounted, and the bridge promises to be as stable as it is efficient. Neither of these, however, has reached the limits of the system. When, for instance, it was proposed to erect a railway bridge across the Rhine at Cologne, Mr. Fairbairn gave two designs: one for a bridge in four spans, which it was estimated would cost when complete 230l. per foot, and one in two spans, the expense of which would have been 2801. The latter would have consisted of two tubes, carrying the railway with the roadway between them and footways outside, each tube measuring 1140 feet, supported by one pier in the centre; the two spans being thus 100 feet in excess of those of the Menai Bridge. Indeed, there seems no reason why openings of 700 or 800 feet might not be bridged by these means.

Whether or not this is the cheapest mode of accomplishing the object is not quite clear. The Menai, with its double road, cost 4007. per foot; and the Montreal, with its single line, 1717. But the only economy that could be made is in the vertical web that connects the top and bottom. All engineers are pretty well agreed as to the amount of metal which is required to provide a given amount of strength to the top and bottom for a given span, but they differ as to the mode of forming the sides. Thus the great tubes of the Menai Bridge weigh about 1600 tons; 500 tons of this weight is in the top, and a like quantity in the bottom, and consequently 600 in the sides. Half that

*The smaller Montreal tubes weigh 252 tons, made up thus:

quantity would suffice according to some, and consequently all conceivable forms of lattice girders and trusses have been employed for this purpose, and have economised metal to a great extent; but it has yet to be ascertained whether they are as stable. There is a grand simplicity in a wall of iron, every inch of which is as available in tension as it is in compression, and consequently can take all the varying strains of the traffic without suffering from the inequality; whereas the best designed truss must always be stronger and better in one position than another, and depends more or less on bolts and fastenings, which any inequality or sinking may throw out of work. If such be the case, it is to be regretted, for it is to be feared that the tubular girder can never be other than ugly; while many of those composed of diagonal framings are pleasing in themselves. A mere lattice like that at Cologne is not better than a tube, and is as flimsy as it looks; but a well-designed truss like that of the Charingcross Bridge is a beautiful thing in itself, and, if the bridge really cost only 1307. per foot for four lines of rails, is as cheap an expedient as can well be adopted. The spans, however, are only 154 feet, which, of course, prevents its being compared with the great works just mentioned.*

In the early ages of engineering experience, tunnels seemed far more formidable undertakings than bridges. Men could face what they saw, and undertake what they could calculate; but it was another thing to burrow into the bowels of the earth to encounter rocks or quicksands, or it might be springs and moving clays, and all this in darkness, and in ignorance of what might come next. All these things are now become perfectly understood, and the mode of making them settled. There have, in fact, been more

So the only economy could be effected in the last item, and this is very inconsiderable compared with the whole cost. For according to the published account the masonry of the piers cost 1147. per foot of the bridge, leaving only 577. for the iron work, and the only saving that could be effected would be one-third of this.

* For the railway bridge now erecting between proposed, in order not to interfere with the traffic, Southwark and London bridges Mr. Hawkshaw to make the central arch 300 feet span,-which would have been a really grand object, the side arches 150. The authorities decided that if there was one arch larger than the others all the traffic would go through it, and consequently ordered them all to be made equal, so that the bargees might be puzzled which to choose!