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At present the tendency, unfortunately, is to abandon these beautiful and permanent structures, and to adopt wrought-iron cylinders instead. Their cheapness is, of course, the great recommendation, but there is also the rapidity with which they can be designed and erected, which saves both time and thought, and is consequently too great a temptation. But harbour works in general are of so grand and so enduring a character that it may be hoped that something better than these flimsy expedients will soon again be adopted, for we have so few real works of architecture in modern times, that it is a pity to forego any chance that may procure us such examples as these sea-girt lighthouses certainly afford.
Strange though it may at first sight be thought, it seems nevertheless true that men sailed over the sea in ships, and provided ports and piers to shelter them, long before they thought of making roads to facilitate traffic on shore. In early times nations were content-as they are in most parts of the East now—with such loads as could be carried on the backs of beasts of burden. Long strings of camels or mules, or droves of bullocks wandering over the half-cultivated plains, sufficed for all the rude wants of the Phænician epoch. The Romans, living in a more closelycultivated country, and with a more extended empire than had previously been known, seem to have been the first to think of employing wheeled carriages for purposes of transport, and consequently the first who deemed it necessary to make permanent roads or to build bridges.
In those days, however, the mechanical branch of the profession was so immeasurably behind that which we now designate as civil engineering, that the professors of the latter were content to effect by brute force what we now accomplish by infinite scientific contrivance. They drove their roads straight as an arrow up hill and down dale, and paved them with blocks of stone, that not only must have enormously increased the friction, but must have tended to destroy any waggon not provided with springs, and have required a Roman's power of endurance to survive a journey long upon them.
In order to understand this, it is necessary to bear in mind that the resistance to a load drawn along a road is made up of two parts, friction and weight. No human ingenuity has yet succeeded in taking one ounce off the weight, though by distributing it over a very long surface, by means of low gradients, it may to a certain extent be rendered practically innocuous. · All our skill has been applied to the task of getting rid of friction, and on our railroads we have so far succeeded as to diminish the relative importance of these two elements to an extent never before dreamt of. An active horse, for instance, will draw a cart, weighing a ton, with tolerable ease along a well-made level road ; and when he comes to such an incline as shall require a tractive force equal to what would draw two tons on a level, he can double his power for a short distance and overcome it. The same horse, however, will draw ten or even thirty tons along a perfectly level railway ; but a very slight incline will double this, or require the exertion of ten to twenty times greater force to lift the train up the incline, than what is required to move it on the level, and no horse could even for a few yards accomplish this. Indeed up some such inclines as the locomotive now climbs he would require to put forth the power of 100 horses to lift the train while the friction remains constant at one-horse power. With the Romans all this was reversed. Clumsy mechanical arrangements made friction the element to be overcome; so much so that it is difficult for us to understand how a four-wheeled plaustrum, without a perch, was ever coaxed round a curve-how it turned nobody knows-and with 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 employed. 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 fourwheeled 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 a 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 through 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 science seemed nearly insuperable. With all their architectural skill, the Egyptians never 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 a part, as near as can now be ascertained. In modern times the bridge over the Dee at Chester is the
largest 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 than a straight line, with elliptical arches, as may be seen by comparing it with Waterloo Bridge, which, with all its grandeur, fails in reaching the perfection of its younger rival, though this may perhaps be partly owing to the Doric columns, which were absurdly added with an idea of ornamenting its piers.
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
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most gigantic arches ought to be far easier to build in this mate rial. 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 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 remarkable for its tenacity; nor were they 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 its most perfect form for