A book of bridges

by Walter Shaw Sparrow

PONT DE TOURS—A FAMOUS BRIDGE OF THE XVIII CENTURY. IT IS IN KEEPING WITH THE SPIRIT OF WATTEAU

The concept of metal bridges may have come to Europe from China. In the seventeenth century Kircher saw and described a Chinese bridge which seems to have been a genuine suspension bridge of metal, a true forerunner of the Pont de la Caille, over the Pass of Usses, and of the immense Pont de Beaucaire, which in four spans unites Beaucaire to Tarascon, covering a distance of more than 438 metres.[140] Who can explain why backward China has hit upon many fertile ideas before the more enterprising nations? Why has she not learnt to rule the world? Perhaps her body has been too numerous for her brain. On my table lies the photograph of a bridge which may be similar to the one admired by Athanase Kircher. It is an iron swing bridge in Western China, near Auhsien. There are three piers, two of stone, and the other a makeshift of two timber piles joined together at top by a log upon which the footway rests. The carpentry of the footway is primitive: across the long bearing beams, which are not at all thick or heavy, a great many slim laths lie unevenly; and up the middle of the bridge, from end to end, is a narrow path made with long and flat planks which rest upon the transverse timbering. As for the iron suspension, it is a chain of thick and short rods which are linked firmly together. These rods, thus looped at each end and interlocked, run in two lines from abutment to abutment, making a sort of parapet at each side of the bridge. Bamboo rods suspend the footway to the iron chains, which pass over the abutments to be fastened securely on the ground.

There are four abutments, but my photograph shows only one; and it omits also the main thing of all—the means by which the metal chains are anchored. Still, the abutment is entertaining. It is a stone pillar about five feet high, perhaps a little more or less; it seems to be old, and from two holes pierced through it we learn that several experiments were made before the right leverage was obtained. The first hole was too low down, so another was drilled about 12 inches above it, and through this second hole the chain was passed, then tugged down to its anchorage. Even then the suspension was not effective, the hole or “saddle” being still not high enough above the footway, and the builders knew not what to do. Not only was there insufficient space for a third hole, but very few makers of suspension bridges have been reasonable enough to pass their metal chains over the summits of stone pillars and towers. The Chinese workmen at Auhsien were not more foolish than many European engineers have been, for their perforated pillars are not a bit worse than the perforated towers through which suspension chains pass at Clifton and at Budapest, not to mention many other familiar examples. So determined were the Chinese to overcome their difficulties without using the summit of their pillar, that they cut away the stone until they came to the second hole or saddle, and then they thrust a lump of iron under the taut chain. Next, to increase the tension still more, they put up a smaller pillar perhaps a yard from the first one, forcing it under the iron rods, which at this point strain downward to their anchorage. Curiously enough, the lesser pillar—a sort of understudy—is used as an architect would employ it: along the top a groove is hollowed, and the chain rests in the groove and then dips down at a sharp angle. Perhaps, then, the smaller pillar is fairly new, while the larger one is old.

The Rev. O. M. Jackson[141] knows this bridge very well; he lived for five years at Auhsien, and on one occasion the whole bridge was washed away by a spate. For months the iron chains lay here and there on the river-bed; and as floods are frequent, and the bridge is not a high one, very little of the workmanship has had a chance of growing hoar. The pillars have the best chance; and I suppose the iron chains are worth saving from the river whenever the bridge is reconstructed.

I have lingered over Auhsien Suspension Bridge not because of its craftsmanship, but because it marks a primitive phase in the evolution of metal bridges. Perhaps the example seen by Kircher was less rude; and perhaps the principle of its construction may have been precisely like that in the bamboo swing bridges of Western China. In these there are four huge cables of twisted bamboo[142]: two of them carry the footway, while the upper ones serve a double purpose: a strong netting on each side braces them to the lower cables, giving another support to the footway, and forming a sort of hammock a good deal taller than an average man. It is within this deep hammock that everybody walks across a bamboo swing bridge, which in a high wind is as enjoyable as a rowing-boat. At each abutment there is a gabled entrance gate, where the four cables are screwed up.[143] Displace the bamboo cables for iron chains, and we get at once, perhaps, an idea of the bridge that Kircher regarded as “merveilleux.”

As Kircher’s book was published in 1670, an iron bridge ought to have been built in Europe before the middle of the eighteenth century. An attempt to build one was made in 1755 at Lyons, but it failed. An arch was put together in a builder’s yard and then the project was abandoned as too costly! But the idea was handed on somehow to an English ironmaster, Abraham Darby, of Coalbrookdale, who in 1779 won a great success by bridging the Severn with a very useful arch of cast-iron, having a rise of 50 feet, and a span of 100 ft. 6 ins. The cost of it is not known, but the weight of metal employed was 378½ tons. The design is bold, and the arch handsome. Every pontist should get a photograph of Coalbrookdale Bridge. Already it is out of date, and its value as history will not save it from destruction.

A few years later, in 1796, Rowland Burdon followed the example set by Abraham Darby, but not as a mere copyist, his Wearmouth Bridge being an arch of open cast-iron panels, which act as voussoirs. The span is 236 feet, with a rise of 34 feet; the springings are 95 feet above the river-bed; at first the footway was rather narrow, but in 1858 it was widened by Robert Stephenson. Rowland Burdon used 260 tons of iron, and his work cost only £27,000.

Soon afterwards, in a great cast-iron arch thrown over the Spey, Telford made new experiments, and, as Professor Fleeming Jenkin has said, his bridge at Craigellachie marked “a great advance in the conception of what was the safest form in which to apply cast-iron to an arch.” But more than this was expected from an engineer of Telford’s reputation, and nothing more came from him, unfortunately. In fact, Telford divorced his work from the good sense of good design, which Darby and Burdon had endeavoured to respect. At each abutment he put up a silly tower pierced with arrow-slits and armed with battlements, advertising a farce of warlike make-believe which scores of foolish engineers would copy and adapt, while leaving their bridges entirely unfortified.