VI
SOME TYPICAL TIMBER BRIDGES
Asthere is no room here for a pedigree of timber bridges, let us choose a few examples which are particularly famous in history. It will be enough if we take three: (1) a prehistoric lake-village, (2) the Pons Sublicius of the Romans, and (3) the wonderful work done in the eighteenth century by two Swiss carpenters, the brothers Grubenmann.
Lake and marsh villages were the highest form of prehistoric bridge-building; their thronged platforms, dotted with round huts, not only put a defence of water between home life and prowling foes, but heralded all the housed bridges that the world has seen during its periods of written history. Whether we study Old London Bridge, or the criss-cross bridges with frail shops in Kashmír (p.71), or the booth-bridges of China (p.210note), or the roofed timber bridges of Switzerland (p.291), we are concerned with a pedigree that starts out from the first Neolithic lake-dwellings. But the later stone period, known as Neolithic, is not very old. Between it and ourselves there is a span of about nine thousand years, or a few thousand years more.[46]But British lake-dwellings are attributed to a time still later, the Bronze Age, whose date in the British Isles may be fixed tentatively at from 1200 to 1400B.C.[47]Further, as pit-dwellings lasted to the days of Tacitus among some Germanic tribes,[48]so a British lake-village here and there defied progress till the coming of the Romans. There was one at Glastonbury, and its “Late Celtic” routine of life has been studied carefully from its remains.
Standing on an artificial island formed by a series of timber bridges, it occupied nearly three and a half acres, and its round huts, about sixty in number, were intermingled with a few square cabins that marked the most recent enterprise. Low walls were erected with upright posts driven into the artificial island at a distance of about a foot from each other; then this framework was wattled and plastered with clay. A few rough slabs of lias stone made a doorstep, a piece of timber lay across the threshold, a wood fire crackled on a central hearth, and every household wanted to feel entirely safe, for a tall and tight palisade enclosed the little colony. In this primitive defence a great many poles were set up side by side; they ranged in height from five to ten feet. Wolves and war were feared verymuch, evidently; and yet the villagers were devoted to that self-decoration with which men and women for many a long period had tried to rival the patterned colours given by Nature to birds, and beasts, and insects, and fish, and snakes, and flowers, and stones. They loved rings, cut from amber and jet and glass; wore bracelets, some of bronze, others of Kimmeridge shale; glass beads had their vogue, and clothes were fastened together with bronze safety-pins, or with split-ring brooches of bronze. Perhaps the women were truly feminine, and wore a monstrous headgear, outraging their good looks in fashionable efforts to renew their beauty.
Drawing closer to this village perched up on primitive bridges, we find in it some weavers and spinners, a few wood-carvers who were true artists, some carpenters who had lathes, and some clever smiths who made iron knives, awls, spades, bill-hooks, gouges; and a few ambitious potters decorated their work and gave it a careful finish. Harvests were grown somewhere, as women used querns to grind the corn. Good little people! They wanted to be pacific and artistic; fighting did not set their genius; and so they vanished. How could they hope to protect the gift of life when British war-chariots and Roman soldiers began to fight in the neighbourhood, obeying the dread mysterious law of fruitful carnage? They slunk away from the fierce midwifery of war, fearing the long self-sacrifices of a painful renaissance.
Their gentle enterprise lasted from about the second orthird centuryB.C.to the Roman occupation. Among the remains of their village several skulls have been found, mild-looking skulls of a long shape, like those which have been taken from the long barrows. It was an Iberic tribe that trifled with peace and art, showing an epicene fervour akin to that of our cooing sentimentalists. Perhaps the Romans allowed the village to fade out of being, or perhaps they cleared it away as a futility, for neither Roman coins nor Roman wares have been found on the site, though remnants of Roman villas and potteries have been unburied in thevicinity.[49]
It is certain that most of the Roman bridges were built with timber. Thousands of trees were cut down when a paved road was constructed, so that cheap material for bridge-building was always at hand when the road was carried over ravines and rivers.[50]Besides, if a great many stone bridges had been built by the Romans, in Britain and elsewhere, many remains of the piers would have been foundin all big rivers. We know, too, that the Romans were tolerant in their attitude to native bridgemen, since the criss-cross piers of the Gauls outlived the Roman Empire by many centuries.
We know not, neither can we learn, how the Romans themselves made timber bridges. Even their Pons Sublicius, a sacred monument, hallowed by historical traditions and by its connection with religious ceremonies, was described imperfectly. To this day experts quarrel over its technique and over its position on the Tiber. Colonel Emy has tried to reconstruct it, but his attempt differs from that of Canina, and we cannot choose between them. The utmost we can say is this—that the Pons Sublicius was a tree-bridge resting on piles, and dating from the times of Ancus Marcius, who reigned fromB.C.640-616. If the chief priests did not build it, they certainly kept it in repair, always using wood with a pious regard for a venerated past; and with their help it existed as late as the reign of Constantine (A.D.306-337), when it was mentioned in the “Notitia,” and when a bridge was named after it at Constantinople. But the Pons Sublicius became obsolete as a highway for traffic, and then a good understudy bridge of stone—the Pons Lapideus—was built close at hand, and was known sometimes as Pons Sublicius, a title of honour. Sir William Smith believed that these bridges were outside the city, beyond the Porta Trigemina, and that the wooden one was built by Ancus Marcius in order to connect the town side of the Tiber with a new fortress erected on the Janiculus.
We pass on now to the brothers Grubenmann, whose best work was destroyed during the war of 1799. Ulric and Jean Grubenmann were village carpenters, born at Teufen, in the canton of Appenzell. Ulric seems to have been the abler of the two; certainly he was a man of true genius who spanned great distances by his unrivalled use of corbelled and trussed timber bearings. It was in 1755 that he began his suspension bridge at Schaffhausen, and in 1758 this work was complete. There were two spans in a distance of 364 feet, and they formed an elbow that pointed upstream. The abutment near Schaffhausen was 171 feet from the angle, and from the angle to the opposite shore was 193 feet. Ulric had decided that the bridge should cross the Rhine in one magnificent flight from abutment to abutment, but the town authorities interposed and told him to find use for a stone pier belonging to a bridge which a flood had ruined in 1754. Being a Swiss by birth and by training, Ulric Grubenmann followed an ancient tradition in Swiss carpentry, covering his bridge with a solid roof; and so perfect was the bridge, so admirably scarfed, trussed, strutted, braced, bolted up, and suspended, that only two faults could be found with it: the roof was too heavy, and the parts were too dependent on each other. An injury to one portion of the structure might have been disastrous to the whole bridge—a vital consideration in a warfaring time.
Grubenmann’s methods were simple. “The braces proceeding from each abutment,” said Telford, “are continuedto the beam which passes along the top of the uprights, and the lowest of these general braces are actually united under that beam, thereby forming a continued arch between the abutments, the chord line of which is three hundred and sixty-four feet, and the versed sine about thirty feet. These braces are kept in a straight direction by the uprights, which are placed seventeen feet and five inches apart. If this bridge had been formed in a straight line between the abutments I can see no reason why this form of construction should not have supported a roadway of about eighteen feet in breadth, as well as a slight roof; because, in that case, all the weight arising from the braces which proceed from the middle pier would have been saved, and the roof might have been made much simpler and lighter.”
While Ulric Grubenmann was working at Schaffhausen, his brother Jean built a similar bridge at Reichenau, two hundred and forty feet in a single span; and some years later the two brothers constructed their Wittingen Bridge over the Limmat, near Baden, giving to it a span of three hundred and ninety feet. They were famous now, and their influence travelled from Europe to America, where it found in Bludget an able interpreter, Bludget’s bridge over the Portsmouth River being similar in technique to the bridge at Schaffhausen. Since that time the evolution of timber bridges has remained in the United States of America, where it has ranged from the criss-cross of logs for bearing piles to the most intricate combinations of lattices and trusses. Very often there is far too muchintricacy, and no thought at all is given to military considerations (p.352). “Many wooden American bridges are trusses which almost defy analysis, the designs being, however, obviously suggested by an attempt to combine at least two of the three main types of bridges. No advantage whatever is gained by a combination of this kind; on the contrary, great disadvantage is almost sure to follow its adoption, namely, that it will be impossible that each part of the structure should, under all circumstances, carry that portion of the load which the designer entrusted to it. For suppose a bridge constructed partly as a girder and partly as a suspension bridge, the girder being very stiff and deep, the chain perfectly flexible with considerable dip. Let the chain and girder be each fit to carry half the passing load. It is perfectly conceivable that the deflections of the two should be so different that the girder would, under the actual load, break before the chain was sensibly strained, or the difference in the relative dip of the chain and depth of the girder might be such as to cause the former to give wayfirst.”[51]