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An Encyclopedia of the History of Technology part 49

An Encyclopedia of the History of Technology part 49. This one of a kind encyclopedia presents the entire field of technology from rudimentary agricultural tools to communication satellites in this first of its kind reference source. Following an introduction that discusses basic tools, devices, and mechanisms, the chapters are grouped into five parts that provide detailed information on materials, power and engineering, transportation, communication and calculation, and technology and society, revealing how different technologies have together evolved to produce enormous changes in the course of history | PART THREE TRANSPORT BRIDGES The building of bridges is almost as old as the making of roads or at least tracks occurring at about the same time as man s first settlement in agricultural communities and the very beginnings of permanent villages in the Neolithic Age some 10 000 years ago or more. At first bridges were of very limited span simple beams of single logs laid across gaps or streams and suited only to pedestrian traffic. The branch of a vine or similar creeper on which a man could swing across a gap could hardly be classed as a suspension bridge but it was surely the antecedent of the first single rope spans. As more and more animals became more and more domesticated the dog the ox the sheep the donkey and the ass it became increasingly necessary to widen the single log beam bridge and to give it a reasonably flat upper surface. Where long enough logs were not available to span the gap to be bridged it was but a small advance to build a pier and double the span or several piers and hence to multiply it. In the fifth century BC Herodotus describes a multi-span bridge at Babylon having a hundred stone piers joined by timber beams averaging over 1.5m 5ft in length the whole bridge being 200m 660ft long. In other cases where the piers consisted of wooden piles the technique was possibly adopted from the Bronze Age lake dwellers of Switzerland who developed it about 2500 BC or from other northern peoples who built similar structures towards the retreat of the last Ice Age such as those at Star Carr in Yorkshire. To a limited extent stone slabs were used in place of logs to make clam bridges if single span or clapper bridges if of several spans. Some have survived in Britain until today particularly in the Cotswolds and on Dartmoor and Exmoor. Existing since prehistoric days they have long outlasted their timber equivalents but they are few and stone is too brittle to be used successfully except for the smallest spans. It first appears as early as 3500 BC in .