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would build his church (Matthew 16:18) and referred to himself as the ‘stone which the builders rejected’ – prophesized in the 118th Psalm – that would ‘become the head of the corner’ – which is to say the foundation stone of the church of redemption, (Matthew 12:10).

      Whatever the intention behind the bridge’s foundation rituals, as it turned out, they brought no good to the Jews of Prague. During Easter 1389, as the bridge was nearing completion, the clergy in the city inflamed the latent anti-Semitic feelings of the population by announcing that Jews – long held responsible by Christians for the death of Christ – had desecrated the host, the Eucharistic wafer that becomes the body of Christ during the mystery of the Roman Catholic Mass. Murderous chaos followed which resulted in the Jewish ghetto being pillaged and burnt, and much of the Jewish population of Prague – estimated at around 3,000 people – being murdered.

      Four years later the bridge – still incomplete – became itself the focus of a grim event that, in later centuries, did much to define the character and spiritual aspirations of the city. On the night of 20 March 1393, John Nepomuk was killed by being thrown from the bridge on the orders of Wenceslaus, King of Bohemia, who in 1378 succeeded his father, Charles IV, as ruler of Prague. Nepomuk was a principled cleric who displeased Wenceslaus by refusing to divulge to him the secrets of his Queen’s confession. In consequence Nepomuk was tortured (apparently his tongue was removed in no uncertain manner) before being tossed from the Charles Bridge. His sufferings and manner of martyrdom led Nepomuk to be canonized, made the patron saint of Prague, the official holy protector against floods, and inspired within Bohemian architects for years to come a morbid interest in tongues. As motifs, as plan forms or vault patterns, stylized tongues, small or vast in scale, enliven the sacred architecture of the region.

      The Charles Bridge stood firm and largely unaltered for nearly 300 years, maintained by a toll collected by the crusading military order of the Knights of the Red Cross and Star whose mother-house was located next to the bridge. Then, in the 1680s, the bridge’s lurid past caught up with it. This was a time of Roman Catholic resurgence in Bohemia, following the dramatic defeat in 1620 of Protestant forces at the Battle of the White Mountain, and if the bridge’s foundation had anything to do with the ancient arts of magic, alchemy or the Kabbalah, then the Catholics felt something had to be done about it. And it was. From 1683 until about 1714, the bridge’s parapets were loaded with statues carved of stone, mostly of saints and clerics – including, of course, an image of St John Nepomuk. The bridge was turned into a Roman Catholic shrine – walking along it became a mini-pilgrimage – with the flamboyantly posturing parade of saints, carved in ostentatious baroque manner, being a tremendous late flowering – in theology and in art – of the Counter Reformation. Virtually every one of these statues has now gone, their weathered and battered hulks carted off to the Lapidarium museum and replaced by replicas. But the fourteenth century bridge endures, a tribute to its robust construction, to the skill of the master mason Peter Parler who supervised building works and – perhaps – to the strange ritual of its foundation.

      STRUCTURAL PRINCIPLES

      During the late fourteenth century in Europe, when the Charles Bridge was being built, the technical approach to bridge building was starting to change. Masons like Peter Parler tended to have an almost intuitive understanding – honed by years of experience and exposure to the trade ‘mysteries’ of their craft – of the structural forces engaged in bridge construction. Their responses to these forces of nature, and to the manner in which loads are contained or transmitted by structures of different forms or materials, were usually pragmatic and the result of empirical observation, practical experiment and trial and error. This resulted in safe and conservative designs with few great and dramatic leaps forward – which is what makes the unusually wide-span elliptical arches of the mid fourteenth century Ponte Vecchio in Florence so novel and interesting (see page 150). Throughout the fifteenth century things started to change, gradually at first, as bridge building became more theoretical and finely calculated. But it was not until the late sixteenth century that scientific understanding of the theory of bridge construction started to dominate the business of bridge building.

      Crucial to this new understanding was the ground-breaking research and analysis undertaken in the late sixteenth and early seventeenth centuries by mathematician, astronomer and philosopher Galileo Galilei. This allowed late Renaissance engineers to calculate the ways in which the shape and size of structural members – for example beams and trusses – and the materials from which they were made would affect their ability to carry and transmit loads. Significantly Galileo identified the ‘scaling problem’. He established the principle that as a beam increases in length it decreases in strength, unless its thickness and breadth increase disproportionately. He also demonstrated that this escalation of scale has very definite limits dictated by nature. Quite simply, if a beam is increased in scale beyond a natural limit it will be capable of supporting no loads at all and break under its own weight.

      In the late sixteenth century the scientific and mathematical approach to construction was in fact being explored by many and evolved at a rapid rate. For example, the architect Andrea Palladio’s Quattro Libri dell’Architettura of 1570 included the first published illustration of a triangulated truss – a robust structure for transferring loads through a rigid system of triangular forms. Other important publications pioneering, promoting or explaining theories of bridge construction included Machinae Novae of 1595 by Fausto Veranzio, which includes information on tied-arch bridge construction, the oval lenticular or lentil-shaped truss, and the iron chain-link suspension bridge. A key later work containing much technical information is Traitre des Ponts of 1716 by Hubert (Henri) Gautier.

      BRIDGE DESIGN AND CONSTRUCTION

      The permutations of materials and structural principles employed in bridge construction are seemingly many, varied and complex – timber, brick, stone, cast and wrought iron, steel, hydraulic cement, mass concrete and steel-reinforced concrete, arches of diverse form, beams, cantilevers, pylons, cables and masts. But in its aim bridge construction is straightforward and construction simple. The object is to link two points of land as safely and efficiently as possible. If the obstacle being bridged is running water, then the ideal is to achieve wide spans with minimal support rising from the water to make the bridge easier to build and maintain, to avoid disturbing navigation, and to reduce the risk of the bridge being swept away.

      In essence bridge construction is of two basic types. The carriageway – be it for vehicles or pedestrians – is either supported from below or suspended from above. If the ‘dead’ load of the carriageway (its weight) and the ‘live’ load of the carriageway (the weight of the use it carries plus the ‘environmental load’ comprising the weight and pressure of rain, snow and wind) are supported from below it must be carried on arches or vaults of varied types; on beams either cantilevered from, or supported by, abutments and piers; or set within a lattice-like engineered truss wrought of timber or metal. There are two models of nature for support from below: rock formations that arch over; and timber logs or beams laid across, chasms or rivers.

      If the ‘dead’ and ‘live’ loads are supported from above, the carriageway must be suspended from well-anchored cables or chains stretching over masts to form inverted arches of strong catenary shape, or from natural features – a system known in China from the second century BC. Cables can also be stayed or anchored firmly to a single support to create a cable-stayed bridge. The prototype in nature for these types of suspension bridges is a walkway formed by, or supported by, hanging vines and vegetation.

      These different approaches are determined by a variety of circumstances but all are responses – in various and appropriate ways – to the four basic types of forces that act on bridges, either singly or in combination: tension, or a tendency to stretch or pull apart; compression which pushes together and compacts; shear, which is a sliding force; and torsion which is a twisting force.¹¹ The form of the bridge, and the materials used in its construction, also create different – and utilize different – structural forces.

      A bow-string truss or tied-arch

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