Dan Cruickshank’s Bridges: Heroic Designs that Changed the World. Dan Cruickshank
Чтение книги онлайн.
Читать онлайн книгу Dan Cruickshank’s Bridges: Heroic Designs that Changed the World - Dan Cruickshank страница 13
Название: Dan Cruickshank’s Bridges: Heroic Designs that Changed the World
Автор: Dan Cruickshank
Издательство: HarperCollins
Жанр: Архитектура
isbn: 9780007412334
isbn:
The scale and quality of the construction that the engineers and masons achieved can only be properly appreciated by considering for a moment the tools and machines at their disposal. They had picks and chisels made from hardened iron for working stone, which were adequate, with skill, application and time, for cutting limestone and sandstone with precision. They also used water power to operate stone saws and lifting machines, though not to any great extent.’21 Construction work, from cutting and transporting stones to raising them into place, was a very important part of the economy of the Roman state. It was a way of creating work and jobs, keeping people employed and out of trouble, and of getting money to flow through society. For example, Emperor Vespasian (AD 69-79) refused to let builders use water-driven hoists ‘lest the poor should have no work.’22 So in place of mechanical power, the Romans preferred – almost as a matter of state policy – to achieve lifting largely by muscle power, operating cranes or systems of pulley blocks hung from legs or poles and worked by winch or capstan. Using these devices, heavy stones were lifted by slings, or gripped by pincers or a kind of ‘lewis’, a lifting tool comprising metal bars inserted and wedged into a dovetailed cavity cut in the top of a block of masonry.
Detail of the Pont du Gard showing piers rising from the lower arcade. The projecting stones were supports for timber scaffold during construction.
Metal was also used in construction, usually in the form of wrought-iron cramps or bars set in lead and then placed in cut recesses to bond stones together. As Vitruvius explains when discussing how to avoid the problem of crumbling mortar: ‘…leave a cavity behind the [facing wall]…on the inside build walls two feet thick…and bind them to the fronts by means of iron clamps and lead.’ Work executed in this way, Vitruvius claimed, ‘will be strong enough to last forever.’23
Vitruvius also had some specific things to say about aqueducts, reminding his readers of the importance of an adequate and reliable water supply and stating that for aqueducts or ‘conduits’ the masonry should be ‘as solid as possible and the bed of the channel have a gradient of not less than a quarter of an inch for every hundred feet, and let the masonry structure be arched over, so that the sun may not strike the water at all.’24 Vitruvius also recommended that, when the water carried in the conduit reaches the city it should be held in a ‘reservoir with a distribution tank in three compartments.’ The system was intended to segregate water used for different purposes and to prevent people tapping into the main flow and stealing public water for private use.25 Vitruvius also recognized that water quality was very important, so recommended that it was best to conduct water through clay pipes rather than lead pipes because ‘water from clay pipes is much more wholesome than that which is conducted through lead pipes, because lead is found to be harmful [and] hurtful to the human system.’26 The evidence for this was the health of ‘plumbers, since in them the natural colour of the body is replaced by a deep pallor’ caused by lead fumes that ‘burn out and take away all the virtues of the blood from their limbs.’27 Vitruvius’ recommendations seem to have been used by the Rome official Sextus Julius Frontinus who in about AD 80 wrote a treatise on the Aqueducts of Rome, in which he described the condition of the city’s water supply, actions needed to prevent water leaks and theft, and, in general, promoted Vitruvian theory.
An interpretation of a stone-lifting pulley block.
A view of a Roman period water supply system, including covered aqueducts. Both engravings date from 1521 and are based on descriptions by the first-century BC Roman architect Vitruvius.
‘The sight of the aqueduct entering the town is among the greatest surviving urban scenes from the ancient world. It was dedicated to Hercules – the legendary founder of the city – and still seems the work of divine heroes.’
Extraordinarily enough, a physical example of Vitruvius’ recommendations and theory survives to this day in Spain. The aqueduct in Segovia was built about 100 or so years after the Pont du Gard, perhaps in about AD 50–100 and, unlike the Pont du Gard, continues to fulfil the function for which it was built. Thanks to the skill and robustness of its construction, generations of maintenance, self-effacing reconstruction, and the soundness of Vitruvius’ thinking, the conduit and aqueduct retains its Roman identity and still carries water 15 kilometres from the Fuente Frio River to the old city of Segovia. Much of this length is at or near ground level, but due to the terrain near the city, 800 metres of the conduit is supported on arches springing from piers up to 28.5 metres high.28 Few things are more moving, in the world of ancient engineering, than to see and hear the water – after nearly 2,000 years – still coursing along its worn but serviceable granite conduit perched high off the gnarled, sun-baked and leaping arches of the aqueduct.
The aqueduct, and the conduit it supports, is an admirable machine for gathering and delivering water to an elevated city, the aqueduct’s height above ground varying to accommodate the terrain and to keep the decline of the conduit as little as possible. When the water arrives in the city it is first collected in a tank known as El Caserón (the Big House) and from there runs along a channel to a tower known as the Casa de Aguas (the water house) where it is allowed to decant and impurities settle. The reasonably pure water then travels nearly 730 metres, at a fall of only one-in-a-hundred, to an outcrop near to which the Roman city was built. Then the aqueduct, rising to its full height of 28.5 metres and comprising two levels of semi-circular headed arches, carries the water into the city, to what is now the Plaza de Díaz Sanz. As with the Pont du Gard, the stone blocks – in this case granite – with which the aqueduct is built are unmortared, their precise construction and weight being enough to keep all standing. Particularly satisfying is the masonry of the voussoirs forming the arches, which are a single block deep. They combine with the horizontal courses of the spandrels to form a perfect example of robust masonry construction, where every block is not just doing its job but is seen to be doing its job in a most reassuring manner. It is possible to ponder these stones for hours without getting in the least bored, wondering at the creation of poetic beauty through purely expressed function.
The sight of the aqueduct entering the town – a seemingly endless arcade of granite with the conduit perched high on the immensely tall and seemingly impossibly slender piers of the lower arcade – is among the greatest surviving urban scenes from the ancient world. It was dedicated to Hercules – the legendary founder of the city – and still seems the work of divine heroes. Goodness knows what the local people felt 2,000 years ago; this mighty work is the power of Rome personified – remorseless and eternal.
The Puente de Alcántara makes a telling contrast with the cyclopean Pont du Gard and Segovia aqueduct so that, together, they encompass the whole spectrum of Roman bridge building. In comparison to the massiveness of the latter two, the Puente de Alcántara possesses a lightness of touch as it springs across the void that it was created to tame. It is, in its daring СКАЧАТЬ