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Cordouan lighthouse and the first dioptric lighthouse lens
ОглавлениеLike the British, who boasted some impressive granite edifices among their lighthouse engineering achievements, the French were not slouches in designing monuments to their profession. By far the most famous lighthouse on the French coast was, and still is, Cordouan. The story of Cordouan, involving continual trial and error in construction and illumination techniques, was repeated in other lighthouses around the world, the most famous being Eddystone, where five completely new lighthouses were built between 1698 and 1882. The Eddystone story has been told many times and focuses primarily on the civil-engineering achievements of its architects and builders. Cordouan’s significance to lighthouse illumination is that it was the first lighthouse to show a Fresnel dioptric light and four-wick burner. The history of Cordouan also illustrates how successive generations of French engineers applied the latest illumination technology, often before it was known to work.
Cordouan was chosen for the debut of Fresnel’s lens for political reasons. It was the pride of the French lighthouse authority. They described it as the ‘Phares de Roi’ or ‘Lighthouse for Kings’. The history of the site predates medieval times as there was certainly a monastic light there between AD 814 and 840 and a lighthouse is shown there on the earliest sea charts. In 1584, Louis de Foix designed and built a new tower at the command of King Henri III. This construction became the basis of the modern lighthouse, though there have been many changes made over the succeeding years to both structure and equipment. De Foix was the right man for the job as he had extensive experience of civil engineering work. He had previously worked on the Escorial Palace in Madrid, which was completed in 1600, and constructed water works, bridges, roads and fortifications throughout Europe.
His first remit was to inspect the remains of the existing lighthouse and cost the repairs but these proved to be higher than a completely new building. The De Foix design followed French tradition that public buildings should reflect the splendour of France and ‘excite the admiration of posterity’. The tower was barrel shaped containing two vaulted rooms, a great hall 52ft (15.8m) in diameter called the King’s Room, with an ornate chapel on the floor above – hardly the usual design for a lighthouse. On top of this chapel was a lantern, though the open fire was in a turret above. Above the turret a chimney carried away the smoke and helped to draw a draught of air over the fire to assist combustion. The pillars of the turret had the disadvantage that they obscured the fire light from certain directions, though at one stage they were used to support glass screens to try to shelter the flames from the wind. Inside the building an enclosed stairway to the turret provided access for the keepers to carry up billets of wood, and importantly keep dirt and dust away from the ornate chambers. Externally this ornate workmanship was continued, a factor that was to slow down the building and showing of the light.
Fig 5
Cross section showing the configuration of bull’s eye and prisms making up Fresnel’s first- to sixth-order dioptric lenses specified in 1822, together with the larger hyper radial lens added in the 1880s.
The original contract between Henri III and De Foix stipulated that the light should be exhibited within two years. Site problems and the complicated building procedures prevented the building being completed in time, but eventually the light was shown from the unfinished tower in 1610. In the meantime, De Foix had disappeared without trace after experiencing political and religious problems, falling into debt and imprisonment and becoming overwhelmed by adversity. His son Pierre attempted to continue the work but he too ran into financial difficulty, and it was left to their master mason François Beusher to complete the work, which he managed by 1614. De Foix, skilled as he was, had failed to take into account the vagaries of weather, tides and the transportation of component parts from the shore. But his main and unforeseen problem was the disappearance of the sand island that he was building on, solved only by the unplanned, extensive building of rampart walls to enclose the tower and keep the sea at bay. He was not alone in suffering these problems; many other lighthouse builders suffered equally when placing lights on offshore sites.
Cordouan had cost more than three times the original estimate but was the pride of France. That was not enough to protect the edifice. In 1662, a storm and a severe lightning strike removed the top of the tower, while the heavy seas eroded the sand from under the structure. The engineer Claude Chantillon was sent to assess the damage and make immediate repairs to prevent the loss of the tower. Remedial work was an ongoing fight with the attacking elements. In 1665, the engineer M. Dominique spent three years at the lighthouse with a team of masons and labourers repairing the lighthouse and sea defences for, by then, the original island had disappeared. In 1717, masons removed part of the upper structure as the flames and smoke from the open oak-log fire had damaged it; weathering had also degraded much of the De Foix ornamentation. This work resulted in the fire being set at a lower level, which reduced its effective range.
Many complaints later forced M. Boucher, the supervisor of the lighthouse at Bordeaux, to write to the British Admiralty in 1724 for details of fire grates and the type of coal burned. As the rising price of imported coal combined with transport difficulties the French Ministry of Marine took steps to switch to oil lamps and reflectors. In May 1782 the ministry accepted a tender by Tortille Sangrain, a Parisian street-lighting maker, who had already supplied lights to other French lighthouses, to fit a glazed lantern and oil lamps at Cordouan. Sangrain supplied a 12-sided lantern of iron with the roof overlaid with copper. It contained 80 spherical 217-mm (8.5-in) diameter metal reflectors arranged in five tiers. Each reflector had an oil lamp with a single 18-mm (0.7-in) wide flat wick fed from a small reservoir. To remove the smoke generated by the 80 lamps, four ventilators were fitted to the lantern roof at the cardinal points, so that whichever way the wind blew the smoke would be removed. This light was first exhibited on 12 November, but navigators declared that it was feeble compared with the previous coal-fed light. Boucher thought that the keepers were negligent cleaning the reflectors and ordered them to remove soot from them regularly throughout the night. This soon caused the polished surface to deteriorate. Sangrain was not perturbed, saying that his new lights were always greeted at first by adverse comment from prejudiced pilots. He claimed that his lantern produced a 2-yard (2-m) solid ball of light.
But Sangrain could not silence his seafaring critics. Adding to his troubles, the lightkeepers complained that the excessive smoke choked them, making it impossible to get inside the lantern to clean the many reflectors and lantern glass. Sangrain asked that a commission be set up to observe and report their findings. This was duly done, with six experienced sea captains agreeing that the light was indeed less effective than the previous coal-fed light, one of which was exhibited from the tower at the same time for comparison. A well-known master mason and engineer, Joseph Teulère, was ordered by the Minister of Marine to visit the lighthouse and carry out an examination of the equipment to see why it was not as effective as it should be. He soon reported that in his opinion the workmen assembling the lamp array had not understood how to set the reflectors and lamps correctly. After a great deal of argument Sangrain removed the reflectors, replacing them with fewer but larger ones. These changes were still not satisfactory and this together with complaints about other lights on the French coast forced the Minister to set up an enquiry into coast lights in general.
This enquiry developed from discussions with the seafaring community into a series of practical tests using parabolic reflectors and the better oil lamps that were now becoming available. Teulère was involved in the trials of this equipment at Dieppe and subsequently designed the replacement light for Corduoan. In addition to the design of the light Teulère removed the remainder of the top of the tower in 1788, replacing it with a taller, less ornate stone column supporting a new lantern, which remains to this day. The new 12-sided, well-ventilated lantern contained a triangular light array of parabolic reflectors and oil lamps all rotated by clockwork. On 29 August 1790 it was shown for the first time, but what should have been a bright flashing light was immediately condemned. Sadly, the promised bright 20-mile (32-km) range flash every minute did not meet expectations. Immediately, Teulère inspected his light and found that the lamps were out of focus. This he corrected but the complaints continued. Subsequently, he changed wick sizes and battled with soot and the fact that the size of glass chimneys that he required could not be produced. The latter was a major stumbling block, as the large oil lamps produced much oily soot that stuck to the glazing thus lowering the light output. In spite of all the improvements and the many trials carried out before expert witnesses, Teulère could not overcome the prejudice against his light. There was continual pressure to return to a coal fire but this was resisted partly due to the cost of coal. Normally the best coal was imported from England, but war between the two countries had stopped all imports. However, Teulère’s light, with its sooty flames and corroded reflectors remained in service until 1823, when it was replaced with Fresnel’s new lens system.
By the early 1820s, therefore, lighthouse illumination technology had taken a major step forward and France was poised to wrest leadership in the field from Britain. After Cordouan, Fresnel was not content to rest on his laurels and continued his work to develop different types of lighthouse optics, advancing France’s lead. Some lights needed to be exhibited either all round the horizon or to cover dangerous sectors of coastline. Fresnel developed the drum, or beehive fixed lens, using the central lens as a continuous horizontal belt with prism segments above and below in parallel continuous rings to refract the stray light from the horizontal belt. The beam was displayed as a cylinder of light that could be seen at equal strength through the lens arc of visibility. Where a sector light (one which illuminated a specific stretch of coastline or sea) was needed a ruby or green glass panel was placed in front of the appropriate lens element.
The Fresnel lens system was further improved as French glassmakers perfected glass casting and grinding techniques. This combined with new engineering skills and enabled larger and more complicated lens systems to be manufactured. Fresnel refined his lenses by first adding mirrors above and below to capture stray light, later realizing that by correctly angling refractive prisms he could make them reflective, thus making the first catadioptric system combining refraction and reflection. It was not until Thomas Stevenson introduced the holophotal lens in 1851 that Britain began to reassert its status as the leader in lighthouse illumination technology.