Carolyn O'Grady makes an illuminating visit to the lighthouse at Portland Bill in Dorset
The Portland Bill promontory and Chesil Beach in Dorset is the graveyard of many vessels that failed to navigate the hazardous meeting of tides around the mass of rocks and the Shambles sandbank. It vividly illustrates the toll in lives exacted by the sea.
It is also the story of how people fought back. In l716, two lighthouses lit by coal fires were built on the Bill. They were badly kept and some years later another was constructed. In l798, when Napoleon threatened invasion, two cannons were installed. This lighthouse was replaced by the present one early this century. A typical example, with a red-circled tower, and a small keeper's house at its foot, it stands at the end of the promontory and a wonderful sea vista from its top floor.
Last year Portland Bill lighthouse underwent further changes. Like most of the 70-odd lighthouses which dot the British coastline, it was automated, though an attendant still checks that everything works and is kept clean.
Gone are the days of the lonely lighthouse keeper, marooned in his tower, watching out for fog and keeping that life-saving light burning. Now a computer does most of the work, monitored from the Trinity House depot in Harwich.
Peter Fitch, the resident attendant at Portland Bill lighthouse, noticed that after its automation many schools which had paid regular visits stopped coming. He guesses that they assumed there would be very little to see and no one to guide them around. In fact Mr Fitch, like many attendants, still gives tours.
A lighthouse visit is an exciting and educational experience. A teacher could strike off a fair number of attainment targets across the curriculum as he or she follows a group up the circular stairs.
The lantern at the top of the lighthouse provides a perfect illustration of using the reflection and refraction of light. Of a type first produced in l822, it is a beautiful object, weighing 334 tons and floatings in a trough of mercury, though this fact is not apparent to the visitors. The lantern is made up of a system of lenses and prisms designed to refract and reflect the light from the central lamp, whether it be, as now, a metal halide lamp or as previously an oil wick burner, in such a way that virtually all the light produced is collected and projected over the sea in a very intense beam that can be seen up to 21 miles away.
Other intriguing answers to tough questions are also evident. In the early days, it was often difficult to distinguish the beam of one lighthouse from another. Now that problem is solved by giving each lighthouse a distinctive flash pattern, which is shown on navigational maps. At Portland Bill four flashes every 20 seconds are interspersed with a period of darkness.
This is achieved by dividing the lantern into four panels. The period of darkness occurs when the back face passes in front of the lamp.
Also found on lighthouses is the fog signal, now automated. The old air-driven fog signal still remains at Portland Bill lighthouse and Mr Fitch will explain how it was specifically designed to give off a very low frequency sound which travels further than those with higher frequencies. Air-driven fog horns were developed in the 19th century, before which bells or even cannons were used.
Mr Fitch will show visitors the lighthouse's subsidiary light which advises sea vessels if they are in the danger zone, and the stand-by generator. There is also a video by the Corporation of Trinity House, the authority for England, Wales and the Channel Islands which provides navigation aids. This looks at the history of lighthouses since the time of Henry VIII and at navigation aids which, apart from on-shore lighthouses, include floating light vessels, buoys - many of which have special radar beacons to notify ships of their position - and offshore lighthouses. The history is fascinating, going back to the 3rd century BC.
Children are often interested in the life of lighthouse keepers not so long ago. The old "service room", where the keeper would have sat watching the paraffin lamp to make sure it didn't go out, still exists and prompts many questions on the lifestyles of the keepers: the shifts they worked and their duties.
School parties can wander round the presently rather sparsely equipped visitors' centre set up in conjunction with the Crown Estate. It is hoped to extend this.
Mr Fitch emphasises that children get most out of the visit when they come well prepared and with their interest already awakened. He sends an information pack and video to schools which book in advance.
A good collection of lighthouse and light vessel equipment can be seen at the Trinity House National Lighthouse Centre at Penzance. Displays include an audio-visual history of the Eddystone Lighthouse, a reconstruction of a rock lighthouse interior and models of navigation aids.
Portland Bill Lighthouse, Easton, Portland, Dorset DT5 2JT. Tel: 01305 820495. Pounds 1.50 adults; 75p children * Trinity House National Lighthouse Centre, Wharf Road, Penzance, Cornwall TR18 4BN. Tel: 01736 360077 * For a list of operational lighthouses which take school parties contact Trinity House, Tower Hill, London EC3N 4DH. Tel: 0171 480 6601