Part 4: Lighthouses
You will hear part of a lecture on the history of lighthouses.
Lời thoại bài nghe (transcript)
Lecturer: Good morning, everyone. In today's lecture in our series on coastal engineering, I'm going to look at lighthouses: how they developed, the problems their builders had to solve, and their role today.
Lecturer: Let's begin with the earliest coastal lights. For many centuries, the only warning sailors received was a fire lit on a hilltop or on a stone platform near the entrance to a harbour. These fires burned wood at first, and later coal, which gave a brighter flame and lasted much longer. The difficulty, of course, was that an open fire was very hard to keep going in bad weather, which was exactly when ships needed it most. Heavy rain could put it out completely.
Lecturer: The next challenge was where to build. Many of the most dangerous hazards were not on the shore at all, but on rocks some distance out to sea. Building on a rock that disappears under the water twice a day was an enormous undertaking. Workers could usually only work for two or three hours at a time, at low tide, and on some sites construction was only possible during the summer months. The first towers on these rocks were made of timber, and a number of them were simply swept away by storms within a few years.
Lecturer: The breakthrough came when builders began to use stone blocks that were cut so that each one locked into the blocks above, below and beside it, rather like the pieces of a puzzle. This meant that the tower behaved as one solid mass, and the sea could not pull individual stones away. Engineers also gave these towers a wide, curved base. You'll often read that this curve was chosen simply because it looked elegant, but in fact it was modelled on the shape of a tree trunk, which spreads out at the bottom. The curve meant that the force of the waves was spread over a large area instead of striking a flat wall.
Lecturer: Now, let's turn to the light itself. Even after candles and oil lamps replaced open fires, most of the light was wasted, because it spread out in every direction, including straight up into the sky. Polished metal mirrors placed behind the lamp helped a little. But the real advance was a new type of lens made of many rings of glass, arranged around the lamp in a shape that looked rather like a beehive. This lens bent the light into a narrow horizontal beam, so that a single lamp could be seen from more than twenty miles away.
Lecturer: Another problem was identification. If every lighthouse showed the same steady light, a captain at night had no way of knowing which one he was looking at. So each light was given its own pattern of flashes. One might flash twice every ten seconds, for example, while its neighbour showed a single long flash. To produce these patterns, the whole lens had to turn. It weighed several tonnes, so it was mounted on a platform that floated in a bath of mercury, which allowed it to rotate smoothly with almost no friction. A clockwork mechanism kept it moving, and this had to be wound by hand every few hours.
Lecturer: That brings me to the keepers. Life in a lighthouse, particularly an offshore one, could be very lonely. Keepers usually worked in teams of three, so that someone was always on watch, and they might spend several weeks without setting foot on land. Their duties were mostly practical: trimming the wick, cleaning soot from the glass and recording the weather in a logbook every few hours. Interestingly, these logbooks have recently become valuable to climate scientists, because they contain detailed observations going back, in some cases, more than a century.
Lecturer: Finally, what about lighthouses today? From the middle of the twentieth century, they were gradually automated, and the last keepers left most stations several decades ago. The lamps are now monitored remotely, and many are powered by solar panels. Since almost every vessel now carries satellite navigation, some people argue that lighthouses are no longer necessary. However, maritime authorities point out that electronic systems can fail, and a visual signal remains an essential backup. Many towers that are no longer in use have found new roles. Some have become museums and others have been turned into holiday accommodation, while a few serve as research stations for scientists who study seabirds.
