The fuels for the lighthouse
A quite interesting Junior book Signposts of the sea by Althea, and illustrations by Timothy Hunkin.
The most interesting paragraphs of this book are about the evolution of the fuels used to generate light for the lighthouse, especially in one stage, people burnt candles in the lighthouse, and sometimes the light given out by a lantern is still calculated by "candle power"! This remind me of the origin of "horse power".
The earliest lighthouses used wood fires for their lanterns. One of the first lighthouses was built in about 300 BC on the island of Pharos at the port of Alexandria, in Egypt. Different ancient writers vary in their description of it, but it was probably built of white stone, and it was said to be about 400 feet high, and a wood fire was kept burning on the top night and day. This gave a warning light at night and the column of smoke could be seen from a very great distance in the day time.
As wood became scarce, coal started to be used instead. One of the last lighthouses to use a coal fire for its lantern was Flat Holm in the Bristol Channel which used it right up until 1820.
Then, after coal fires they started to use candles. The light in Smeaton's Eddystone Lighthouse was a candelabra of two rings containing 24 candles. To remind the keepers to trim the candle wicks at intervals. The light given out by a lantern is sometimes still measured in candle power, for example, in Briton the automatic lighthouse at Orford Ness in Suffolk is the most powerful with approximately 7.5 million candle power! However the range of light is more important than the power and so the measurement usually given is 'nominal range.' The nominal range at Orford Ness is 31 miles.
After candles, oil burners were used and these gave a better light, though still not powerful enough. People tried many different systems to make the light brighter, including metal reflectors. Eventually the 'optic' was invented, which you can see in the photograph opposite. There is a bull's-eye lens in the middle, surrounded by lots of concentric rings of glass prisms, which all help to concentrate the light and make it very bright. Behind the light there are reflectors made of segments of glass. The optic is turned round and round which makes the light seem to flash on and off. Some shore lighthouses have a bulb which flashes on and off instead of the optic being turned round.
In the daytime the lighthouse keepers clean the lantern and the prisms and reflectors so that the light will shine brightly. Then they pull curtains around the inside of the lantern windows. This is to prevent the sun shining through the glass prisms and causing a fire by focussing the sunlight.
Lighthouses first used electric light in 1862 and now most of the shore and rock lanterns are lit by electricity which gives an even more powerful and efficient light.
In fog, the light cannot be seen from long distances so lighthouses are also fitted with a fog signal, and they make a deep wailing sound, or a deep grunt.