How Do Lighthouses Work? Lenses, Rhythms and the Keepers Who Left

A flame radiates in every direction, and almost all of its light goes up into the sky, down into the tower and back into the room. The lighthouses of the eighteenth century put mirrors behind the lamp to throw some of it seaward, and the parabolic reflector, polished silver on copper, could gather perhaps a fifth of the light into a beam; a tower would carry twenty or thirty lamps and reflectors on a frame to cover the horizon, and the whole thing burned whale oil by the gallon. Augustin Fresnel, a physicist working for the French lighthouse commission, took the other approach in 1822: put the glass in front of the lamp, and shape it so that every part of it bends the light toward the horizon.

A lens large enough to capture a lamp's light would be metres thick and weigh tonnes. Fresnel's insight was that only the curved surface of a lens does the bending, so he cut the lens into concentric rings, each with the curvature it would have had in the full lens and each thinned to a few centimetres, and stacked them into a drum of glass around the lamp, with prisms above and below to catch the light going up and down and turn it out to sea too. A first-order lens, the largest, is nearly three metres tall, catches about 80 percent of the lamp's light and throws it in a beam that can be seen from beyond the horizon. The lens is made in panels, each producing one beam, so that as the whole assembly rotates on a bath of mercury or on bearings the beams sweep the sea and the light appears to flash. The parts:

• •The lamp: oil wicks, then paraffin vapour mantles from 1901, then electric bulbs, and now LEDs of a few hundred watts
• •The lens: Fresnel rings and prisms in a brass frame, in orders from first, for landfall lights, to sixth, for harbours
• •The rotation: clockwork driven by a falling weight that the keeper wound every few hours, then electric motors
• •The tower: masonry on rock, or cast iron on screw piles in sand, or, offshore, interlocking granite blocks as at Eddystone and Bell Rock
• •The fog signal: a horn or siren, sounded when the light cannot be seen

Every lighthouse has a character, the pattern of its light, published in the lists of lights and printed on the chart beside it: a fixed white, a flash every five seconds, a group of three flashes every fifteen, an occulting light that is mostly on and briefly off, or a colour. The pattern is set by the number of lens panels and the speed of rotation, and it lets a navigator identify a light at night and take a bearing from it, which with a second light gives a position. Sector lights show red or green over the water where a ship should not go and white where it should, so that a vessel in the channel sees white and one straying onto the rocks sees red; the colours of buoys and the rhythms of lights are an international language that a sailor from any country can read.

The great engineering was the offshore towers. Winstanley's wooden Eddystone light of 1698, on a reef off Plymouth, was swept away in a storm in 1703 with its builder in it; John Smeaton's of 1759 was built of granite blocks dovetailed into each other and into the rock, in the shape of an oak trunk, and stood for 120 years; Robert Stevenson's Bell Rock light of 1810, on a reef that is under water at high tide, was built in two-hour shifts as the tide allowed and has stood since. The Stevensons, the family of Robert Louis, built most of Scotland's lights over four generations, and the ones on Skerryvore and Muckle Flugga are among the most exposed buildings in the world, taking waves that break over the lantern forty metres up.

A lighthouse had three keepers, one on watch, tending the lamp, winding the clockwork, polishing the lens and recording the weather, and their life, on a rock for a month at a time, produced its own literature and its own mysteries; the three keepers of the Flannan Isles light vanished in December 1900 and were never found. Automation began in the 1920s, electrification, remote monitoring and long-life lamps made the keeper redundant, and Britain's last, at North Foreland, left in 1998. Satellite navigation has made the lights less essential and not unnecessary, since a receiver can fail and a rock cannot, and the international rules still require them; around 20,000 lighthouses are lit worldwide, most of them now solar-powered, flashing a character that was chosen when the lens was cast.

A lighthouse works by surrounding a lamp with a Fresnel lens, rings and prisms of glass that bend most of the light into horizontal beams, and rotating it so that the beams sweep the sea in a pattern of flashes, the light's character, which a navigator reads from the chart to identify it and take a bearing. Towers on offshore rocks were built of interlocking granite, keepers tended the lamps until automation ended the job, and the lights remain as the backup that a satellite cannot be.