Leading light

Many a boat was dashed to pieces on Bell Rock in the North Sea, until a beacon was built to guide sailors to safe waters. The world's oldest surviving sea-washed lighthouse remains the finest achievement of visionary engineer Robert Stevenson, born 250 years ago this month

- Bella Bathurst

Robert Stevenson is the Stevenson most people haven't heard of. They have heard of the other Robert Stephenson, early locomotive engineer. They have heard of Robert Louis Stevenson, writer. But before RLS made the Stevenson name famous with Kidnapped and Treasure Island, his grandfather made it great, by building impossible towers using untried methods in malignant seas: including the light on the Bell Rock.

The Inchcape reef is a lump of sandstone rock lying like a slice of cheese turned on its side, 12 miles from Dundee on Scotland's east coast. The reef lies in one of the busiest sea routes in the world, used by all vessels, from small fishing boats to naval convoys. At the end of the 18th century, the east coast remained almost completely unlit, had few safe harbours, and was particularly feared for its haars (or sea frets) - the spectral fogs that could drift in without warning, muffling visibility down to the size of a room.

But what made Inchcape so murderous was its silence. It is submerged at high tide and visible only as a ruff of white water at low tide. By 1800 it was estimated that an average of 10 vessels a year were either wrecked on it, or wrecked trying to avoid it. When the warship York sank on the reef in 1799 with the loss of all hands, it was unanimously agreed, both in Parliament and on the coasts, that something must be done.

Over the centuries, all sorts of contraptions to light or mark the reef had been tried: rafts, poles, pillars, towers, cages. So many bells had been raised and then swept aside, either by the sea or by wreckers, that the rock was silted with old metal like a scrapyard. No one disputed that it was essential to have it marked, but no one was daft enough to want to do it.