For most of the seventeenth century, the Isles of Scilly were drawn on English charts roughly ten miles north of where they actually were. Ships running for the English Channel under cloud and dead reckoning had no real idea how close they were sailing to a granite reef that should not have been there. In May 1680, Trinity House began a survey of the coasts to fix the problem and was granted permission to erect lighthouses on Scilly. They chose St Agnes because it sat closest to the worst of it - the tangle of skerries and tide rips now called the Western Rocks. The tower they built was only the second lighthouse station Trinity House had ever established, after Lowestoft in 1609, and only the second built to mark the western approaches, after Lizard in 1619. A field of work was beginning that would, in time, make the British coastline one of the most precisely marked in the world. It started here, on a windy down above a beach littered with wreck timber.

Until 1790, the light burned coal. A keeper climbed the tower with baskets of fuel and tended an open chauffer through the night, sending up a smear of orange flame that could be smothered by the wrong wind. Then came the Argand lamp, that small but consequential Swiss invention that turned a flame into something steady and bright. Twenty-one of them, fitted with parabolic reflectors and mounted on a three-sided revolving array, replaced the coal. In 1806 the lantern at the top of the tower was rebuilt to the form visitors see today. In 1880 the optics were upgraded again - fourteen reflectors burning two-wick mineral oil, arranged in two tiers on a square frame that spun to give a flash every thirty seconds instead of the old leisurely minute. Each refinement was a small revolution, the technology of safety advancing one lamp at a time.

The trouble with St Agnes was that it sat too far inland. On a clear night the lantern could be seen from far out at sea, but the western approaches do not specialise in clear nights. Mist could roll in from the Atlantic and swallow the down whole, leaving sailors with no warning of the rocks lurking beyond. The remedy was Bishop Rock, that astonishing iron-then-granite tower bolted to a single tooth of stone forty miles southwest of Land's End. When Bishop Rock was completed in 1858, St Agnes lost its status as England's westernmost light. It carried on for another half-century, downgraded but still useful, until 1911, when a new steel lattice lighthouse at Peninnis on St Mary's took over its duties. The old tower was decommissioned, but never demolished. It still stands, painted white and maintained as a daymark - a landmark by day, a memorial by night.

Visitors who walk up to it today find a stout, friendly-looking thing, more cottage than castle, with those curious gun ports cut into walls thick enough to laugh at a storm. From the foot of the tower, the view runs west across the Western Rocks toward the slender finger of Bishop Rock on the horizon - the lighthouse that replaced it, framed by the sea that took so many ships before either of them existed. The St Agnes light is silent now. It has not flashed for over a century. But its shape on the skyline, white against the green of the down, still does the work it was built for: telling sailors where they are, and warning them what lies beyond.