The Salisbury timepiece may well be the world's oldest surviving clock. But it hardly looks like a clock at all: the wheels and gears are mounted in an open, cube-shaped iron frame, about 4ft wide. Having no face or hands, it signals the time only by striking the hours. Incredibly, there are no nuts and bolts in its wood-and-iron mechanism (they had not yet been invented in the 14th century); instead, the parts are held together by dowels and pegs.
Two large weights drive the clockwork. As the weights fall, a rope unwinds from a wooden barrel, turning a large-geared wheel. Its motion is controlled by a verge-and-foliot "escapement" - a long iron bar that swings back and forth, alternately stopping and releasing the main wheel. The escapement allows the clock (with regular winding) to tick away at a precise, steady rate.
The machine's origins are unknown, but the cathedral's records show that a man was paid to wind the clock as early as 1386. "How long it was here before that, we can't say," remarks John Plaister, the cathedral's current clock-keeper. "But I've no fear to say it's the oldest working clock in England."
The world's first recognisable mechanical clocks appeared in Europe early in the 14th century, and there is another very early one in Wells Cathedral, 35 miles west. It may even have been built by the same man who made the Salisbury machine, for records show that it was there in 1390. But although the two clocks have similar mechanisms, they look quite different. The Wells clock has a magnificent face, with elaborate paintings of the moon, sun and stars, and a large outer dial showing Roman numerals from 1 to 12 twice over, making a 24-hour display. A smaller circle within it shows the minutes. Above the dials, a miniature medieval tournament springs to life every quarter-hour, with four knights on horseback trotting around in a circle.
In the age of digital watches and atomic clocks, such conceits seem fanciful. But if it hadn't been for the fascination with time that inspired the medieval clockmakers to create these pioneering contraptions, it's conceivable that the Millennium - with all its attendant Domes and Ferris wheels - would be passing almost unnoticed.