Tower and Turret Clocks
Long before most people owned a timepiece, a clock high in a church or town tower told a whole community when to work, pray and rest.
Why clocks went up towers
A tower clock, often called a turret clock, is a large mechanical clock built into a church, town hall or gatehouse. For centuries it was the only shared clock most people had, and it mattered as much for its bell as for its dial.
The first mechanical clocks grew out of alarms, in which a slowly falling weight set off a bell at a chosen moment. According to the University of Bristol's study of parish clocks, a monk named Robertus Anglicus wrote in 1271 that clockmakers were trying to build a wheel that turned once every 24 hours but had not yet managed it. By the end of that century, several clocks were clearly working in England.
Height was not just for show. Those early clocks were driven by heavy weights hanging from ropes, and the weights needed a long drop to keep the clock running between windings. The Bristol researchers say towers and steeples provided exactly that height.
Bell height mattered too. How far the hour strike could be heard depended on where the bell hung, not on the clock mechanism itself, so a bell high in a tower carried the time across a whole parish.
How a weight-driven clock works
The power source in an early tower clock is simple. A weight hangs from a rope wrapped around a barrel. As the weight falls, the barrel turns, and gearing carries that turning motion through the going train, the set of wheels that keeps time.
Left alone, the weight would crash to the floor in seconds. The escapement stops that from happening by letting the gears move forward only in tiny, regular steps. From the late 13th century to the late 17th century, the dominant design was the verge and foliot, in which a crown wheel pushes against two pallets on a rod called the verge, rocking a weighted bar called the foliot back and forth.
Regulating these clocks was hands-on work. Keepers made them run faster or slower by adding or removing material from the weights or the foliot, or by sliding small weights toward or away from the ends of the foliot bar. Most 16th-century English church clocks were wound once a day.
See also: a guide to the parts inside a clock on OnlineClock.app.
The oldest working clock
Salisbury Cathedral in England has what Guinness World Records lists as the oldest surviving working clock, thought to date from 1386 or possibly earlier. It has no face at all. Its only job was to strike a bell on the hour, calling people to services.
The clock first stood in a separate bell tower near the spot now occupied by the cathedral's Bell Tower Tea Rooms. In 1884 it was replaced by a newer clock and put aside. The cathedral says it was rediscovered in 1929 and restored to its original condition in 1956.
Guinness says it struck the hours for 498 years and has ticked more than 500 million times. It still runs inside the cathedral today, driven as it always was by falling weights, which have to be wound back up once a day.
The pendulum takes over
The biggest leap in accuracy came from the pendulum. Galileo started working on the idea of a pendulum-regulated clock in the 1580s, and Christiaan Huygens completed it in the late 1650s, using curved metal cheeks that steered the bob along a path that kept wide swings evenly timed.
The University of Bristol credits Salomon Coster with completing the first pendulum clock, a small household clock, in 1658. Pendulum turret clocks followed within a few years, and many old tower clocks were converted, with a pendulum replacing the foliot. Pendulum clocks cost more at first and needed scarce skilled makers, but they spread quickly.
The anchor escapement, developed during the 1660s and 1670s, soon became standard for new turret clocks because it needed only a small pendulum swing. From then on, the length of the pendulum set the timekeeping. Clocks still needed tall towers, though, because the driving weights still needed room to fall.
Big Ben and the gravity escapement
Tower clocks face a problem that small clocks do not: their huge hands are exposed to the weather, and wind pressing on the hands can disturb the pendulum. The answer came from Edmund Beckett Denison, a lawyer and amateur clockmaker, who invented the double three-legged gravity escapement in 1854.
His chance came after the old Palace of Westminster burned in 1834. Charles Barry led the rebuilding, and Augustus Pugin designed the clock tower and its faces. Barry first sought advice from the clockmaker Benjamin Vulliamy, but the Lords turned instead to the Astronomer Royal, George Airy, and to Denison. The two then clashed: Airy wanted the clock tied to electrical signals from Greenwich, Denison wanted bells, and Airy eventually stepped aside.
Denison designed the mechanism, and Edward John Dent was chosen to build it. When Dent died suddenly, his adopted son Frederick Dent finished the job. The Linda Hall Library records that the clock was inaugurated on May 1, 1859, that Denison also wrote the tune now known as the Westminster quarters, and that he never patented his escapement. It spread so widely that tower clocks everywhere were soon using it.
UK Parliament's figures give a sense of scale. Each of the four dials is 7 m across, the copper minute hands are 4.2 m long, and the hour hands are 2.7 m. The pendulum is 4.4 m long with a beat of 2 seconds. The clock is still fine-tuned with old pennies on the pendulum: adding one makes it gain two-fifths of a second a day.
See also: the chime patterns clock bells ring on OnlineClock.app.
Winding: from muscle to motors
Winding a big tower clock was hard labor. According to the Cumbria Clock Company, two men wound the Westminster clock by hand every week before 1913, turning the handles thousands of times in a job that took about four hours and wore footprints into the clock room floor. Denison himself said the task was hardly fit for anybody except convicts.
In 1913 Dent and Co. installed an electro-mechanical winder that cut the job to about 40 minutes. The system used clutches, cams and an epicyclic gear train so the clock kept running while its weights were raised. By the 1970s only part of it was still in use, and it later fell into disrepair until the Cumbria Clock Company restored it during the Elizabeth Tower conservation project.
Denison's original mechanism still runs the clock, refurbished several times along the way. At heart it works on the same principle as Salisbury's: a falling weight, a regulator, and a bell to tell everyone within earshot what time it is.
Questions people ask
What is the difference between a tower clock and a turret clock?
They mean the same thing: a large mechanical clock built into a tower, church or other public building. The two terms are used interchangeably.
What is the oldest working tower clock?
Guinness World Records lists the faceless clock at Salisbury Cathedral, thought to date from 1386 or possibly earlier, as the oldest surviving working clock. Its exact age is an estimate.
How are tower clocks powered?
Traditionally by falling weights that must be wound back up. Big Ben, for example, was wound by hand every week until an electro-mechanical winder was installed in 1913.
Why do tower clocks use a gravity escapement?
Wind and weather on large outdoor hands can disturb timekeeping. Denison's gravity escapement keeps those outside forces away from the pendulum, which is why it became standard on tower clocks after 1854.