Time, a central theme in modern life, has for most of human been thought of in very imprecise terms.

The day and the week are easily recognized and recorded – though an accurate calendar for the year is hard to achieve. The forenoon is easily distinguishable from the afternoon, provided the sun is shining, and the position of the sun in the landscape can reveal roughly how much of the day has passed. By contrast the smaller parcels of time – hours, minutes and seconds – have until recent centuries been both unmeasurable and unneeded.

Sundial and water clock: from the 2nd millennium BC

The movement of the sun through the sky makes possible a simple estimate of time, from the length and position of a shadow cast by a vertical stick. (It also makes possible more elaborate calculations, as in the attempt of Erathosthenes to measure the world). If marks are made where the sun’s shadow falls, the time of day can be recorded in a consistent manner.

The result is the sundial. An Egyptian example survives from about 800 BC, but the principle is certainly familiar to astronomers very much earlier. However it is difficult to measure time precisely on a sundial, because the sun’s path throug the sky changes with the seasons. Early attempts at precision in time-keeping rely on a different principle.

The water clock, known from a Greek word as the clepsydra, attempts to measure time by the amount of water which drips from a tank. This would be a reliable form of clock if the flow of water could be perfectly controlled. In practice it cannot. The clepsydra has an honourable history from perhaps 1400 BC in Egypt, through Greece and Rome and the Arab civlizations and China, and even up to the 16th century in Europe. But it is more of a toy than a timepiece.

The hourglass, using sand on the same principle, has an even longer career. It is a standard feature on 18th-century pulpits in Britain, ensuring a sermon of sufficient length. In a reduced form it can still be found timing an egg.

A tower clock in China: AD 1094

After six years’ work, a Buddhist monk by the name of Su Song completes a great tower, some thirty feet high, which is designed to reveal the movement of the stars and the hours of the day. Figures pop out of doors and strike bells to signify the hours.

The power comes from a water wheel occupying the lower part of the tower. Su Song has designed a device which stops the water wheel except for a brief spell, once every quarter of an hour, when the weight of the water (accumulated in vessels on the rim) is sufficient to trip a mechanism. The wheel, lurching forward, drives the machinery of the tower to the next stationary point in a continuing cycle.

This device (which in Su Sung’s tower must feel like a minor earthquake every time it slams the machinery into action) is an early example of an escapement – a concept essential to mechanical clockwork. In any form of clock based on machinery, power must be delivered to the mechanism in intermittent bursts which can be precisely regulated. The rationing of power is the function of the escapement. The real birth of mechanical clockwork awaits a reliable version, developed in Europe in the 13th century.

Meanwhile Su Sung’s tower clock, ready for inspection by the emperor in 1094, is destroyed shortly afterwards by marauding barbarians from the north.

Clockwork in Europe: 13th – 14th century AD

Europe at the end of the Middle Ages is busy trying to capture time. The underlying aim is as much astronomical (to reflect the movement of the heavenly bodies) as it is to do with the more mundane task of measuring everybody’s day. But the attraction of that achievement is recognized too. A textbook on astronomy, written by ‘Robert the Englishman’ in 1271, says that ‘clockmakers are trying to make a wheel which will make one complete revolution’ in each day, but that ‘they cannot quite perfect their work’.

What prevents them even beginning to perfect their work is the lack of an escapement. But a practical version of this dates from only a few years later.

A working escapement is invented in about 1275. The process allows a toothed wheel to turn, one tooth at a time, by successive teeth catching against knobs projecting from an upright rod which oscillates back and forth. The speed of its oscillation is regulated by a horizontal bar (known as a foliot) attached to the top of the rod. The time taken in the foliot’s swing can be regulated by moving weights in or out on each arm.

The function of the foliot is the same as that of the pendulum in modern , but it is less efficient in that gravity is not helping it to oscillate. A very heavy weight is needed to power the clock, involving massive machinery and much friction.

Nevertheless the foliot works to a degree acceptable at the time (a clock in the Middle Ages is counted a good timekeeper if it loses or gains only a quarter of an hour a day), and in the 14th century there are increasingly frequent references to clocks in European cities. A particularly elaborate one is built between 1348 and 1364 in Padua by Giovanni de’ Dondi, a professor of astronomy at the university who writes a detailed description of his clock. A 14th-century manuscript of his text has the earliest illustration of a clock mechanism with its escapement.

The world’s three oldest surviving examples of clockwork date from the last years of the 14th century.

The famous clock in Salisbury cathedral, installed by 1386 and still working today with its original mechanism, is a very plain piece of machinery. It has no face, being designed only to strike the hours. Striking is the main function of all early clocks (the word has links with the French cloche, meaning ‘bell’).

In 1389 a great clock is installed above a bridge spanning a street in Rouen. It remains one of the famous sights of the city, though its glorious gilded dial is a later addition and its foliot has been replaced by a pendulum (in 1713). The historical distinction of the Rouen clock is that it is the first machine designed to strike the quarter-hours.

In 1392 the bishop of Wells instals a clock in his cathedral. The bishop has previously been in Salisbury, and the same engineer seems to have made the new clock. It not only strikes the quarters. It steals a march on Rouen by having a dial, showing the movement of astronomical bodies.

With escapements, chiming mechanisms and dials, clocks are now set to evolve into their more familiar selves. And the telling of time soon alters people’s perceptions of time itself. Hours, minutes and seconds are units which only come into existence as the ability to measure them develops.

Domestic clocks: 15th century AD

After the success of the clocks in Europe’s cathedrals in the late 14th century, and the introduction of the clock face in places such as Wells, kings and nobles naturally want this impressive technology at home.

The first domestic clocks, in the early 15th century, are miniature versions of the cathedral clocks – powered by hanging weights, regulated by escapements with a foliot, and showing the time to the great man’s family and household by means of a single hand working its way round a 12-hour circuit on the clock’s face. But before the middle of the 15th century a development of great significance occurs, in the form of a spring-driven mechanism.

The earliest surviving spring-driven clock, now in the Science Museum in London, dates from about 1450. By that time clockmakers have not only discovered how to transmit power to the mechanism from a coiled spring. They have also devised a simple but effective solution to the problem inherent in a coiled spring which steadily loses power as it uncoils.

The solution to this is the fusee.

The fusee is a cone, bearing a spiral of grooves on its surface, which forms part of the axle driving the wheels of the clock mechanism. The length of gut linking the drum of the spring to the axle is wound round the fusee. It lies on the thinnest part of the cone when the spring is fully wound and reaches its broadest circumference by the time the spring is weak. Increased leverage exactly counteracts decreasing strength.

These two devices, eliminating the need for weights, make possible clocks which stand on tables, clocks which can be taken from room to room, even clocks to accompany a traveller in a carriage. Eventually, most significant of all, they make possible the pocket watch.

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