A clock (from the Latin word cloca, meaning "bell") is an instrument for measuring time. In its most common form, in use since at least the fourteenth century, it displays the time in hours, minutes, and often seconds, during a 12 or 24 hour period.
Clocks that are used for telling the time at very high accuracy are usually called chronometers. A common, portable timekeeping instrument for personal use is the pocket watch or wristwatch.
By definition, a "true" clock has an announcing or striking mechanism that sounds after each set interval of time. The sound could be the ringing of a bell, chimes, or gong. A silent clock without a striking mechanism is traditionally known as a timepiece, a term sometimes used by horologists and other specialists to describe devices such as ordinary wristwatches (Baillie et al., p. 307; Palmer, p. 19; Zea and Cheney, p. 172).
HistoryA replica of an ancient Chinese incense clock.
The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes.
Sundials and other techniques
The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely used in ancient times. A well-designed sundial can measure local solar time with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations—it requires the sun to shine and doesn't work at all during the night—encouraged the use of other techniques for measuring time.
Candles and sticks of incense that burn down at, approximately, predictable speeds have also been used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.The massive clock on St. Stephen's Tower, London (commonly known as Big Ben]). The 5 foot 4 inch (1.63 m) person "holding on" to the six-o'clock marking has been inserted into the picture at correct scale. The hour hand is 9 feet (2.7 m) long, and the minute hand is 14 feet (4.3 m) long.
Vitruvius reported that the ancient Egyptians used a clepsydra, a time mechanism using flowing water. Herodotus had mentioned an ancient Egyptian time-keeping device that was based on mercury. By the ninth century C.E., a mechanical timekeeper had been developed that lacked only an escapement mechanism. Later years saw the rise of automated water clocks in Arabia, China, and Korea.
Early mechanical clocks
None of the first clocks survive from thirteenth century Europe, but various mentions in church records reveal some of the early history of the clock.
Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be strictly regulated. This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination. Important times and durations were broadcast by bells, rung either by hand or by some mechanical device such as a falling weight or rotating beater.
The word horologia (from the Greek hora, hour, and legein, to tell) was used to describe all these devices, but the use of this word (still used in several romance languages) for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a horologe but the mechanism used is unknown. In 1198, during a fire at the abbey of St Edmundsbury (now Bury St Edmunds), the monks "ran to the clock" to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.
These early clocks may not have used hands or dials, but “told” the time with audible signals.
A new mechanism
The word "clock" (from the Latin word for "bell"), which gradually supersedes "horologe, " suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the thirteenth century.
Between 1280 and 1320, there was an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take their driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power—the escapement—marks the beginning of the true mechanical clock.
These mechanical clocks were intended for two main purposes: For signaling and notification (e.g. the timing of services and public events), and for modeling the solar system. The former purpose was administrative, the latter arose naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system.
Simple clocks intended mainly for notification were installed in towers, and did not always require dials or hands. They would have announced the canonical hours or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including Italian hours, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata.
In 1283, a large clock was installed at Dunstable Priory; its location above the rood screen suggests that it was not a water clock. In 1292, Canterbury Cathedral installed a "great horloge." Over the next 30 years there are brief mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a new clock was installed in Norwich, an expensive replacement for an earlier clock installed in 1273. This had a large (2 meter) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two technicians for two years.
Early astronomical clocks
The clocks constructed by Richard of Wallingford in St Albans by 1336, and by Giovanni de'Dondi in Padua from 1348 to 1364, no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena.
Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a wheel of fortune and an indicator of the state of the tide at London Bridge. Bells rang every hour, the number of strokes indicating the time.