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Clocks
A clock (from the Latin cloca, "bell") is an instrument for measuring time. The clock in its most common, modern form (in use since at least the 14th century) displays the time in hours, minutes, and often seconds during a 12- or 24-hour period. more...
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Clocks used for technical purposes, of very high accuracy, are usually called chronometers. A common portable timekeeping instrument for personal used is the pocket or wrist watch.
True clocks also have an announcing or striking mechanism that sounds after each set interval of time, usually by ringing a bell (which, as previously indicated, originally gave the clock its name), 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 ordinary wrist watches and other timekeeping devices lacking a striking mechanism (see Baillie et al., p. 307; Palmer, p. 19; Zea & Cheney, p. 172).
History
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. The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely known in ancient times.
Candles and sticks of incense that burn down at, approximately, predictable speeds were also used as to estimate the passage of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary time.
Water clocks
The historian 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 9th century AD 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
No clocks survive from medieval 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.
Read more at Wikipedia.org
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