Water Clock

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FROM 'GOOGLE'

 

water clock

 

 

 

 

A water clock or clepsydra (Greek kleptein, to steal; hydor, water) is any timekeeper operated by means of a regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where it is measured.

 

Water clock overview

Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and day-counting tally stick.[1] Given their great antiquity, where and when they first existed are not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC.[2]

The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and improved accuracy. These advances were passed on through Byzantium and Islamic times, eventually making their way to Europe. Independently, the Chinese developed their own advanced water clocks, passing their ideas on to Korea and Japan.

Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. It is important to point out that the need for the common person to 'know what time it is' largely did not exist until the Industrial Revolution, when it became important to keep track of hours worked. In the earliest of times, however, the purpose for using a water clock was for astronomical and astrological reasons. These early water clocks were calibrated with a sundial. Through the centuries, water clocks were used for timing lawyer's speeches during a trial, labors of prostitutes, night watches of guards, sermons and Masses in church, to name only a few. While never reaching the level of accuracy based on today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.

 

Egypt

The oldest water clock of which there is physical evidence dates to c. 1400 BC and was used in the Temple of Amen-Re.[3] The oldest documentation of the water clock is the tomb inscription of the 16th century BC Egyptian court official Amenemhet, which identifies him as its inventor.[4][5] These simple water clocks, which were

of the outflow type, were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. There were twelve separate columns with consistently spaced markings on the inside to measure the passage of "hours" as the water level reached them. The columns were for each of the twelve months to allow for the variations of the seasonal hours. These clocks were used to determine hours at night, but may have been used in daylight as well.

 

Babylon

In Babylon, water clocks were of the outflow type and were cylindrical in shape. Use of the water clock as an aid to astronomical calculations dates back to the Old Babylonian period (c. 2000 BC–c. 1600 BC).[6]

While there are no surviving water clocks from the Mesopotamian region, most evidence of their existence comes from writings on clay tablets. Two collections of tablets, for example, are the Enuma-Anu-Enlil (1600–1200 BC) and the MUL.APIN (7th century BC).[7] In these tablets, water clocks are used in reference to payment of the night and day watches (guards).

These clocks were unique, as they did not have an indicator such as hands (as are typically used today) or grooved notches (as were used in Egypt). Instead, these clocks measured time "by the weight of water flowing from" it.[8] The volume was measured in capacity units called qa. The weight, mana (the Greek unit for about one pound), is the weight of water in a water clock.

It is important to note that during Babylonian times, time was measured with temporal hours. So, as seasons changed, so did the length of a day. "To define the length of a 'night watch' at the summer solstice, one had to pour two mana of water into a cylindrical clepsydra; its emptying indicated the end of the watch. One-sixth of a mana had to be added each succeeding half-month. At equinox, three mana had to be emptied in order to correspond to one watch, and four mana were emptied for each watch of the winter solstitial night."[8]

 

India

N. Kameswara Rao suggests that pots excavated from Mohenjodaro might have been used as water clocks; they are tapered at bottom, have a hole on the side, and are similar to the utensil used to perform abhishekam (pour holy water) on shivalingam.[9]

N. Narahari Achar[10] and Subhash Kak[11] suggest that the use of the water clock in ancient India is mentioned in the Atharvaveda from

the 2nd millennium BC.

Ghati or Kapala (clepsydra or water clock) is referred to in Jyotisha Vedanga, where the amount of water that measures a nadika (24 minutes) is mentioned. A more developed form of the clepsydra is described in chapter xiii, 23 of the Suryasiddhanta.[12]

At Nalanda, a Buddhist university, four hours a day and four hours at night were measured by a water clock, which consisted of a copper bowl holding two large floats in a larger bowl filled with water. The bowl was filled with water from a small hole at its bottom; it sank when completely filled and was marked by the beating of a drum at daytime. The amount of water added varied with the seasons and this clock was operated by the students of the university.[13]

The description of a water clock in astrologer Varahimira's Pancasiddhantika (505) adds further detail to the account given in the Suryasiddhanta. The description given by mathematician Brahmagupta in his work Brahmasphutasiddhanta matches with that given in the Suryasiddhanta. Astronomer Lallacharya describes this instrument in detail.[14] In practice, the dimensions were determined by experiment.

 

China

 

 

The water powered mechanism of Su Song's astronomical clock tower.

In China, as well as throughout eastern Asia, water clocks were very important in the study of astronomy and astrology. The oldest reference dates the use of the water-clock in China to the 6th century BC.[15] From about 200 BC onwards, the outflow

clepsydra was replaced almost everywhere in China by the inflow type with an indicator-rod borne on a float.[15]

Huan Tan (40 BC–30 AD), a Secretary at the Court in charge of clepsydrae, wrote that he had to compare clepsydras with sundials because of how temperature and humidity affected their accuracy, demonstrating that the effects of evaporation, as well as of temperature on the speed at which water flows, were known at this time.[16] In 976, Zhang Sixun addressed the problem of the water in clepsydrae freezing in cold weather by using mercury instead.[17]

The use of clepsydrae to drive mechanisms illustrating astronomical phenomena began with Zhang Heng in 125.[18] This led to the invention by Yi Xing and Liang Lingzan in 725 of a clock driven by a water-wheel linkwork escapement.[19] The same mechanism would be used by Su Song in 1088 to power his astronomical clock tower.[20] Su Song's clock tower, over 30 feet tall, possessed a bronze power-driven armillary sphere for observations, an automatically rotating celestial globe, and five front panels with doors that permitted the viewing of changing manikins which rang bells or gongs, and held tablets indicating the hour or other special times of the day.

This is for the knowledge of those in study of Astrology.

A.V.Pathi, Chapel Hill,North Carolina, 27514, USA