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