A
water clock or
clepsydra
(
Greek κλέψτε
kleptein, 'to
steal'; ὕδωρ
hydro, 'water') is any
timepiece in which time is measured by the
regulated flow of liquid into (inflow type) or out from (outflow
type) a vessel where the amount is then measured.
Water clocks, along with
sundials, are
likely to be the oldest time-measuring instruments, with the only
exceptions being the vertical
gnomon and the
day-counting
tally stick. Where and when
they were first invented is not known, and given their great
antiquity it may never be.
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 in China as early as 4000 BC.
The
Greeks and
Romans further advanced water clock design to
include the inflow clepsydra with the earliest feedback system,
gearing, and an
escapement mechanism,
which were connected to fanciful
automata
and also resulted in improved accuracy. Further advances were made
in
Byzantium and particularly the
Islamic world, where water clocks incoporated
complex segmental and
epicyclic
gearing,
water wheels, and
programmability, advances which eventually
made their way to
Europe.
Independently, the
Chinese developed their own advanced water clocks, incorporating
gears, escapement mechanisms, and water wheels, 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. These early
water clocks were calibrated with a
sundial.
While never reaching a level of accuracy comparable to today's
standards of timekeeping, the water clock was the most accurate and
commonly used timekeeping device for millennia, until it was
replaced by more accurate
pendulum
clocks in 18th century Europe.
Egypt
The oldest water clock of which there is physical evidence dates to
c. 1417-1379 BC, during the reign of Amenhotep III where it was used in the
Temple of
Amen-Re
at Karnak. 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.
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 by priests to determine the time at
night so that the temple rites and sacrifices could be performed at
the correct hour. These clocks may have been used in daylight as
well.The water clock was originally invented by the greek inventor
Ctesibius who lived in Alexandria, Egypt at the time.
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).
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). 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. 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."
India
N.
Kameswara Rao suggests that pots excavated
from Mohenjodaro
might have been used as water clocks; they are
tapered at the bottom, have a hole on the side, and are similar to
the utensil used to perform abhishekam (pour holy water) on
shivalingam.
N. Narahari Achar and
Subhash Kak
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.
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.
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. In practice, the dimensions were determined
by experiment.
China
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. 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.
Huan Tan (40 BC–AD 30), a Secretary at the
Court in charge of clepsydrae, wrote that he had to compare
clepsydrae 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. In 976,
Zhang Sixun addressed the problem of the water
in clepsydrae freezing in cold weather by using liquid mercury
instead.
Again, instead of using water, the early
Ming
Dynasty
engineer Zhan Xiyuan (c. 1360-1380) created
a sand-driven wheel clock, improved upon by Zhou Shuxue (c.
1530-1558).
The use of clepsydrae to drive mechanisms
illustrating astronomical phenomena began
with
Zhang Heng (78-139) in 117, who also
employed a
waterwheel. Zhang Heng was the
first in China to add an extra compensating tank between the
reservoir and the inflow vessel, which solved the problem of the
falling
pressure head in the reservoir
tank. Zhang's ingenuity led to the creation by
Yi Xing (683–727) and Liang Lingzan in 725 of a
clock driven by a waterwheel linkwork
escapement mechanism. The same mechanism would be
used by
Su Song (1020-1101) in 1088 to power
his
astronomical clock tower, as
well as a
chain drive. 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.
Today, in
Beijing's Drum
Tower
an outflow clepsydra is operational and displayed
for tourists. It is connected to automata so that every
quarter-hour a small brass statue of a man claps his cymbals.
Greco-Roman world

An early 19th-century illustration of
Ctesibius's clepsydra from the 3rd century BC.
The hour indicator ascends as water flows in.
Also, a series of gears rotate a cylinder to correspond to the
temporal hours.
In
Greece
, a water
clock was known as a clepsydra (water thief). The Greeks
considerably advanced the water clock by tackling the problem of
the diminishing flow. They introduced several types of the inflow
clepsydra, one of which included the earliest feedback control
system.
Ctesibius invented indicator
system typical for later clocks such as the dial and pointer. The
Roman engineer
Vitruvius described early alarm clocks, working
with gongs or trumpets.
A commonly used water clock was the simple outflow clepsydra. This
small earthenware vessel had a hole in its side near the base. In
both Greek and Roman times, this type of clepsydra was used in
courts for allocating periods of time to speakers. In important
cases, when a person's life was at stake for example, it was
filled. But, for more minor cases, it was only partially filled. If
proceedings were interrupted for any reason, such as to examine
documents, the hole in the clepsydra was stopped with wax until the
speaker was able to resume his pleading.
In the
4th century BC, the clepsydra is known to have been used as
stop-watch for imposing a time limit on clients' visits in Athenian
brothels. Slightly later, in the early 3rd century
BC, the Hellenistic physician Herophilos
employed a portable clepsydra on his house visits in Alexandria
for measuring his patients' pulse-beats. By
comparing the rate by age group with empirically obtained data
sets, he was able to determine the intensity of the disorder.
Between 270 BC and 500 AD,
Hellenistic (
Ctesibius,
Hero of
Alexandria,
Archimedes) and
Roman horologists and
astronomers were developing more
elaborate mechanized water clocks. The added complexity was aimed
at regulating the flow and at providing fancier displays of the
passage of time. For example, some water clocks rang
bell and
gongs, while
others opened doors and windows to show figurines of people, or
moved pointers, and dials. Some even displayed
astrological models of the universe. The 3rd
century BC engineer
Philo of
Byzantium referred in his works to water clocks already fitted
with an escapement mechanism, the earliest known of its kind.
The biggest achievement of the invention of clepsydrae during this
time, however, was by Ctesibius with his incorporation of gears and
a dial indicator to automatically show the time as the lengths of
the days changed throughout the year, because of the temporal
timekeeping used during his day.
Also, a
Greek astronomer, Andronicus of
Cyrrhus, supervised the construction of his Horologion, known
today as the Tower of
the Winds
, in the Athens
marketplace
(or Agora) in the first half of the 1st
century BC. This
octagonal clocktower showed scholars and shoppers both
sundials and mechanical hour indicators. It
featured a 24-
hour mechanized clepsydra
and indicators for the eight winds from which the tower got its
name, and it displayed the
seasons of the
year and astrological dates and periods.
Korea

An incomplete scaled-down model of
Jang Yeong-sil's self-striking water clock.
In
Korea
, timekeeping was both a royal duty and a royal
prerogative from its Korean
Three Kingdom Period (c. 37 BC) onwards.
In 1434 during the
Choson (or Joseon
) Dynasty,
Chang Yongsil (or Jang Young Sil),
Palace Guard and later Chief Court Engineer, constructed the
Chagyongnu (self-striking water clock or striking clepsydra) for
King Sejong. What made the
Chagyongnu self-striking (or automatic) was the use of jack-work
mechanisms, by which three wooden figures (jacks) struck objects to
signal the time. This innovation no longer required the reliance of
human workers, known as "rooster men", to constantly replenish it.
By 554,
the water clock spread from Korea to Japan
.
Water clocks were used and improved upon throughout Asia well into
the 15th century.
Islamic and Arabic water clocks
In the
medieval Islamic world
(632-1280), the use of water clocks has its roots from Archimedes
during the rise of Alexandria
in Egypt
and
continues on through Byzantium. The
water clocks by
Al-Jazari, however, are
credited for going "well beyond anything" that had preceded
them.
In al-Jazari's 1206 treatise, he describes one of his water clocks,
the
elephant clock. The clock
recorded the passage of temporal hours, which meant that the rate
of flow had to be changed daily to match the uneven length of days
throughout the year. To accomplish this, the clock had two tanks,
the top tank was connected to the time indicating mechanisms and
the bottom was connected to the
flow control regulator.
Basically, at daybreak the tap was opened and water flowed from the
top tank to the bottom tank via a float regulator that maintained a
constant pressure in the receiving tank.
The most sophisticated water-powered
astronomical clock was
Al-Jazari's
castle
clock, considered to be an early example of a programmable
analog computer, in 1206. It was a
complex device that was about 11 feet high, and had multiple
functions alongside timekeeping. It included a display of the
zodiac and the solar and lunar orbits, and a
pointer in the shape of the crescent moon which traveled across the
top of a gateway, moved by a hidden cart and causing
automatic doors to open, each revealing a
mannequin, every hour. It was possible to re-program the length of
day and night everyday in order to account for the changing lengths
of day and night throughout the year, and it also featured five
robotic musicians who automatically play music when moved by levers
operated by a hidden camshaft attached to a water wheel. Other
components of the castle clock included a main reservoir with a
float, a float chamber and flow regulator, plate and valve trough,
two pulleys, crescent disc displaying the zodiac, and two falcon
automata dropping balls into vases.
The first
water clocks to employ complex segmental and epicyclic gearing was invented earlier by
the Arab engineer Ibn Khalaf al-Muradi in
Islamic
Iberia
circa 1000. His water clocks were
driven by
water wheels, as was also the
case for several Chinese water clocks in the 11th century.
Comparable water clocks were built in
Damascus
and Fez
. The
latter (
Dar al-Magana) remains until
today and its mechanism has been reconstructed. The first European
clock to employ these complex gears was the astronomical clock
created by
Giovanni de Dondi in
circa 1365. Like the Chinese, Arab engineers at the time
also developed an
escapement mechanism
which they employed in some of their water clocks. The escapement
mechanism was in the form of a constant-head system, while heavy
floats were used as weights.
Modern water clock designs

Bernard Gitton's Time-Flow clock
Only a few modern water clocks exist today. In 1979, French
scientist Bernard Gitton began creating his Time-Flow Clocks, which
are a modern-day approach to the historical version.
His unique glass tube
designs can be found in over 30 locations throughout the world,
including one at the The
Children's Museum of Indianapolis
in Indianapolis
, Indiana
, and the Shopping Iguatemi in Porto Alegre
, Brazil
.
There are
other modern designs of water clocks, including the Royal Gorge water clock in Colorado
, the Woodgrove Mall in Nanaimo
, British
Columbia
, and in the Abbotsford Airport
in Abbotsford, British
Columbia
.
Gitton's design relies on gravity powering multiple
siphons; for example, after the water level in the
minute or hour display tubes is reached, an overflow tube starts to
act as a siphon and thus empties the display tube. Actual time
keeping is done by a calibrated pendulum powered by a water stream
piped from the clock's reservoir. The pendulum has a
carefully-constructed container attached to it; this measures the
water that is then poured into display system.
Today,
the use of water flow to power a clock is a rarely-practiced art,
whose purpose is more for show and novelty than for functional
accuracy, an example being the Hornsby Water Clock
in Sydney
, Australia.
Notes
- , pp. 59–61
- "A copper vessel (in the shape of the lower half of the water
jar) which has a small hole in its bottom and being placed upon
clean water in a basin sinks exactly 60 times in a day and at
night." - chapter xiii, 23 of the Suryasiddhanta.
- "A copper vessel weighing 10 palas, 6 angulas in height and
twice as much in breadth at the mouth--this vessel of the capacity
of 60 palas of water and hemispherical in form is called a ghati."
This copper vessel, which was bored with a needle and made of 3 1/8
masas of gold and 4 angulas long, gets filled in one nadika."
- Needham, Joseph (1986). Science and Civilization in China:
Volume 4, Physics and Physical Technology, Part 2, Mechanical
Engineering. Taipei: Caves Books Ltd. Pages 510–511.
- Picture of water clock in Beijing
- This engraving is taken from "Rees's Clocks, Watches, and
Chronometers 1819-20. The design of the illustration was modified
from Claude Perrault's illustrations in his 1684 translation of
Vitruvius's Les Dix Livres d'Architecture (1st century BC), of
which he describes Ctesibius's clepsydra in great length.
- Goodenow, J., Orr, R., & Ross, D. " Mathematical Models of Water Clocks." Rochester
Institute of Technology
- John G. Landels: “Water-Clocks and Time Measurement in
Classical Antiquity”, "Endeavour", Vol. 3, No. 1 (1979), pp. 32-37
(35)
- John G. Landels: “Water-Clocks and Time Measurement in
Classical Antiquity”, "Endeavour", Vol. 3, No. 1 (1979), pp. 32-37
(33)
- Howard R. Turner (1997), Science in Medieval Islam: An
Illustrated Introduction, p. 184. University of Texas Press, ISBN
0292781490.
- Routledge Hill, Donald, "Mechanical
Engineering in the Medieval Near East", Scientific
American, May 1991, pp. 64–69. (cf. Donald Routledge Hill, Mechanical Engineering)
- Ahmad Y Hassan, Transfer Of Islamic Technology To The West, Part
II: Transmission Of Islamic Engineering, History of Science
and Technology in Islam
- Hassan,
Ahmad Y, Transfer Of Islamic Technology To The West, Part
II: Transmission Of Islamic Engineering, History of Science
and Technology in Islam
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- Nam, Moon-Hyon and Jeon San-Woon. “Timekeeping Systems of Early
Choson Dynasty.” Proceedings of First International Conference on
Oriental Astronomy, From Guo Shoujing to King Sejong, Seoul,
October 6-11, 1993, Seoul, Yonsei University Press, 1997.
305-324.
- Needham, Joseph, Major, John S., & Gwei-Djen, Lu. “Hall of
Heavenly Records: Korean Astronomical Instruments and Clocks,
1380-1780.” Cambridge [Cambridgeshire] ; New York : Cambridge
University Press, 1986. ISBN 0-521-30368-0
- Hyeonjong Shillock (Veritable Records of King Hyeonjong),
1669
- Jungjong Shillok (Veritable Records of King Jungjong),
1536.
- Sejong Shillock (Veritable Records of King Sejong), Chapter.
65, A.D. 1434 and Chapter. 80, A.D. 1438.
- Mesopotamian water clocks
- Brown, David R., Fermor, John, & Walker, Christopher B.F.,
“The Water Clock in Mesopotamia.” Archiv für Orientforschung, 46/47
(1999/2000)
- Chadwick, R. “The Origins of Astronomy and Astrology in
Mesopotamia.” Archaeoastronomy. BULL. CTR ARCH. V. 7:1-4, P. 89,
1984. KNUDSEN Bibliographic Code: 1984BuCAr...7...89C
- Fermor, John, “Timing the Sun in Egypt and Mesopotamia.” Vistas
in Astronomy, 41 (1997), 157-167. Elsevier Science. DOI:
10.1016/S0083-6656(96)00069-4.
- Walker, Christopher and Britton, John. “Astronomy and Astrology
in Mesopotamia.” BMP, 1996 (especially pp. 42-67)
- Present-day water clocks
- Gitton, Bernard. “Time, like an everflowing stream.” Trans.
Mlle. Annie Chadeyron. Ed. Anthony Randall. Horological Journal
131.12 (June 1989): 18-20.
- Taylor, Robert. " Taiwan's Biggest Cuckoo Clock?: Recreating an
Astronomical Timepiece". Sinorama Magazine. 3-15-2006
- Xuan, Gao. "Principle Research and Reconstruction Experiment of
the Astronomical Clock Tower in Ancient China." Proceeding of the
11th World Congress in Mechanism and machine Science. August 18-21,
2003. Tianjin, China.
- Other topics on water clocks and related material
- Goodenow, J., Orr, R., & Ross, D. " Mathematical Models of Water Clocks." Rochester
Institute of Technology
- Landels, John G. "Water-Clocks and Time Measurement in
Classical Antiquity." Endeavour 3(1):32-37. 1979. ISSN
0160-9327
- Mills, A.A. “ Newton’s Water Clocks and the Fluid Mechanics of
Clepsydrae.” Notes and Records of the Royal Society of London.
37(1):35-61. 1982. ISSN 0035-9149
- Neugebauer, Otto. The Exact Sciences in Antiquity. Dover
Publications, NY 1969.
- Sarma, S.R., “Setting up the Water Clock for Telling the Time
of Marriage.” in Studies in the History of the Exact Sciences in
Honour of David Pingree, éd. Ch. Burnett, J.P. Hogendijk, K.
Plofker, M. Yano, Leiden-Boston, 2004, pp. 302-330.
- Snell, Daniel. “Life in the Ancient Near East, 3100-332 B.C.E.”
ISBN 0-300-07666-5.
- Non-English resources
- Bilfinger, Gustav, Die babylonische Doppelstunde: Eine
chronologische Untersuchung (Wildt, Stuttgart, 1888).
- Borchardt, Ludwig. 1920. “Die Altägyptische Zeitmessung.” (Old
Egyptian time measurement). Berlin/Leipzig.
- Daressy, G., "Deux clepsydres antiques", BIE, serie 5, 9, 1915,
pages 5-16
- Ginzel, Friedrich Karl,
“Die Wassermessungen der Babylonier und das Sexagesimalsystem”,
Klio: Beiträge zur alten Geschichte, 16 (1920), 234-241.
- Planchon, "L'Heure Par Les Clepsydres." La Nature.
pp.55-59.
- Thureau-Dangin, François, “La clepsydre chez les Babyloniens
[Notes assyriologiques LXIX]”, Revue d’assyriologie et
d’archéologie orientale, 29 (1932), 133-136.
- Thureau-Dangin, François, “Clepsydre babylonienne et clepsydre
égyptienne”, Revue d’assyriologie et d’archéologie orientale, 30
(1933), 51-52.
- Thureau-Dangin, François, “Le clepsydre babylonienne”, Revue
d’assyriologie et d’archéologie orientale, 34 (1937), 144.
See also
External links