The clepsydra and the final goal of the clockmakers’ art

In Ancient Greece, a water clock was known as “water thief“, clepsydra in Greek (κλέπτειν kleptein, ‘to steal’; ὕδωρ hudor, ‘water’).

It is likely to be the oldest time-measuring instrument, with the only exceptions being the vertical gnomon and the day-counting tally stick. Where and when it was first invented is not known, and given its 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.

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 water clocks were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom to mark the hours. There were twelve separate columns with consistently spaced markings on the inside to measure the passage of “hours” as the water level reached them. It took about an hour for the water level to sink to each level. 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.

A full-size reconstruction may be seen in the New Walk Museum, and illustrates how it could act as a timekeeper independent of the Sun. The vessel is filled with water to a mark near the rim, and then allowed to empty via a narrow jet near the base. With a cylindrical container the rate of flow diminishes as the head of water within the pot decreases, so the water surface drops more slowly with time. The ancient Egyptian designer (Amenhemhet, about 1550 B.C.) has cleverly compensated for this by employing a conical vessel, which chosen angle gave rise to an optimized approximation to a constant descent rate of the water surface.

In Babylon, water clocks 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 measured time “by the weight of water flowing from” it. The volume was measured in capacity units called qaMana (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.”

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.

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.[17] 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).[19]

In both Greek and Roman times, a commonly used water clock was the simple outflow clepsydra. This small earthenware vessel had a hole in its side near the base.

This type of clepsydra was used in courts, in the manner of a stop-clock, to allocate fixed periods of time to speakers: the greater the crime, the longer the period an advocate could argue his client’s case. For fairly minor offences, the vessel was partially filled with water to last about a 20 minute period of time. In important cases, for instance, when a person’s life was at stake, the vessel was fully filled. These devices gave rise to the expression “Your time has run out”. 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.

There is an interesting legend presumably about a water clock, involving the Indian mathematician and astrologer Bhaskaracharya (1114-1185) and his daughter Lilavati, after whom was named his book on arithmetic. According to a Persian translation, when Lilavati became of marriageable age, Bhaskaracharya cast her horoscope to determine the most propitious day and hour for her wedding to take place. The signs told him that if she was not married at a particular place at a particular time, the bridegroom would die shortly after the wedding. To prevent this, Bhaskaracharya made a small hole in the bottom of a cup which he then put into a jug of water. He had calculated that the cup would sink to the bottom at the appropriate hour for the wedding. Though he warned his daughter not to disturb this arrangement, Lilavati’s curiosity led her to lean over the device, and in doing so, a pearl fell off her garments and blocked the hole in the cup. The cup never sank and poor Lilavati was not able to be married. Her father wrote for her a Mathematics manual, which was supposed to console her and to keep her occupied as she studied its contents.

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 first water clocks to employ complex segmental and epicyclic gearing was invented by the Arab engineer Ibn Khalaf al-Muradi in Islamic Iberia circa 1000. His water clocks were driven by water wheels.

The water clocks by Al-Jazari (including an elephant-clock), however, are credited for going “well beyond anything” that had preceded them. The most sophisticated water-powered astronomical clock was Al-Jazari’s castle clock, considered by some to be an early example of a programmable analog computer, in 1206. It was a complex device that was about 11 feet (3.4 m) 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 musician automata 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 final goal of the clockmakers’ art was to construct a clock showing the relative movements of the Sun, Moon and stars as accurately as possible. The underlying problem is that Sun (civil) time, lunar (tidal) time and star (sidereal) time are not integrally related to one another, and any small discrepancy in ratio soon builds up into an obvious error. The Moon rises on average some 50 minutes later each day, whilst the starry background appears to complete one turn in about 23 hours 56 minutes.

Adapted from various sources


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