Rōkoku

A water clock or clepsydra (; ; ) is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount is then measured.

Water clocks are one of the oldest time-measuring instruments. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon, Egypt, and Persia 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, claim that water clocks appeared in China as early as 4000 BC. Water clocks were also used in ancient Greece and ancient Rome, described by technical writers such as Ctesibius and Vitruvius.

Designs

A water clock uses the flow of water to measure time. If viscosity is neglected, the physical principle required to study such clocks is Torricelli's law. There are two types of water clocks: inflow and outflow. In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container. This container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed. An inflow dasher water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines and tell how much time has passed.
Some modern timepieces are called "water clocks" but work differently from the ancient ones. Their timekeeping is governed by a pendulum, but they use water for other purposes, such as providing the power needed to drive the clock by using a water wheel or something similar, or by having water in their displays. A water clock is one of the oldest instruments ever, and one of the most important.

The Greeks and Romans advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy. Further advances were made in Byzantium, Syria, and Mesopotamia, where increasingly accurate water clocks incorporated 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 17th-century Europe.

Regional development

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.

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 – ''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'' or mina (the Greek unit for about one pound), is the weight of water in a water clock.

In 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 mana had to be added each succeeding half-month. At the equinox, three mana had to be emptied in order to correspond to one watch, and four mana was emptied for each watch of the winter solstitial night."

India

According to N. Kameswara Rao, pots excavated from the Indus Valley Pakistan site of Mohenjo-daro (around 2500 BC) may 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 ''abhiṣeka'' (ritual water pouring) on lingams. 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.

The Jyotisha school, one of the six Vedanga disciplines, describes water clocks called ''ghati'' or ''kapala'' that measure time in units of ''nadika'' (around 24 minutes). A clepsydra in the form of a floating and sinking copper vessel is mentioned in the ''Sürya Siddhānta'' (5th century AD). At Nalanda

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