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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.

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. Some modern timepieces are called "water clocks" but work differently from the ancient ones. As the container fills, the observer can see where the water meets 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 water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed. This container has markings that are used to show the passage of time. In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container.

There are two types of water clocks: inflow and outflow. If viscosity is neglected, the physical principle required to study such clocks is Torricelli's law.

3 Temperature, water viscosity, and clock accuracyĪ water clock uses the flow of water to measure time.

2.6.1 Klepsydra springhouse of the Athenian acropolis.

Water clocks were also used in ancient Greece and ancient Rome, described by technical writers such as Ctesibius and Vitruvius. Some authors, however, claim that water clocks appeared in China as early as 4000 BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. 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. Water clocks are one of the oldest time-measuring instruments. 'water thief') is any 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. The bottom is a reconstruction of a clay original.Ī water clock or clepsydra (from Ancient Greek κλεψύδρα ( klepsúdra) ' pipette, water clock' from κλέπτω ( kléptō) 'to steal', and ὕδωρ ( hydor) 'water' lit. The top is an original from the late 5th century BC. A display of two outflow water clocks from the Ancient Agora Museum in Athens.