In the long history of time-measurement tools, the clepsydra represents a fundamental milestone: for the first time, a system allowed for the calculation of duration independently of external events, unlike the sundial and the gnomon. Over time, from the Egyptians to the most prestigious scholars of Arabia and China, the water clock was perfected to become an increasingly precise and majestic instrument.
Although older Babylonian tablets already mention the water clock, its first archaeological manifestation dates back to around the 15th century BC: a unique specimen was discovered in Karnak, in the Temple of Amun, at the beginning of the 20th century. This represents the first known reference to a time-measurement system of this kind, based on the constant flow of a liquid into a conical vessel adorned with graduations, featuring an orifice for the water to drain. This water clock is still on display at the Cairo Museum.
However, it was the Greeks, a millennium later, who gave the clepsydra its true distinction—and even its name, originating from the Greek “klepsydra,” which later became “clepsydra” in the Latin world. The word is formed from “kleptein” (“to steal”) and “hudor” (“water”), literally meaning “water thief.” Nevertheless, the use of the water clock is also attested among the Gauls, the Chinese, and, on the other side of the Atlantic, among Native Americans.
The significance of the clepsydra lies in its ability to evaluate durations independently of external elements such as the sun, which is necessary for the operation of sundials and gnomons. Capable of functioning in all circumstances, including at night and indoors, the water clock allows for the measurement of brief durations (between 5 and 10 minutes for primitive models) with satisfactory precision. In Greece, it was used to limit speaking time during political speeches and judicial pleadings. In the Roman Empire, it served to calculate the night watch rotations of soldiers.
During the Middle Ages, Arab and Chinese scholars drew inspiration from Greek studies to further perfect the mechanism of the water clock. Some of their clepsydras have remained famous, notably the one presented to Charlemagne by the Caliph of Baghdad, Harun al-Rashid, in 807, which was adorned with automata that came to life as the water rose in the vessel. Or the great water clock, standing some ten meters high, designed by Su Sung for the Emperor of China in the 11th century—the clock tower was topped with a sphere displaying the position of the stars, which, along with dials indicating the hour, day, and month, made it the most complex water clock ever invented.
In its most primitive form, the clepsydra—based on the same principle as an hourglass—consists only of a single bowl with a hole at its lowest point, through which the water drains. The elapsed duration is measured by means of graduations engraved inside the container.
Later, it was perfected with two superimposed vessels, slightly offset, with the top one featuring a hole that allows water to flow into the one below. Inside the latter, roughly equidistant graduations allow for the measurement of time intervals.
These water clock models use conical-shaped vessels. Why not cylindrical? Because it has been observed that at a constant flow rate, the conical shape is the only one that compensates for the loss of water ejection speed due to the combined effect of the falling liquid level and the related reduction in pressure. The cone provides a measurement of duration that is approximately equal from the bottom of the vessel to its rim.
The Greek inventor Ctesibius evolved the traditional two-vessel clepsydra model into a more complex instrument, in which the flow of water becomes an energy source that enables the movement of gears and time indicators. This modernized water clock, which can henceforth be described as a true hydraulic clock (it operates as a closed system and this time truly measures time, not just duration), was adopted by the Romans. Here is how it works:
This clepsydra comprises two vessels: one positioned higher up and another below containing a float, a notched rod immersed in the lower vessel, and a gear system that allows a hand to rotate around a dial. Water is poured continuously into the upper vessel, while the surplus is drained through an overflow pipe. Water flows toward the lower vessel at a constant rate because the upper vessel is always full. As it fills the lower vessel, the water causes the float to rise, which in turn pushes the notched rod upward. This rod rotates the toothed wheel mechanism, which indicates the hour on the dial by means of a hand.
In fact, it was this model of water clock that definitively moved this instrument into the realm of modern timepieces. The system would be perfected by the Arabs and the Chinese, without undergoing any major revolution during the following centuries.
If building a clepsydra is an experiment that interests you, take a look at this page!
Between the sundial and the hourglass: the water clock
Operating principle of a clepsydra
The clepsydra of Ctesibius: a step toward modernity