A clock that works at night
Sundials depend on sunlight. A priest wanting to schedule a night ritual, a soldier checking the guard watch or a speaker facing a time limit needed something else. The simplest answer was water. If water flows out of a vessel through a small hole at a steady rate, the amount that has left is a measure of the time that has passed. The Greeks called such a device a clepsydra, meaning roughly “water thief,” and the name is now used for water clocks in general.
Outflow clocks
The basic outflow clock is a jar with a small hole near the bottom. It is filled and left to drain, and marks on the inside indicate hours as the level falls. One of the oldest surviving examples comes from the Egyptian temple complex at Karnak and is usually dated to the reign of Amenhotep III in the fourteenth century BCE. Its sides slope outward, and inner marks are spaced for different months. Egyptian texts also mention water clocks from earlier periods, and their use at night is regularly given as the main purpose.
The sloping shape is a clever detail. Water drains faster when the vessel is full, because the pressure at the hole is greater. A vessel that is wider at the top than at the bottom compensates for this by making the level fall at a more even rate, though the match is only approximate. Marks for different seasons dealt with another problem, discussed below.
Inflow clocks
A different design reverses the process. Water drips at a steady rate into a vessel, and a float rises with the level, carrying a pointer along a scale. An inflow clock can be read at any moment without waiting for the vessel to empty. It also invites a more elaborate construction, with a float connected to a figure, a rod or a set of gears.
In Hellenistic Alexandria, the engineer Ctesibius, in the third century BCE, is credited with improvements to such clocks. The main trick was keeping the water level in the supply tank constant by means of an overflow, so the flow into the measuring vessel stayed steady. His designs, preserved through later writers, also included moving figures and sound effects. Details are uncertain because we rely on descriptions by Vitruvius and others, and not on surviving machines.
Courts, armies and monasteries
The best-known use in the Greek world was in the courts of Athens. Speakers in a trial were given a set amount of water; when the vessel ran out, their time was up. A standard measure of water was kept to limit the speeches. Romans used similar devices in the law courts and the army, where a water clock helped to divide the night into watches. Roman writers mention that the public water clock was sometimes not trusted, since the water could be adjusted by someone with an interest in the result.
Other uses included regulating shifts, timing religious rites and allotting irrigation water. In parts of Iran, for instance, a bowl with a hole in it floated in a larger vessel, sank after a fixed time, and was used to share the water from underground channels among farmers. This is a good example of timekeeping as a social tool, not only a scientific one.
Seasonal hours and the need to adjust
Ancient people often divided daytime and night-time into twelve parts each, so the length of an hour changed with the season. A water clock that drips at a constant rate measures equal intervals, so it must be adjusted. Some designs had different scales for different months, and others changed the flow or the water level by hand. These adjustments show how much tinkering a “simple” water clock required.
Chinese and Islamic water clocks
In China, water clocks were used for centuries to regulate the schedules of the court and the army, and large versions were developed with astronomical features. The best-known is the clock tower built by Su Song in Kaifeng in the late eleventh century. Water filled the scoops of a large wheel, which turned in controlled steps through a mechanism that restrained it, and the movement drove an armillary sphere and a celestial globe along with figures that announced the hours. A description in a book by Su Song survives, and scholars have built reconstructions. Today it is often described as a forerunner of mechanical clocks, though connecting it to the later European verge escapement is a matter of debate.
Islamic engineers also built elaborate water-driven devices. Ibn al-Razzaz al-Jazari, writing in 1206, described several, including a large castle clock with moving figures and a system that controlled the flow of water to keep the schedule. The mechanical ingenuity of these devices connects to the tradition of automata that blended entertainment with engineering. Building them also relied on practical knowledge from aqueduct engineering and the handling of pipes and valves.
Limits of water
A water clock has many weaknesses. Flow varies with temperature, since cold water is thicker, and mineral deposits can clog the hole. The vessel needs refilling and sometimes cleaning. In cold climates the water could freeze, and in dry climates it evaporated. Accuracy is hard to judge, and even good water clocks probably varied by many minutes a day. They still did the job for which they were built, and the gap they filled is clear: they worked when the Sun did not.
Fading out
Water clocks never disappeared overnight. Even after mechanical clocks spread in medieval Europe, water clocks remained in use in some places for many years. Eventually, weights and gears offered a regulator that did not leak, freeze or need continuous refilling. The older technology left its mark, however: many ideas about gear trains, floats and feedback in later clocks first appeared in water-driven devices, and the same people often worked on both.