Water Clocks and the Clepsydra

Before gears and springs, people measured time by letting water drip, and the best of these clocks were astonishing machines.

Time that works at night

A sundial is useless after dark or under clouds. A water clock solves that problem by measuring time with a steady flow of water instead of sunlight. Historians at the University of St Andrews suggest that is probably why water clocks were developed: they worked without sun and could divide time into steady, repeatable intervals.

There are two basic kinds. In an outflow clock, water drips out of a marked container, and the falling level shows how much time has passed. In an inflow clock, water drips into a container, and the rising level, or a float riding on it, shows the time.

The Greeks called these devices clepsydras, meaning water thieves, and the name stuck. Water clocks were used for thousands of years, and NIST notes that the simplest kind was still in use in North Africa in the 20th century.

Egypt: the first water clocks

According to NIST, one of the oldest known water clocks was found in the tomb of the Egyptian pharaoh Amenhotep I, who was buried around 1500 BCE.

Scientific American describes a water clock from the Temple of Amun at Karnak dating to about 1400 BCE. Its scales divided the night into 12 parts. Egyptian hours were not fixed lengths: they stretched and shrank with the seasons, so a clock like this had to be read differently through the year.

Early designs included stone vessels with sloping sides that let water drip out at a nearly constant rate, and containers that slowly filled, with hour markings on the inside.

Greece: timing speeches in court

One of the water clock's most familiar jobs in ancient Greece was keeping courtroom speeches in check. The University of St Andrews' history of water clocks notes that judicial water clocks were in common use in the law courts of Athens by around 430 BCE, and writers including Aristotle, Aristophanes and Demosthenes mention them.

The amount of water poured in depended on the type of speech or trial, so a more serious case got a bigger allowance of time. NIST gives about 325 BCE as the date by which the Greeks were using water clocks more generally.

Another simple design was a metal bowl with a small hole in it, placed in a basin of water. It filled slowly and sank after a set interval, at which point an attendant called time and reset it. Bowls like these are the ones NIST says were still in use in North Africa in the 20th century.

See also: a free online stopwatch for timing your own speeches on OnlineClock.app.

Ctesibius and the self-regulating clock

The biggest leap came from Ctesibius of Alexandria, an inventor of the 3rd century BCE. None of his writings survive, but the Roman architect Vitruvius described his clocks, and St Andrews' historians draw on that account.

His key improvement was a float that rose steadily as water flowed in, carrying a pointer that marked the hours. To keep the flow constant, a large reservoir was held at a fixed level by an overflow pipe, so the water pressure stayed the same. The water entered through a small hole drilled in gold or a gemstone, materials chosen because they would not wear away or corrode and clog.

These clocks could even handle seasonal hours. Vitruvius describes adjustable wedges that changed the flow for winter and summer, and one design used a disk pierced with 365 holes of different sizes, turned one hole per day, with the largest hole for the winter solstice and the smallest for the summer solstice.

In another version attributed to Ctesibius, a rising float lifted a small figure whose pointer showed the time on a column marked with seasonal hours. When the tank filled at the end of the day, a siphon emptied it onto a small waterwheel, and gearing used that turn to rotate the column, ready for the next day's hour lengths.

Toothed bars and drums could also turn the float's movement into entertainment, driving moving statues, sounding trumpets and setting off other effects.

Mechanical water clocks

According to NIST, Greek and Roman engineers built more elaborate water clocks between about 100 BCE and 500 CE, some ringing bells or moving pointers and figures. In Athens, the Tower of the Winds held a 24-hour mechanized clepsydra in the first century BCE. Much of its stone water system survives in the Roman Agora, and St Andrews notes that its overflow fed the tower's fountains.

In China, mechanized astronomical clock-making developed between about 200 and 1300 CE. NIST notes that Chinese water clocks of the third century were already driving mechanisms that showed astronomical events.

Water had limits, though. Its flow is hard to control precisely, and NIST points out that water clocks could never reach excellent accuracy, which eventually pushed clockmakers toward other methods.

Su Song's astronomical clock tower

The most spectacular water clock of all was built in 1088 under the direction of the Chinese scholar and official Su Song, who lived from 1020 to 1101. Britannica describes it as housed in a tower about 11 m tall and driven by a waterwheel and a chain drive, and says Su Song built it as the basis for reforming the calendar.

The tower worked in three layers. According to the National Museum of Natural Science in Taiwan, an armillary sphere for observing the sky sat at the top, a rotating celestial globe in the middle, and the timekeeping and power machinery at the bottom. There, figures on a tiered pagoda struck a bell, gong or drum at set intervals and held up signs showing the hour, the quarter hour or the watch of the night. The museum credits Han Gonglian as Su Song's partner in designing and building it.

The heart of the machine was its waterwheel, which the Science Museum in London describes as acting as an escapement: it held back the power and released it in small, regular steps, the same job a mechanical escapement does in a European clock. When scholars rediscovered Su Song's written description in the 1960s, the museum notes, it challenged the belief that mechanical clocks began entirely in Europe.

Su Song's book, known as Xin Yixiang Fayao, or New Design for an Armillary Sphere and Celestial Globe, made modern reconstructions possible. The museum in Taichung, Taiwan, built what it calls the first true-to-scale working model of the tower, and the Science Museum in London holds a one-sixth scale model of its escapement, made in 1965.

See also: the wider story of how timekeeping developed on OnlineClock.app.

Keep exploring

Sources

1. A Walk Through Time: Early Clocks, NIST
2. How did we come to divide the day into hours, minutes and seconds?, Scientific American
3. Timekeeping in the Ancient World: Water-clocks, MacTutor, University of St Andrews
4. Su Song, Britannica
5. Water-Powered Astronomical Clock Tower, National Museum of Natural Science, Taiwan
6. Scale model of Su Song's water-balance escapement, Science Museum Group Collection