Water clock

A water clock tells time using liquid.

One kind lets water drip out of a jar. The jar has lines on it. You see the water level go down the lines. This shows how much time passed.

Another kind lets water fill a jar. As the jar fills up, the water hits the lines. This also shows the time.

People used these very long ago. In Egypt, priests used them at night. In China, they used them to study the stars.

Some clocks even used a water wheel. This helped them move parts. These clocks were used for many years.

A water clock is a very old tool for measuring time.

It works by using the flow of liquid. There are two main ways these clocks work.

One way is called an outflow clock. A jar is filled with water. The water drips out of a small hole. The jar has lines on the inside. An observer watches the water level go down the lines. This shows how much time has passed.

The other way is an inflow clock. Water flows into a marked jar. As the jar fills up, the water reaches the lines. This also tells the time.

Different people used these in many places. In ancient Egypt, priests used them to time night rites. In China, people used them to study the stars. Some Chinese clocks were very big. They used a waterwheel to move parts. One famous clock tower used a chain drive. It even had figures that rang bells to show the hour.

In Persia, farmers used water clocks. They needed to know how long to take water from wells for their crops. These clocks helped keep things fair for everyone.

A water clock is an ancient tool used to measure time.

These devices are also called clepsydras. They work by using the steady flow of liquid. There are two main ways these clocks work. In an outflow clock, water drains slowly from a container. The container has markings on the inside. An observer watches the water level drop past these lines. In an inflow clock, water fills a marked container instead. As the liquid rises, it meets the lines to show time.

Different cultures built these clocks in many ways. In Egypt, stone vessels had sloping sides. A small hole at the bottom let water drip out at a constant rate. These clocks had twelve columns of markings for the months. Priests used them to time temple rites at night. In Babylon, people used cylindrical outflow clocks. They measured time by the weight of the water. They used units called mana to track the flow.

Many clever inventors improved how these clocks worked. Ctesibius and Vitruvius wrote about designs in Greece and Rome. In China, the philosopher Huan Tan noted how temperature changed the flow. To stop water from freezing, Zhang Sixun used mercury in 976. Mercury stays liquid even in very cold air. Another engineer, Zhang Heng, added a special tank to keep the pressure steady. This helped the water flow more evenly into the clock.

Some water clocks became very large and complex. In 1088, Su Song built a huge astronomical clock tower. It used a waterwheel and a chain drive to move parts. The tower had a bronze sphere for looking at stars. It even had mannequins that rang bells or gongs. In India, students at Nalanda mahavihara used copper bowls. These bowls floated and sank in a larger basin of water. Students operated these clocks to measure four-hour intervals.

Water clocks were useful for many everyday jobs. In Persia, farmers used them to manage water from wells. This helped them know how long to irrigate their gardens. It kept the sharing of water fair for everyone. These clocks were used for thousands of years. They were eventually replaced by mechanical clocks around 1300. Even today, some parts of these old ideas live on.

You can still see an outflow clock working in Beijing's Drum Tower.

A water clock, also known as a clepsydra, is a device used to measure time through the regulated flow of liquid.

These instruments are among the oldest timekeeping tools in human history. They function by measuring the volume of liquid that moves into or out of a vessel over a set period. This method allows observers to track the passage of time by watching the liquid level relative to specific markings. Because they rely on the physical properties of fluids, they were essential for many ancient civilizations. They provided a way to track hours, seasons, and even astronomical events long before modern electronics existed.

There are two primary mechanisms used in these devices: inflow and outflow. In an outflow clepsydra, a container is filled with liquid and allowed to drain slowly through a small opening.

As the water leaves the container, the level drops past calibrated lines marked on the vessel's interior. An observer can determine how much time has passed by seeing which line the water level reaches. Conversely, an inflow clepsydra works by filling a marked container. In this version, the liquid rises as it enters, and the observer tracks the time by watching the water meet the rising markings. The physical principle governing this flow, if viscosity is neglected, is known as Torricelli's law.

Different cultures developed unique variations of these clocks to suit their specific needs. In ancient Egypt, outflow clocks were often made of stone with sloping sides. These vessels featured twelve separate columns of markings to account for the twelve months and varying seasonal hours.

In Babylon, cylindrical outflow clocks were used to measure time by the weight of the water rather than just its volume. They used capacity units called qa and weight units called mana, or mina, to track the flow. In ancient India, the Sürya Siddhānta describes a copper vessel that sinks in a basin of water. At the Nalanda mahavihara university, students used a copper bowl that floated and sank in a larger basin to measure four-hour intervals.

Technological advancements in the Greeks and Romans led to much more complex designs. Engineers like Ctesibius and Vitruvius documented improvements that included early feedback systems, gearing, and escapement mechanisms. These additions allowed water clocks to drive fanciful automata, which are moving mechanical figures. In China, the technology reached incredible levels of complexity. The Han dynasty philosopher Huan Tan observed that temperature and humidity could affect accuracy by changing the speed of the flow. To solve the problem of freezing, the engineer Zhang Sixun used mercury instead of water in 976. Because mercury remains liquid at much lower temperatures than water, the clock could function in cold environments.

One of the most impressive examples of engineering was the astronomical clock tower built by Su Song in 1088. This massive structure used a waterwheel and an escapement mechanism to power a bronze armillary sphere for celestial observations. It even featured a chain drive and rotating celestial globes. The tower included front panels with doors that revealed mannequins. These figures would ring bells or gongs to indicate the hour. This level of automation showed how water power could be used to drive intricate mechanical systems.

Water clocks were not just for scholars; they were vital for daily survival and fairness. In ancient Persia, water clocks called pengan or fenjan were used in arid desert regions.

Farmers relied on them to manage the distribution of water from qanats, which are underground channels, to their fields. Because water was a scarce resource, it was essential to know exactly how long a farmer could divert water for irrigation. To ensure justice, a manager known as a mir āb was elected to oversee the water clock. This prevented disputes and ensured that every shareholder received their fair share of the water supply.

While these devices were eventually replaced by mechanical verge escapement clocks around 1300, their legacy remains. They represent a long history of humans trying to understand and master the movement of the natural world. From the stone vessels of Egypt to the complex gears of the Song dynasty, the water clock was a bridge between simple observation and advanced engineering. They allowed humanity to organize society, conduct religious rites, and study the stars with increasing precision.