Tickipedia: 500BC-0AD

Time, one of the seven fundamental physical quantities in the International System of Units, is a dimension in which we all exist. It marches on as certain as the sun rises and sets, and its behaviour has fascinated scientists as far back as history recalls. In this edition of Tickipedia, we investigate the evolution of water clocks, the most advanced timekeeping devices in the world for over two millennia.

A later version of the water clock shows how it evolved, this model usable as a cyclic hour glass

There were three main kinds of water clock in the ancient world, in progressing order of complexity. The first, invented by the Egyptians, was the outflow water clock, from which you could measure time by observing how much water had flowed out of a large container. In ancient Greece, these simple devices were called clepsydra (‘water thief’) and were commonly used as timers at public events, for example allocating speaking lengths to lawyers during a trial. When the clepsydra was empty, your time was up.

The second type of water clock, the inflow clock, was also used by the Greeks. It had two containers: one, a reservoir, that emptied into the other, a basin, the fullness of which showed the time. Plato used such a mechanism as an alarm clock in the 5th century BC; when the water reached a certain level, a lid with metal balls would overflow, clanging onto a copper plate and waking the students at his academy.

The Persians used a different form of inflow clock from at least 500BC. This employed an empty bowl with a hole at the bottom that was placed into a larger container of water. The bowl would slowly fill, and the time taken until it sank was considered one ‘unit’. This method was used to fairly distribute a common water supply to different farmers, at a time when agriculture was highly dependent on irrigation. The sinking bowl proved to be so practical and reliable that it was used for timekeeping in the Middle East until the mid-twentieth century.

But the problem with both inflow and outflow clocks was that they were inconsistent. The flow rate of water depends on the quantity in the container, and it decreases as the container empties. This problem was solved by the mysterious genius Ctesibius in the third century BC. Though none of his writings have survived, he was considered the father of pneumatics by his contemporaries.

Ctesibius's solution was the inflow water clock. It introduced a third container between the reservoir and the measurement basin, an overflow tank; water could enter it from the reservoir at a faster rate than it left, meaning it was always full and overflowing. Thus water would leave it at a constant rate, and the resulting measurements would remain steady and accurate. Ctesibius soon improved the design again, creating a buoyant stoppage device for the overflow tank to prevent inflow from the reservoir when the tank was full—a perfect feedback control system that wasted no water.

The three-container model was gradually improved by Ctesibius and others, and soon included water-powered escapements and gear trains. Its accuracy was not beaten for 1,800 years until the invention of the pendulum clock in 1656. The Greeks and Romans used it to create a range of impressive automata; one design featured a sculpture owl that would flap its wings on the hour and make a hooting sound using a water organ (another of Ctesibius' inventions).