Water Clocks

Clepsydra

Water clocks, along with sand clocks, were one of the earliest forms of clock that didn't rely on the sun or other astronomical phenomena. As such they could be used at night and/or indoors. The Greeks called them "clepsydra", a name which means "water thief". It's thought that the actual use of these clocks was not normally to track "time of day" but to measure duration.

The earliest water clocks - which date back to at least 1500 BC - were very simple and basically consisted of a bowl with a hole! There were three main ways of using this to tell time:

A bowl with a small hole in the base would be filled with water. As the water dripped out, the level would fall and the elapsed time could be read off according to markings on the bowl.
One water container would drip into a marked bowl which could be read accordingly.
A bowl with a hole was placed in a water container and as it gradually sank it would fill and the elapsed time could be read according to the usual markings.

Mechanical Water Clocks

As technology advanced, water clocks became more elaborate. The principle remained the same - something emptying, something filling, or something sinking. However a variety of ingenious mechanical mechanisms were created for taking this motive force and making use of it. Other innovations included more sophisticated control of water flow than a drip.

These mechanical water clocks would display the time in a way closer to the form we know today, often by means of dials or pointers. Some of them also generated motion in the form of moving figures and ringing bells.

Towers

Mechanical water mechanisms were used as the basis of some public clocks. One of the most famous was the Horologion of Andronikos of Kyrrhos, also known as the Tower of the Winds. The Tower of the Winds was built in the Athens marketplace, probably in the 1st century BC. It is an octagonal structure with the faces illustrating the winds along with sundials, mechanical time indicators and assorted astrological data.

Another famous water driven clock tower was Su Sung's Tower, also known as the "Cosmic Engine". The Chinese emperor employed Su Sung to build this famous astronomical clock tower. This was completed in 1094 AD and was a 30ft high tower with a 10ft high observation platform atop. The water powered mechanism showed the time, controlled mannequins ringing bells and kept the observational equipment correctly aligned with the heavens.

You can still buy water clocks today, though they are mainly sold as novelty products. They are also the basis of many a school science project.