In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.
A scale model of Su Song's Astronomical Clock Tower, built in 11th century Kaifeng, China
It was driven by a large waterwheel, chain drive, and escapement mechanism.
Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick
Given their great antiquity, where and when they first existed are not known and perhaps unknowable
The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC
Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain
Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.
Greek astronomer, Andronicus of Cyrrhus, supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.
The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and also resulted in improved accuracy
These advances were passed on through Byzantium and Islamic times, eventually making their way to Europe
Independently, the Chinese developed their own advanced water clocks（钟）in 725 A.D., passing their ideas on to Korea and Japan.
Automatic clock of al-Jazari, 12th century.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade
Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, where every hour of work or rest is monitored, and work may start or finish at any time regardless of external conditions
Instead, water clocks in ancient societies were used mainly for astrological reasons
These early water clocks were calibrated with a sundial
While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe.
In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water clock.
An elephant clock in a manuscript by Al-Jazari (1206 AD) from The Book of Knowledge of Ingenious Mechanical Devices.