Water clocks are some of the oldest devices on the planet used for measuring the passage of time. Earliest records have them popping up in the 16th-century B.C. in both ancient Egypt and Babylonia. These devices showed up in India and China in ancient times as well, with some researchers even suggesting these clocks appeared in China 6,000 years ago.
Early versions of these clocks didn’t use gears, valves, or metal, but instead were made using simple bowls and pots. All water clocks work on either an inflow or outflow model. Inflow clocks receive a constant, steady flow of water. As they fill up, various markings indicate the time. Outflow clocks work just the opposite. As water flows out, markings indicate the time.
By the time the Greeks and Romans set about making their own water clocks, they learned to incorporate gears and other mechanics. This allowed them to make their own versions of water-powered cuckoo clocks, using their own fanciful automation that triggered alarms and the movement of dolls. These water clocks were referred to as clepsydrae—meaning water thieves. These engineers realized that as a vessel reached the end of its reserves, the flow would be reduced—with less water pushing down.
Clocks were important at this time for allotting speaking time to politicians and lawyers. For important cases a clepsydra would be filled to the brim, but, for trivial matters, the clock would be set more modestly. As time went on, the clepsydra became more common, even serving as a timer for brothels.
Before the common era, scientists would find uses for water clocks in measuring heart rate and making astronomical models.
Mechanical clocks using weights and then springs eventually took over in the ensuing millennium, but water clocks remained a popular curiosity into the 19th and early 20th centuries.