A water clock is a timekeeping device that uses a flow of water to measure time. It is not as accurate as most modern time-keeping devices, but when these clocks were first developed, they were quite adequate for the times. It is believed that they may be among the oldest of devices used to keep time, since written accounts of them date to around 4000 BCE, with physical examples from Egypt dating to 1500 BCE.
There are a number of ways in which a water clock can work, but generally, such clocks are classified as either inflow, or outflow. An outflow clock keeps time by allowing water to slowly drain away; an example from antiquity was a bowl marked with lines which had a small hole in the bottom. The bowl was filled and allowed to drain slowly, and the lines were used to keep track of time. In the case of an inflow clock, the flow of water into a container such as a cylinder is used to gauge the passage of time.
Water clocks may have originally been developed for the purpose of making astrological observations, when the passage of time can become an important factor. They were also apparently used to measure the passage of time during speeches and other events. The origins of this technology are not known, although Egypt appears to have some of the oldest examples; they were also used in Asia, the Middle East, and Greece, where the clock was known as a clepsydra, or “water stealer.”
A clock's accuracy could be fine tuned by pressurizing the water, or using various tools to alter the rate of flow. Some became quite sophisticated, turning into elaborate clock towers. As other timekeeping devices arose, the technology began to be displaced, and water clocks today are largely a curiosity, rather than a usable time piece, thanks to the fact that more precise methods of measuring time have been developed.
The need for more accurate time pieces began to emerge during the age of exploration, when mariners urgently needed accurate clocks so that they could measure longitude. During this era, more precise and reliable clocks began to be developed, and with the rise of the Industrial Revolution, when the need to keep accurate time became even more important, even better clocks were developed. By the 19th century, the water clock had been largely phased out, although they continued to be used in some remote areas where the rhythms of people's days did not need to be so precisely regulated.