A water clock (or clepsydra) is an instrument where time is measured by regulating a flow of liquid. The oldest water clocks simply consisted of a pierced bowl placed in a larger pot filled with water (see left image, Persian artefact from 400 BC). These existed back in the 16th century BC. The passage of time was observed by counting how many times the bowl overflowed and its content had to be poured back in the larger pot. This was used for example in Persia to ensure fair irrigation practices.
This basic system has been improved upon many times across different continents, yielding more precise measurements and eliminating the need for manually resetting the mechanism. The second and third image show the largest water clock ever made, built by French physicist-turned-artist Bernard Gitton in 1988. It is more than 9 meters tall and shows the current hour and minute.
- Wikipedia article on water clocks.
- Holmes (2016) Object of Intrigue: Ancient Persian Water Clocks.
- English (2012) Inspiration: The Water Clocks of Bernard Gitton.
- Wikipedia article Water clock (Indianapolis).
- Left image from Wikimedia commons, middle image by youtube user Michael Shen, right image by youtube user Ryan Schade.