'Sinking bowl' water clock, formed of a copper bowl, in the base of which is a miniscule hole bushed with gold. Sri Lanka, unknown date. Clockmakers' Museum No. 646

The water clock, or clepsydra, is one of the earliest forms of timekeepers and allows the passage of time to be measured by regulating and measuring the flow of water into or out of a vessel. In use this 'sinking' bowl is placed on the surface of water. Through the hole in the bottom which slowly allows water into it, the bowl gradually sinks until it is fully submerged after a period of approximately 24 minutes.