Also known as a water clock. An instrument that measures time by the flow of a quantity of a fluid (water or mercury).
The Egyptians and the Chinese used clepsydras in the fifteenth century BC. Clepsydras are also still used in the XVIIIth century. Improvements were made to the system by incorporating a cylindrical float chamber that moved a hand around a dial, marked into hours. Famous clepsydras include the one, now vanished, by Ctesibius, a copy of which can be seen at the Munich Science Museum, and the one which Haroun al-Raschid gave to Charlemagne in 809.