Clepsydra (Gr. from to steal, and water), a hydraulic clock in use among the ancients, which measured time by the quantity of water that escaped from a small orifice in a reservoir. The simplest kind consisted of a transparent vase, filled with water, graduated, and having a small opening in its bottom. As the liquid gradually escaped, its height in the vase marked the hour. Clepsydras were in use in Egypt under the Ptolemies, and a great improvement in them was made by Ctesibius, a mathematician of Alex-andria, about 235 B. C. In his instrument the water was made to drop upon wheels which were thereby turned, and the motion was communicated to a small statue, which gradually rose and pointed with a rod toward the hours marked on a diagram. Clepsydras measured the time allowed for speakers in courts of justice, and they are frequently referred to in the writings of Aristophanes, Aristotle, Plato, Demosthenes, Cicero, and Pliny, like the sands of the hour glass in modern literature. Soon after the decline of Rome they were spread throughout Europe, but their use was abandoned after the invention of pendulum clocks.
At the beginning of the 9th century Charlemagne received a magnificent clepsydra as a present from the caliph Haroun al-Rashid. The fact that the quantity of water escaping from the orifice varies according to the height of the column and the shape of the vase furnishes a difficult mathematical problem, which occupied the most skilful geometers of the 18th century. For its solution Bernoulli received a prize from the French academy of sciences.