CLEP'SYDRA (Gk. κλεψύδρα, klepsydra, from κλέπτειν, kleptein, to steal + ὕδωρ, hydōr, water). An ancient instrument for measuring time by the efflux of water through a small orifice. Two kinds have been in use. In the simplest form the water was allowed to escape from one vessel into another. This form was used in the Athenian courts, where a speaker was allowed a certain number of amphoræ of water for his speech, the quantity depending on the importance of the suit. The more complicated form was said by some to have been invented by Plato, while others gave the honor to Ctesibius of Alexandria. In this form the water was allowed to flow at a uniform rate into a receptacle, on which was marked a scale of hours. Both forms are said to have been introduced into Rome in B.C. 159, and were widely used. Athenæus (iv. p. 174) applies the name to a variety of the hydraulic organ.