CLEPSYDRA, klĕp-sī'drạ, or WATER-CLOCK, an instrument for the measurement of time by the escape of water from a vessel through an orifice. Its origin is extremely ancient, and has generally been attributed to the Egyptians. Two descriptions of clepsydra have been employed — one in which the water merely escapes through the orifice, the other in which the same level is constantly maintained by the introduction of a fresh supply of water, and a uniformity of efflux secured by retaining throughout an equal amount of pressure on the fluid as it issues from the bottom of the vessel. In one kind of water-clock the measure of time is registered on a dial-plate by means of a hydraulic apparatus acted on by the efflux of water from a cistern. The simpler form was used in Athenian courts where a speaker was allowed so much water for his speech, according to the importance of his case. The more complicated form is said to have been invented by Plato; or by Ctesibius, according to some. Both forms were introduced into Rome in 159 B.C. These instruments are now scarcely ever constructed.