CLEPSYDRA (from Gr., to steal, and , water), the chronometer of the Greeks and Romans, which measured time by the flow of water. In its simplest form it was a short-necked earthenware globe of known capacity, pierced at the bottom with several small holes, through which the water escaped or "stole away." The instrument was employed to set a limit to the speeches in courts of justice, hence the phrases aquam dare, to give the advocate speaking time, and aquam perdere, to waste time. Smaller clepsydrae of glass were very early used in place of the sun-dial, to mark the hours. But as the length of the hour varied according to the season of the year, various arrangements, of which we have no clear account, were necessary to obviate this and other defects. For instance, the flow of water varied with the temperature and pressure of the air, and secondly, the rate of flow became less as the vessel emptied itself. The latter defect was remedied by keeping the level of the water in the clepsydra uniform, the volume of that discharged being noted. Plato is said to have invented a complicated clepsydra to indicate the hours of the night as well as of the day. In the clepsydra or hydraulic clock of Ctesibius of Alexandria, made about 135 B.C., the movement of water-wheels caused the gradual rise of a little figure, which pointed out the hours with a little stick on an index attached to the machine. The clepsydra is said to have been known to the Egyptians. There was one in the Tower of the Winds at Athens; and the turret on the south side of the tower is supposed to have contained the cistern which supplied the water.
See Marquardt, Das Privatleben der Römer, i. (2nd ed., 1886), p. 792; G. Bilfinger, Die Zeitmesser der antiken Völker (1886), and Die antiken Stundenangaben (1888).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)