Clepsydra (Gr., 'water-clock'), an instrument to measure time by the trickling or escape of water. In Babylonia, India, and Egypt, the clepsydra was used from before the dawn of history, especially in astronomical observations. A Hindu form of it was that of a copper basin put to float in a vessel, so that by the gradual influx of water through an aperture beneath, it should sink in a certain time. A more common type is that referred to by Greek and Latin writers, which resembled the modern sand-glass, and was used in courts of law to limit the length of the pleadings. Julius Cæsar (Com. de Bell. Gall., lib. v. 13) speaks of measuring time in Britain 'by accurate water-measures,' and some commentators infer that the clepsydra was used by the Britons. More probably, however, it was brought by the Romans, being regularly used in their armies for allotting out the three hours' watches. Pliny ascribes the invention of the clepsydra to Scipio Nasica, but he, no doubt, merely introduced it from Alexandria or Greece, where it had already been greatly improved and perfected.
The general form of the clepsydra consisted essentially of a float which slowly rose by the trickling of water from above through a small hole in a plate of metal. As the float rose it pointed to a scale of hours at the side of the water-vessel, or, in the more elaborate forms, moved a wheel by means of a ratchet, and thus turned a hand on a dial. The first great difficulty was to secure a constant and uniform supply of water. This was well and simply done by using an intermediate cistern, so that, by means of a waste-pipe near the surface, the water always remained at the same level, even when the amount poured in exceeded the average.
There remained a much greater difficulty, arising from a fact often overlooked—viz. that by the Greek and Roman calendar the hour was not a fixed space of time, being very much longer in summer than in winter. A day meant the interval from sunrise to sunset, and an hour being the twelfth part of it was by no means so simple and measurable a magnitude as the unit of time shown on our modern dials. The problem of measuring the hour, thus varying through the year, was solved by Ctesibius of Alexandria, who invented an elaborate self-adjusting mechanism.—Clepsydra was also the name given to an ancient musical instrument, a hydraulic or water organ, described by Athenæus.