+--------------------------------------------------------------------+ | [Illustration: The Clepsydra, or Water-Clock of the Greeks.] | +--------------------------------------------------------------------+ The CLEPSYDRA is so named because the water escapes from it as it were by stealth, but in a regulated flow so as to permit of the lapse of time being computed thereby, even as by sand running through sand-glasses. The Clepsydra appears to have been at first used to limit the time during which persons were allowed to speak in the Athenian Courts of Justice; 'the first water,' says Æschines, 'being given to the accuser, the second to the accused, and the third to the judges,'--a special officer being appointed in the courts for the purpose of watching the Clepsydra and stopping it when any documents were read whereby the speaker was interrupted. The time, and consequently the water allowed, depended upon the importance of the case. This custom, says Phavorinus, was to prevent babbling, that such as spake should be brief in their speeches. Ctesibius of Alexandria, who lived about 245, invented a much improved water-clock, mentioned by Vitruvius and Athenæus. Another kind of Clepsydra consisted of a vessel of water having a hole in it through which the fluid gradually escaped; a miniature boat floated upon the water and descended as the water decreased, whilst an oar placed in the boat indicated the hour by pointing to certain line-marks on the side of the vessel. The hole through which the water dropped was made, we are told, through a pearl, because it was supposed that the action of the water upon the pearl would not, as upon other substances, enlarge the aperture, nor would the pearl, it was imagined, be choked by the adhesion of any other material. The chief fault of the Clepsydra as a chronometer arose from the inequality of the flow of water, it being found to escape more rapidly when the vessel was full than when it was becoming empty, and also more speedily in hot weather than in cold. The Egyptians are however said to have measured by this machine the course of the sun; by it Tycho-Brahe computed the motion of the stars; and by it Dudley made his maritime observations. Plato furnished the original idea of the hydraulic organ by inventing a Clepsydra, or water-clock, which played upon flutes the hours of the night when darkness precluded their being shown by the index. Clepsydræ are still used in India.