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hydraulis. Ancient instr., also known as water org., said to have been invented by Gr. Ktesibios c.250 BC. Resembled small modern org. Kbd. operated by series of levers, each with a return mechanism, pressed down by fingers to obtain required notes. Water was used to stabilize wind pressure. In 4th cent. AD hydraulic mechanism was replaced by bellows, enabling much louder tone. See also organ.