Water clocks, along with sundials, are, with the exception of the vertical gnomon and the day-counting tally stick, the oldest time-measuring instruments. Where and when water clocks were first invented is not known. Until the development of the pendulum clock (1656), water clocks were the most accurate timekeeping devices.
"The oldest water clock of which there is physical evidence dates to c. 1417-1379 BC, during the reign of Amenhotep III where it was used in the Temple of Amen-Re at Karnak. The oldest documentation of the water clock is the tomb inscription of the 16th century BC Egyptian court official Amenemhet, which identifies him as its inventor. These simple water clocks, which were of the outflow type, were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. There were twelve separate columns with consistently spaced markings on the inside to measure the passage of "hours" as the water level reached them. The columns were for each of the twelve months to allow for the variations of the seasonal hours. These clocks were used by priests to determine the time at night so that the temple rites and sacrifices could be performed at the correct hour. These clocks may have been used in daylight as well" (Wikipedia article on water clock, accessed 12-25-2011).