As sundials were dependent on light and good weather conditions, the Egyptians created water clocks in the 15th century BC to tell the time in all conditions, 24 hours a day. Essentially the first all-weather clock, the device, known as a clepsydra (“water thief”), was a large pot that leaked water through a hole in the bottom at a steady, measurable rate. Along with sundials (including time sticks and obelisks), water clocks are the oldest means of keeping time. Monasteries in medieval Europe used water clocks with bells set to strike at hours for worship and work. Although not accurate by modern standards, the water clock represented a significant technological advance in timekeeping and was not supplanted until the arrival of the pendulum clock.In c. 1088, the Chinese astronomer Su Sung built a vast water clock for the Emperor, powered by a 3.3-m (11-ft) water wheel. The complex device featured one of the earliest escapements – the mechanism by which continuous motion is converted into the uniform “tick-tick” of a mechanical clock.