Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.
The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and also resulted in improved accuracy.
Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less constant allowing reasonably precise, and repeatable, estimates of time passages.
In the hourglass, fine sand pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined, passage of time, the resource is not consumed but re-used.
The word clock is derived (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell".
The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC.
Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain.
A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels.
Spring-driven clocks appeared during the 15th century.
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Sundials continued to be used to monitor the performance of clocks until the modern era.