How did mechanical clocks change over time in ancient China?

What was the purpose of the mechanical clock in ancient China?

During the Song Dynasty in China, the mechanical clock was invented in 976 A.D. Chan Ssu-Hsun produced a mercury clock. It was the world's first functional mechanical clock. Mechanical clocks are significant because they were designed to tell time more precisely than water or sun clocks.

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i'd assume india mainly because of how isolated japan was.   plus it is a whole china away.