Water-balance escapement, Chinese, 1088.
© Science Museum / Science & Society Picture Library
Working model (scale 1:6). The original was used in Su Song's astronomical clock-tower of 1088. Su Song (1020-1101) was a Chinese astronomer and inventor who was ordered by the emperor to construct a complex armillary clock. With his advisers, he built a full-scale working model in wood which was tested before the metal parts were cast. The water-balance escapement represents a 'mising link' between earlier water clocks, whose timekeeping depended upon measuring changes in the water level, and the all-mechanical escapement clock developed in Europe towards the end of the 13th century.