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fl. 700s

Chinese Buddhist Monk, Mathematician, and Astronomer

I-Hsing is most known for his contributions to the development of a water clock with an escapement to control the speed and regularity of the clock's movements. This advancement allowed more accurate timekeeping correct to within 15 minutes a day.

Records on the life of I-Hsing, also known as I-Xing, are scarce and center on his association with the construction of a water clock in eighth century China. Early in the K'ai-Yuan reign of 713-741, the crown recruited Buddhist monk I-Hsing to work with administrative official Liang Ling-Tsan (Liang Ling-Zan) and lead the building of a bronze astronomical instrument, a water clock that would tell time and indicate the movements of the constellations.

Water clocks date back to around 3,000 b.c. in China. This type of clock relies on the constant drip of water from one vessel into another to provide a steady rate from which the passage of time could be determined. While the Chinese were developing water clocks, other countries were using such timekeeping methods as sundials or astronomical observation to tell time. The Chinese technique differed substantially in that it provided a method that did not depend on clear weather. Sun dials and astronomical observation required skies clear enough for the sun to cast a shadow on the dial or for the viewing of at least a portion of the night sky. Water clocks had no such requirements and continued operating regardless of weather.

Water clocks also allowed observers to maintain a constant unit of time. For many centuries—even after the development and refinement of water clocks—many societies used socalled temporal hours, time periods which varied from season to season and even day to day.

I-Hsing's mathematical and astronomical skills provided the theory for the water clock that he and Liang would oversee. Liang's engineering skills, on the other hand, gave form to I-Hsing's ideas. The final water clock included a celestial sphere, essentially a globe marked with representations of equatorial constellations as well as degrees of movement. As viewed from Earth, the stars appear to revolve around the planet, and the celestial sphere mimicked that movement. Outside the globe, the men added two rings: one to represent the sun's movement and one to follow the moon's travels.

To run the clock, I-Hsing and Liang employed a wheel that would turn as water steadily dropped from a tank onto the wheel. As this driving wheel turned the celestial sphere, sun ring, and moon ring moved in accordance with their current locations in the sky. In addition, the two designers added a bell-and-drum system to alert those within earshot of the current time. This intricate device took a place in the Wu Ch'êng Hall of the crown palace.

In 1092, Chang Ssu-Hsün followed in the footsteps of I-Hsing and Liang and created another great astronomical clock in China. Building upon the former designers' escapement, he constructed a 33-ft (10-m) tall water-clock tower. Water clocks continued to be used and refined in China, while European nations began to develop mechanical clocks. Eventually the accuracy of the mechanical clocks exceeded that of the water clocks. Water clocks began to disappear, and now are rarely found except in museums.