Tickipedia: 0AD-1000AD

Time, one of the seven fundamental physical quantities in the International System of Units, is a dimension in which we all exist. It marches on as certain as the sun rises and sets, and its behaviour has fascinated scientists as far back as history recalls. In this edition of Tickipedia, we investigate the evolution of water-powered clocks in China during a period of unprecedented technological development.

Chinese inventors fine-tuned the water clock design with extra tanks and balance mechanisms

Last time, we looked at how basic inflow and outflow water clocks were developed in ancient Egypt, Persia and Greece in the centuries BC. Even by this time, visionary clockmakers such as Ctesibius had done wonders with water, crafting complex automata and various ornamental timepieces, and in Europe, experiments and embellishments involving water clocks continued for many centuries into the new millennium. But, thanks to the advent of the Dark Ages, the most exciting developments were left to be discovered in the Far East.

At the height of Imperial China, a second-century polymath called Zhang Heng independently created a three-tiered water clock (very similar to that made by Ctesibius). Other Chinese inventors furthered the idea by adding extra tanks to make the design even more precise, and by the seventh century came the introduction of balance arms with counterweights that allowed flow to be adjusted according to the seasons—because the Chinese measured hours in proportion to the daylight of a given day.

Zhang was also experimenting with gears and axis shafts, allowing his water clocks to power more complex astronomical instruments such as armillary spheres, celestial models of the sky. In the eighth century, the Buddhist monk Yi Xing used this idea to create the first escapement (a device with a locking action and fixed oscillation rate that supplies power to the timekeeping mechanism), which he used in an armillary sphere. Yi's contraptions were so impressive that one source states: ‘In precision, [his] engine can be compared with nature itself.’ Water-powered astronomical automata such as these blossomed in the following centuries, peaking in the tenth century with the simple yet clever idea of using liquid mercury instead of water to negate problems caused by low winter temperatures.

All this came to a head with the extraordinary achievements of one Su Song, an eleventh century nobleman who is known not only as China's greatest horologist, but also as a successful engineer, astronomer, chemist, cartographer, politician and poet. Su's inventions united hydraulic and mechanical functions better than ever before: the result was an evolution of Yi's escapement that created an incredibly accurate stop-release motion using cups that only exerted the necessary gravity when filled with water.

Su's greatest work was a breathtakingly huge and complex astronomical clock tower, built in the Song Dynasty capital of Kaifeng, which measured forty feet high with an eleven foot escape wheel. Featuring an armillary sphere (itself weighing more than ten tons) and figurines that appeared at little windows to audibly announce the time with gongs, drums and bells, its workings included advanced use of oblique gears and the first endless chain drive mechanism. Even though detailed records of its construction survive, the work was so masterful that no one has been able to build another like it since. In all but power source, this was a mechanical timepiece displaying technological sophistication that would not be matched in Europe until many centuries later.