The difficulty in inventing a mechanical clock was to figure out a way in which a wheel no bigger than a room could turn at the same speed as the Earth, but still be turning more or less continuously. If this could be accomplished, then the wheel became a mini-Earth and could tell the time. For, after all, the time is nothing more nor less than how far the Earth has turned today.
  Accomplishing this mechanical feat was one of the greatest steps forward of the human race. Where would we be today without clocks? The mechanical clock was invented in China in the eighth century A6. But still in 1271, Robertus Anglicus in his commentary on the Sphere of Sacrobosco tells us that in Europe 'artificers are trying to make a wheel which will pass through one complete revolution for every one of the [Earth's], but they cannot quite perfect their work. If they could, it would be a really accurate clock, and worth more than any astrolabe or other astronomical instrument for reckoning the hours . . .'
  By 1310, this had finally been achieved in Europe. And the stimulus for it seems to have been some garbled accounts of Chinese mechanical clocks which came to the West by way of traders. This was the same century that brought to Europe the Chinese inventions of gunpowder, segmental arch bridges, cast iron, and printing.
  Apart from the fact that the Chinese are obviously an inventive people, what other factors can account for the fact that they were the first to invent mechanical clocks? Was there some special reason why they urgently needed to know the hours of the day and the days of the year with a precision not required in Europe? The answer is yes, but few could possibly imagine why.
  The Chinese emperor was a cosmic figure, the equivalent on Earth of the Pole Star. His every move was regulated in conjunction with astrology. His heir was not necessarily his eldest son. Many examples in Chinese history exist of fourth sons, or other lesser offspring, being selected as the next emperor. How, then, was it determined who should be the heir? Part of the process of selection involved the astrological computation of the moment of the child's conception (since in China horoscopes commence at the estimated time of conception rather than at birth). And the moments when conception might take place were carefully set aside for the highest-ranking wives and concubines of the emperor to sleep with him. Access to the emperor's person had to be precisely timed in order for this to work properly. From the Record of Institutions of the Chou Dynasty compiled about the second century Bc, we find the following asto- nishing passage about the emperor's sex life:

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A model of the 'Cosmic Engine', Su Sung's great astronomical clock of 1092. The framework has been left uncovered to reveal the mechanisms. The original clock tower was 30 feet high. At the top is the power-driven armillary sphere for observing the positions of the stars. In the original, this was bronze, and the power for turning it was transmitted by a chain- drive. Mid-right (B) may be seen a celestial globe which was inside the tower and turned in synchronization with the sphere above. The central element in the reconstruction (D) is the water-wheel escapement, which, though turned by water power, was a mechanical escapement. This was a mechanical clock rather than a water clock, even though its power came from failing water or mercury. (Science Museum, London.)