- Korean Water-Clocks: “Chagyongnu,” the Striking Clepsydra, and the History of Control and Instrumentation Engineering*
Historians of science and technology in Korea would agree that the golden age in Korean traditional science and technology is King Sejong’s reign (r. 1418–50 A.D.) of the Choson Dynasty. During this time, due largely to the king’s own interest in science and technology, the “Hall of Worthies” (Chiphyonjon) was reestablished, the Royal Observatory was renovated, and movable type and Han’gul (the Korean alphabet) were invented. King Sejong was particularly interested in developing new national astronomical calendar systems and standard clocks, which were distinct from, and superior to, those used in China. Constructed in 1432–34 by Chang Yongsil, Chagyongnu is a “key machine” that symbolizes the spirit of this golden age and represents the innovative minds of King Sejong and his engineers. Chagyongnu, which literally means a self-striking clepsydra or water clock, was a fully automatic device. Constructing an automatic standard clock that would not require any human agency (such as “rooster men”) was the king’s motivation and goal.
The first self-striking clepsydra in Korea employed jack-work mechanisms, in which three wooden figures (jacks) strike a bell, a drum, and a gong to signal, respectively, each “double-hour,” “night-watch” (a night-watch being one-fifth of a nighttime, i.e., from dusk to dawn), and “point” (five points being one night-watch). Another jack on the clock displayed a specific double-hour during each two hours. The clock was powered by the constant upward motion of rising water in the water-receiving vessels. This continuous movement of water started the motion of small balls at regular [End Page 553] intervals. These small balls in turn triggered the motion of larger balls, which, with levers and other devices, moved the jacks to ring a bell and so on. People could only see the moving jacks and hear the sounds, as the large balls and mechanical devices were hidden inside a big wooden box.
Chagyongnu has been considered a mysterious artifact in the history of Korean science and technology. Was this complicated automatic device technically feasible at that time? If so, how? In 1534, exactly one hundred years after its construction, the original Chagyongnu was repaired and a replica was built by King Choongjong. The original was destroyed in 1592 during the war with Japan, and the jack-work device of the replica was eventually dismantled in the seventeenth century. The only surviving parts now are the clepsydra vessels and water-receiving vessels of King Choongjong’s replica. Since the seventeenth century, the mechanical part of the automatic device has become a forgotten technology.
Fortunately for historians, two detailed documents on Sejong’s Chagyongnu survived. Like other historical documents, however, they are missing critical elements. Joseph Needham and his collaborators carefully examined the documents and suggested a possible working mechanism inside the wooden box. On this basis, they proposed that Islamic clepsydras designed by al-Jazari, in which water was used as a motive power and rolling balls were employed, must have influenced, via China, the design of Korean Chagyongnu. However, even Needham and his colleagues felt that “working solutions for some parts of the mechanism will never be more than conjectural, owing to a lack of information on some key points” (Joseph Needham, The Hall of Heavenly Records: Korean Astronomical Instruments and Clocks, 1380–1780 [Cambridge: Cambridge University Press, 1986, pp. 34–35]).
Moon-Hyun Nam’s monograph attempts to fill this gap. His project is both important and ambitious. By fully utilizing his background in modern control engineering and through his meticulous and painstaking analysis of the existing documents, Nam has reconstructed the design blueprint of King Sejong’s Chagyongnu. According to Nam’s blueprint, the clock employed several kinds of spoon-shaped levers for its double-hour time indicator. Yet, signaling five night-watches by a drum and twenty points by a...