This paper reviews the use of water clocks in the early Joseon Dynasty from the era of King Taejo till the days of King Seongjong. More specifically, it traces the creation of Jagyeongnu (自擊漏, automati- cally-striking clepsydra) the organization and maintenance of Geumru (禁漏) of Seoun-gwan (書雲觀, Bureau of Astronomy), and the repair and im- provement of Jagyeongnu in the post King Sejong era.
From the early days, Joseon Dynasty strove to utilize water clocks as its national standard clock. Jagyeongnu, completed under King Sejong’s reign, was the product of efforts to create the most advanced water clock that solves problems with previous water clocks.
King Sejong and his officials studied works on Chinese historical clep- sydras to fix the problems of the past water clocks and used the knowledge gained in designing Jagyeongnu in 1433. After a year of test operation, Jagyeongnu became the national standard clock on July 1st, 1434.
The main staffs in charge of the operation of national water clocks, including Jagyeongnu, were officials called ‘Geumru’, members of Seoun-gwan. In the proc- ess of the reform of government organization, Geumru was affiliated as junior officials under Seoun-gwan, started with the 40 members and reorganized as 30 peoples in November 1475, and this number was written in Gyeongguk-Daejeon (經國大典), the National Code of Joseon Dynasty.
The operation of an automatic clepsydra such as Jagyeongnu incurred many difficulties. As witnessed in several cases since the era of King Munjong, malfunction could be bred in the compartments of automatic water clocks: the water supply system comprised of water-supply vessels and inflow vessels, power transmission devise using iron balls, and the time casting apparatus. This was why there were continuous suggestions for repairs of water clocks since the full-scale operation of Jagyeongnu.