Self-striking water clock

Jang Yeong-sil built the Jagyeongnu, a water clock that struck bells, drums and gongs on its own; it kept the official time at Gyeongbokgung.

Sejong promoted Jang Yeong-sil, a government slave of unusual talent. In the Jagyeongnu, water flowing steadily into a vessel lifted a rod that released balls; these set figures in motion to strike a bell each hour, a drum at each night watch and a gong at each subdivision. In 1434 it was installed in the Borugak pavilion south of Gyeonghoeru and kept the official time. The same year the hemispherical sundial Angbu ilgu was set up at Hyejeonggyo Bridge and before Jongmyo so that commoners could tell the time. The clock that survives at Deoksugung is part of a version rebuilt under Jungjong in 1536.