Jagyeongnu by Jang Yeong-sil: Construction and Time Signals =========================================================== The Jagyeongnu, built by Jang Yeong-sil and others, converted the flow of water into mechanical signals to announce the time automatically. Its recorded completion in 1433, the start of its use in 1434, and the surviving artifact made in 1536 must be distinguished. - The Jagyeongnu used a mechanical device to do the work of people who read a water clock's markings and announced the time. - In 1433, credit for building the Jagyeokgungnu was recorded, and a new water clock came into use on the first day of the seventh lunar month in 1434. - The movement of a rod floating in water passed through balls and levers to trigger time signals from a bell, drum, and gong. - In 1437, a method was adopted to relay signals from the palace Jagyeongnu to the Uigeumbu. - The surviving water-clock apparatus of the Changgyeonggung Palace Jagyeongnu is part of an improved version made in 1536; Jang Yeong-sil's original mechanism has not survived. Jang Yeong-sil and others completed the Jagyeongnu in 1433. Its automatic time signals solved the problem of having someone watch a water clock and announce the time. The device, used from July 1434, is remembered for changing how Joseon kept time. This article follows the records in the National Institute of Korean History’s Annals of King Sejong, checked in October 2026. What problem was the Jagyeongnu built to solve? The Jagyeongnu was designed to remove the need for a person to read and announce the time. In earlier water clocks, a marked rod rose as the water level climbed. An attendant had to watch its markings and announce the time. A time signal announces a set time through a sound or display. Even if the water clock measured time, the signal would be wrong if the attendant misread it. The Annals of King Sejong records that Sejong was concerned about mistakes by the attendants responsible for time signals. The Jagyeongnu had a mechanism with wooden figures to do that work instead. Annals of King Sejong, 1st day of the 7th month of Sejong’s 16th year Category Earlier water clock watched by a person Jagyeongnu Measuring time Used the flow of water and markings on a rod Used the flow of water and the rod’s movement Determining the time An attendant read the markings A mechanism operated when the rod reached a set height Announcing the time An attendant gave the signal Figures struck a bell, drum and gong Work still requiring an attendant Maintaining the water clock and giving time signals Changing the water and maintaining the mechanism How a technician born a government slave gained an official post Jang Yeong-sil’s appointment to an official post faced opposition because of his social status. He is known as an early Joseon technician who was born a government slave in Dongnae County. A government slave was a male slave attached to a government office. His birth and death years are unknown. An entry in the 1433 annals recounts an earlier discussion about appointing him to the Royal Wardrobe Office. Heo Jo opposed it because of his mother’s status. Jo Mal-saeng argued that Jang Yeong-sil was suited to the office. Sejong later accepted his ministers’ views and appointed him to a byeoljwa post. Making the Jagyeokgungnu was the turning point at which his technical work led to a higher post. Citing his contribution to its construction, Sejong proposed giving him the additional post of hogun. The ministers agreed, and Sejong followed their view. He has made an instrument that will be passed down for a long time, so his contribution is considerable. I wish to give him the additional post of hogun. Sejong, Annals of King Sejong, 16th day of the 9th month of Sejong’s 15th year. The entry A timeline distinguishing construction from use Separate records confirm the Jagyeongnu’s completion and the start of its use. Our History Net’s account of the Jagyeongnu gives 1433 as its completion year. The annals state that the new water clock came into use on the 1st day of the 7th lunar month of 1434. The months and days below follow the lunar dates recorded at the time. Date Recorded event Source 16th day of the 9th month of 1433 Discussion of the contribution made by building the Jagyeokgungnu and granting the post of hogun Relevant entry in the Annals of King Sejong, Sejong’s 15th year 1st day of the 7th month of 1434 Start of use of the new water clock and description of its automatic time-signal mechanism Relevant entry in the Annals of King Sejong, Sejong’s 16th year 28th day of the 6th month of 1437 Order to relay time signals from the palace to Uigeumbu Relevant entry in the Annals of King Sejong, Sejong’s 19th year 1438 Construction of the Okru, which also displayed changes in the heavens Our History Net, “Astronomical Instruments and Calendrical Systems under Sejong” 3rd month of 1442 Investigation after a royal palanquin built under his supervision broke Our History Net, “Jang Yeong-sil” 1536 Installation of an additional Jagyeongnu at Changgyeonggung Palace National Palace Museum of Korea collection description The 1433 entry confirms his contribution to the construction. It alone cannot establish the completion date or testing schedule. The 1434 entry gives specific details about the start of use and the device’s construction. Our History Net, “Jagyeongnu” How the Jagyeongnu worked The Jagyeongnu turned a rising water level into the movement of balls to announce the time. The water-clock section measured the passage of time. The time-signal mechanism turned that movement into sounds and displays by figures. Vessels that supplied and received water The pasuho were vessels that released water, and the susuho were vessels that received it. The 1434 annals record four pasuho and two susuho. The susuho were used alternately when the water was changed. As the water level rose in a receiving vessel, a rod inside it rose too. The sequence from rod to ball When the rod reached a set height, it activated a mechanism that released a ball. A small copper ball acted on a mechanism that released a larger iron ball. The force of the falling ball moved the figures that gave the time signals. Water flows from a pasuho into a susuho. As the water rises, the rod inside the susuho rises. At a set position, the rod activates the mechanism that releases a small ball. The small ball moves a lever mechanism, releasing a large iron ball. The iron ball’s movement sets the figures’ time-signal actions in motion. What did the bell, drum and gong announce? The bell, drum and gong announced different divisions of time. The bell marked the hour, the drum marked a night watch, and the gong marked a subdivision of a watch. Figures holding plaques showing the time also appeared in turn. Display mechanism What it announced Figure striking the bell An hour as a division of the day Figure striking the drum A watch as a division of the night Figure striking the gong A further division of a watch Figure holding a time plaque A display of the corresponding time Nighttime signals also needed seasonal adjustment. The annals describe a structure in which copper plates were changed according to the seasonal divisions. Automatic time signals did not eliminate the need to change the water or make seasonal adjustments. Description of the water clock’s structure in the Annals of King Sejong How were palace time signals relayed through the capital? In 1437, a method was adopted for relaying signals from the palace Jagyeongnu to Uigeumbu. The State Council raised the inaccuracy of the existing water clocks as a problem. It also identified attendants’ mistakes as a cause of incorrect entry and exit times. The State Council asked for metal percussion instruments and drums to be placed at intermediate points to relay the signals. The plan was to hear the palace Jagyeongnu and give signals in sequence. Sejong accepted the proposal. Annals of King Sejong, 28th day of the 6th month of Sejong’s 19th year Stage Role What the record indicates Palace Jagyeongnu Produce the reference time signal Establish the time at the starting point Intermediate relay points Hear the sound and give the next signal Carry the signal beyond the palace Relay to Uigeumbu Connect the signal to time announcements in the capital Enable multiple places to follow the same reference This record helps define the extent of the automation. In the palace, a machine announced the time. Outside the palace, people heard the sound and relayed the signal. Describing this as a system for sharing a common time is an interpretation based on the record. Jang Yeong-sil’s work and the scope of the joint effort Jang Yeong-sil’s work included making water clocks and astronomical observation instruments. Our History Net also describes his study of astronomical instruments in Ming China. That account must be considered alongside the later record Yeollyeosil Gisul. Research into written sources and technical work were divided among those making the instruments. For the ganui, Jeong Cho, Jeong In-ji and others studied its design. Jang Yeong-sil and Yi Cheon supervised the technicians. His work also included taking part in casting the Gapinja type and supervising mineral production. Our History Net, “Jang Yeong-sil” Instrument Main function Relationship to the Jagyeongnu Ganui Observing the positions of celestial bodies Related to aligning clock times with observation results Honcheonui Determining the positions and movements of celestial bodies Part of the astronomical observation effort Angbuilgu Determining the time and seasonal divisions from the sun’s shadow A sundial that uses sunlight Jagyeongnu Automatically announcing the time measured by the flow of water A water clock installed at Borugak Okru Displaying the time and changes in the heavens An astronomical clock installed at Heumgyeonggak in 1438 The Angbuilgu was an instrument whose concave surface showed a shadow to be read. The Jagyeongnu could measure time even at night, without sunlight. The Okru added a mechanism showing changes in the heavens. These instruments served observation and timekeeping in different ways. Our History Net, “Astronomical Instruments and Calendrical Systems under Sejong” From what period are the surviving water vessels? The surviving vessels of the Changgyeonggung Palace Jagyeongnu are part of an instrument made in 1536. The Jagyeongnu made by Jang Yeong-sil under Sejong has not survived. The National Palace Museum of Korea describes the surviving objects as three pasuho and two susuho. Category Jagyeongnu under Sejong Surviving vessels of the Changgyeonggung Palace Jagyeongnu Date Completed in 1433, brought into use in 1434 Additionally installed at Changgyeonggung Palace in 1536 Nature Automatic water clock made by Jang Yeong-sil and others Part of an improved version that followed Sejong’s instrument Water vessels Records list four pasuho and two susuho Three pasuho and two susuho survive Preservation Original instrument has not survived Water vessels survive The remaining vessels alone cannot show the full automatic time-signal mechanism. The susuho bear inscriptions naming the participants in their construction and their posts. The inscription on the large pasuho provides evidence for the construction date. The surviving objects are confirmed as part of the National Palace Museum of Korea’s collection. National Palace Museum of Korea collection description Records and interpretation Facts recorded directly must be distinguished from later accounts. The annals are the key source confirming Jang Yeong-sil’s contribution to making the Jagyeongnu and the start of its use. His childhood and later years are not known to the same degree. Facts confirmed by records The 1433 annals state that Sejong recognized Jang Yeong-sil’s contribution to the construction. The 1434 entry describes the structure of the new water clock. The 1437 entry records the adoption of a method for relaying time signals. Meaning shared by the explanatory accounts Our History Net explains that automatic time signals reduced the attendants’ workload. This interprets the mechanism as connecting time measurement and announcement. It should not, however, be taken to mean that the device was entirely free of error. Conflicting or unconfirmed details The annals and the Asan Jang Clan Genealogy give different accounts of his family background. They disagree on when his ancestors came from China to settle in Korea and on their lineage. The exact date of his appointment to the byeoljwa post at the Royal Wardrobe Office is also difficult to establish. Punishment and dismissal followed the 1442 palanquin accident. His subsequent activities and the date of his death are unknown. Neither a year of death nor a place of retirement can be established from this. Stories about his childhood and scenes in dramas must also be distinguished from historical sources. Our History Net’s review of his life and the records Why automatic time signals matter today The Jagyeongnu is an example of a machine that conveyed measured information at the moment it was needed. It converted the continuous change in water level into signals at set times. Those signals were conveyed by sound and displays by figures. What can be compared with automation today is the connection between functions. The breakdown below is a modern way to analyze the device. It does not mean the technology was the same as electronics or computers. Measurement: The flow of water and the height of the rod indicated elapsed time. Activation: At a set position, a ball was released to move the next mechanism. Announcement: A bell, drum, gong and time plaque announced the time. Operation: The water had to be changed, and seasonal adjustments were needed. Frequently asked questions Was the Jagyeongnu made in 1433 or 1434? Our History Net gives 1433 as the year of completion. That year’s annals also mention the contribution made by building the Jagyeokgungnu. The new water clock came into use on the 1st day of the 7th lunar month of 1434. Did automatic time signals eliminate the need for people? The main task automated was determining and announcing the time. The annals say the susuho were used alternately when the water was changed. Maintenance also included changing parts to suit the seasonal divisions. Are the Jagyeongnu and Borugak the same thing? The Jagyeongnu was a water clock, while Borugak was the building where it was installed. Under Sejong, the Jagyeongnu operated at Borugak, south of Gyeonghoeru Pavilion in Gyeongbokgung Palace. For this reason, it was also called the Borugakru. Did Jang Yeong-sil make the museum’s water vessels himself? The surviving vessels are not the originals made by Jang Yeong-sil. They are part of the Jagyeongnu installed at Changgyeonggung Palace in 1536. Nor has the entire automatic time-signal mechanism from Sejong’s reign survived. Did Jang Yeong-sil die in 1442? The year 1442 appears in accounts of the palanquin accident and his punishment. The incident cannot be treated as a record of his death. His year of death is unknown. FAQ Q. What kind of clock is Jagyeongnu? A. Jagyeongnu is a water clock that measures time using the flow of water and automatically signals the time. Beads and mechanical devices moved figures that rang a bell, beat a drum, and struck a gong. Q. Was Jagyeongnu made in 1433 or 1434? A. Our History Net gives 1433 as the year it was completed. The annals record that a new water clock was used starting on the first day of the seventh lunar month in 1434. Q. How was the flow of water turned into a time signal? A. As the water rose, a rod rose and activated a bead-release mechanism. The movement of the beads was transferred to other mechanical devices, causing the figures to signal the time. Q. What did the bell, drum, and gong each signal? A. The bell signaled the hour, the drum the night watch, and the gong a division of the night watch. Figures holding plaques marked with the time also appeared in turn. Q. Did automatic time signaling eliminate the need for someone to maintain it? A. Reading and announcing the time were automated, but maintenance was still necessary. The water had to be replaced, and parts had to be changed according to the solar terms. Q. Are Jagyeongnu and Borugak the same thing? A. Jagyeongnu is a water clock, while Borugak is the building where it was installed. The Jagyeongnu installed in Borugak was also called Borugaknu. Q. Was the time kept by the royal court's Jagyeongnu also used outside the palace? A. A 1437 entry in the annals records instructions to listen for the sound of Jagyeongnu and pass on a signal from an intermediate point. A system for relaying the signal as far as Uigeumbu was adopted. Q. Is the Jagyeongnu that survives today the original made by Jang Yeong-sil? A. The surviving Jagyeongnu water clock from Changgyeonggung Palace is part of an instrument made in 1536. The original instrument made by Jang Yeong-sil during King Sejong's reign has not survived. Q. Which parts of Jagyeongnu still survive? A. The National Palace Museum of Korea says that three water-supply vessels and two water-receiving vessels survive. The entire automatic time-signaling mechanism has not been preserved. Q. What is the difference between Jagyeongnu and Angbuilgu? A. Jagyeongnu is an automatic water clock that uses the flow of water. Angbuilgu is a sundial that uses the sun's shadow cast on a concave surface to tell the time and solar term. Q. Did Jang Yeong-sil die in 1442? A. The palanquin accident in 1442 cannot be taken as a record of his death. His subsequent activities and the year of his death remain unknown. Sources - The Annals of King Sejong, 16th day of the 9th month in King Sejong's 15th year: Discussing the conferral of the additional office of hogun on Jang Yeong-sil: https://sillok.history.go.kr/id/kda_11509016_003 - The Annals of King Sejong, 1st day of the 7th month in King Sejong's 16th year: The structure and operating principles of the newly made water clock and its inscription: https://sillok.history.go.kr/id/kda_11607001_004 - The Annals of King Sejong, 28th day of the 6th month in King Sejong's 19th year: Ordering the bell marking the transition from night to dawn to be struck upon hearing the palace Jagyeongnu: https://sillok.history.go.kr/id/wda_11906028_003 - National Institute of Korean History, Our History Net, Chronology of Korean History, “Jagyeongnu”: https://contents.history.go.kr/mobile/kc/view.do?code=kc_age_30&levelId=kc_r300785 - National Institute of Korean History, Our History Net, “Astronomical Instruments and Calendrical Systems during King Sejong's Reign”: https://contents.history.go.kr/mobile/tt/view.do?levelId=tt_b21 - National Institute of Korean History, Our History Net, Chronology of Korean History, “Jang Yeong-sil”: https://contents.history.go.kr/mobile/kc/view.do?levelId=kc_n310300 - National Palace Museum of Korea Collection Guide, Changgyeonggung Palace Jagyeongnu Water Clock: https://www.gogung.go.kr/gogung/pgm/psgudMng/view.do?menuNo=800065&psgudSn=370348 Images - At dawn, an attendant tends palace water-clock vessels beside a mechanism with a bell, drum and gong.: https://injoys.com/rails/active_storage/blobs/proxy/eyJfcmFpbHMiOnsiZGF0YSI6MjgyMjEsInB1ciI6ImJsb2JfaWQifX0=--02ed33bf79e4f20fbde4076a857ce899059f1aab/ai-2b05b4af.webp - Two attendants stand with their backs turned beside a drum and gong in a broad palace courtyard.: https://injoys.com/rails/active_storage/blobs/proxy/eyJfcmFpbHMiOnsiZGF0YSI6MjgyMjcsInB1ciI6ImJsb2JfaWQifX0=--63efa05dc2ea8846dfd6da043fe0f6e3e57d5e95/ai-9d531284.webp --- Category: Knowledge Base Source: https://injoys.com/en/articles/jang-yeong-sil-jagyeongnu-making-and-automatic-time-signals License: cc_by Translation-Status: reviewed