Jagyeongnu by Jang Yeong-sil: Construction and Time Signals

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

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In 1437, a method was adopted to relay the palace water clock’s time signal to Uigeumbu.AI-generated image

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.

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.

1. Water flows from a pasuho into a susuho.
2. As the water rises, the rod inside the susuho rises.
3. At a set position, the rod activates the mechanism that releases a small ball.
4. The small ball moves a lever mechanism, releasing a large iron ball.
5. 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.

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

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”

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.

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.

Questions checked against the record

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.