Korean History › Early Joseon › King Sejong the Great
Jang Yeong-sil
fl. 1420s–1442; rain gauge 1441· Person· Culture, religion & science
Well documentedSolid records, physical evidence or scientific consensus.
A government slave who became Sejong's leading engineer, building water clocks, astronomical instruments and, tradition says, the rain gauge.
See it on the Korean History map →What happened
Jang Yeong-sil (장영실) was born a government slave in Dongnae, today's Busan; the annals say his father came from Yuan China and his mother was a gisaeng (official entertainer). Taejong brought him to court; Sejong freed him and gave him office in 1423. He built the self-striking water clock Jagyeongnu (1434), worked on armillary spheres, sundials and the 1434 metal type. In 1442 a royal palanquin he oversaw broke; he was flogged and dismissed, then vanishes from the records.
Why it matters
His career shows Sejong promoting talent across class lines, and his instruments put Joseon among the world's leaders in timekeeping and astronomy in the 1400s.
Legacy and how we know
The Sejong sillok record his origins, promotions and punishment; a 1536 water clock survives, and the 1434 Jagyeongnu was rebuilt in 2007 from the annals' description.
Still debated
Why he disappears after 1442 is unknown, and the annals credit the 1441 rain gauge to Crown Prince Munjong, not to him.
Connections (2)
▸King Sejong the Great promoted Jang Yeong-silWell documented
The connection
Sejong sent Jang Yeong-sil to Ming China to study astronomical instruments, then, over some ministers' objections, freed him from slavery and gave him office in 1423. He put Jang on teams building water clocks, sundials and armillary spheres and promoted him to the third rank before his fall in 1442.
When and how
1421–1442 in Hanyang (Seoul) and Ming China; Sejong, Jang Yeong-sil and scholar-officials such as Yi Cheon and Jeong Cho.
How we know
The Sejong sillok record the debate over his appointment, his promotions and his inventions.
Debates and open questions
Why Sejong let him be flogged and dropped in 1442, after years of favor, is never explained.
▸Jang Yeong-sil was a low-born exception to Yangban aristocracy and the class orderWell documented
The connection
Joseon's class order normally barred slaves from office. Jang Yeong-sil's rise from government slave to third-rank official was a rare exception, granted by royal favor and argued over by ministers. It shows both how rigid the yangban system was and how a king could bend it for talent.
When and how
1423 in Hanyang (Seoul), when Sejong consulted ministers Heo Jo and Jo Mal-saeng on appointing him; similar debates recurred over later promotions.
How we know
The Sejong sillok record his slave origins, the ministers' disagreement and the king's decision.
Debates and open questions
Some historians suspect his low birth explains how quickly he was dropped after 1442, but the records do not say.