Jang Yeong-sil (; ; 1390 – after 1442) was a Korean mechanical engineer, scientist, and inventor during the Joseon Dynasty (1392–1897). Although Jang was born a peasant, King Sejong allowed Jang to work at the royal palace. Jang's inventions, such as the Cheugugi (the rain gauge) and the water gauge, highlight the technological advancements of the Joseon Dynasty.
Early years
Jang Yeong-sil's birth is recorded only in the genealogy of the Jang family and in the Annals of the Joseon Dynasty. According to these records, his father, Jang Seong-hwi, was the 8th generation of the Jang family. Jang Seong-hwi was the 3rd of 5 brothers and all of the brothers previously were ministers of Goryeo. There are many historical records about his elder brother, Jang Seong-bal, who was born in 1344 and his grave located at Ui-seong in the province of Gyeongbuk. The Annals state that Yeong-sil's mother was a gwangi (gisaeng), assigning them (Yeong-sil and his mother) the social status of gwanno, (a servant in civil service district courts).
Civil service
Jang's fame gained him entry into the royal court at Hanseong (present-day Seoul), where selected commoners displayed their talents before the king and his advisers. Jang met Sejong's expectations in crafts and engineering, and allowed Jang to work as a government official in the palace. The talented scientists recruited under Sejong's new program worked at the Hall of Worthies (집현전; 集賢殿; Jiphyeonjeon).
Astronomical instruments
Sejong's first assignment to Jang was to build a celestial globe to measure astronomical objects. Books obtained from Arabian and Chinese scholars were not complete in their instructions, for these devices could also be used for military purposes. After two months of study, Jang made a spherical device that could perform with mediocre accuracy. In 1433, a year after his first attempt, Jang made an armillary sphere known as the honcheonui (혼천의, 渾天儀). Honcheonui depended on a waterwheel to rotate the internal globe to indicate time. Whether day or night, this allowed the instrument to be updated on the positions of the sun, moon, and the stars. Later celestial globes (gyupyo (규표)) could measure time changes according to the seasonal variations. These instruments, along with the sundials and water clocks, were stationed around the Gyeongnghoeru Pavilion in Gyeongbok Palace and put into use by the astronomers. The success of Jang's astronomical machines was marked in 1442 AD when the Korean astronomers compiled their computations on the courses of the seven heavenly objects (five visible planets, the sun, and moon) in Chiljeongsan (칠정산), an astronomical calendar that made it possible for scientists to calculate and accurately predict all the major heavenly phenomena, such as solar eclipses and other stellar movements.
Iron printing press
Although Choe Yun-ui (최윤의) invented the world's first metal printing press in 1234 during the Goryeo Dynasty, Sejong asked scientists at Jiphyeonjeon to build a better printing press. In 1434, scientists accomplished building Gabinja (갑인자, 甲寅字), which was made of copper-zinc and lead-tin alloys. It was said to be twice as fast as the previous printing presses and printed Chinese characters in astounding beauty and clarity. Gabinja was reproduced six times during the next 370 years.
Water clock
Samguk Sagi records that an office overseeing the use of water clocks had been established during the Three Kingdoms Period. The Korean water clock consisted of two stacked jars of water, with water dropping from the top to the bottom at a measured rate. The level of the water indicated the time of the day. This was very inconvenient because a person had to be always be on guard, so that a drum could be banged at each hour to inform the public of the current time.
Self-striking water clocks were not new, having already been invented by the Arabians and the Chinese (in 1091). Upon hearing about the usage of self-striking water clocks in foreign countries, Sejong assigned Jang and other scientists to build a clock emulating such automatic devices. After their initial attempts failed in developing an operational water clock, Jang traveled to China to study the various designs of water clocks. When he returned in 1434, Jang created Korea's first self-striking water clock, the Jagyeokru (자격루), which would mark the hour automatically with the sounds of a bell, gong, and drum, and was used to keep the standard of time in Joseon. This water clock was not preserved well and did not survive; however, reconstructions of the Jagyeokru based on text descriptions have been made.