Jang Yeong-sil

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 his mother was a gwangi (gisaeng), thereby their (Yeong-sil's and their mother) social status was a gwanno, (a servant in civil service district courts).

King Sejong the Great of Joseon instituted a policy of selecting officials based on their talent, not by their wealth or social class. 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. Sejong saw that Jang met his expectations in crafts and engineering, and allowed Jang to work as a government official in the palace. The talented scientists recruited under King Sejong's new program worked at the Hall of Worthies (집현전; 集賢殿; Jiphyeonjeon).

Astronomical instruments

Korean celestial globe first made by the scientist Jang Yeong-sil during the Joseon dynasty under the reign of King Sejong the Great of Joseon

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, he made a spherical device that could perform with mediocre accuracy. One year after his first attempt, in 1433, Young-sil made 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 (i.e. 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 made into use by the astronomers. The success of Jang Yeong-sil'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).

Iron printing press

Although Choe Yun-ui (최윤의) invented the world's first metal printing press in 1234 during the Goryeo Dynasty, Johannes Gutenberg is recognized worldwide as the first to pioneer this technology. In general, metal movable type printing blocks surpassed the wooden counterparts in durability, clarity, and longevity. Even then, the king asked the scientists at Jiphyeonjeon to build a better printing press. In 1434, the scientists accomplished in 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 print the Chinese characters in astounding beauty and clarity. Gabinja was reproduced six times during the next 370 years.

Water clock

Scaled model of Jang Yeong-sil's Self-striking water clock.

Self-striking water clocks had already been invented by the Arabians and the Chinese (in 1091), and a more advanced form was in use by the Koreans. Although it is believed, Samguk Sagi records that an office overseeing the use of water clocks had been established during the Three Kingdoms Period. The Korean version 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 at each hour he or she could bang a drum to inform the public.

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. They failed in developing an operational water clock. Therefore, Jang went to China to study the various designs of water clocks. When he returned in 1434, Jang created Korea's first water clock, Jagyeokru (자격루). This water clock did not survive; however, reconstructions of the Jagyeokru based on text descriptions have been made.

Circling the clock were 12 wooden figures that served as indicators of time. There were four water containers, 2 jars that received the water, and 12 arrows floating inside the lower container. As the water from the upper containers seeped down the pipe to the lower container, one of the arrows would tilt a board filled with small iron balls; a ball would roll down a pipe to a container of larger iron balls. The collision would cause the larger balls to travel down a lower pipe and hit a giant cymbal, announcing the time to the community. Then, the ball would land on another container, which is part of a complex of levers and pulleys that activates the motions of the wooden figures to indicate time visually.

Sundial

A Korean sundial first made by Jang Yeong-sil during the Joseon Dynasty, displayed in Gyeongbokgung.

Jang's invention of the water clock saw its infusion throughout the country. Yet, these were very costly, and the cheaper and more manageable alternative came about to be the sundial. Jang, Ichun, Kimjo, and other scientists made Korea's first sundial, Angbu Ilgu (앙부일구/仰釜日晷), which meant "pot-shaped sun clock staring at the sky". Angbu Ilgu was bronze in composition, and consisted of a bowl marked with 13 meters to indicate time and four legs jointed by a cross at the base. 7 lines crossed the 13 meters in different curves to compensate for the seasonal changes of the course of the sun. Angbu Ilgu and other variants, such as the Hyeonju Ilgu (현주일구/懸珠日晷) and the Cheonpyeong Ilgu (천평일구/天平日晷), were implemented in strategic spots, such as the main streets with heavy traffic, so that the people could be well informed of the time. To compensate for the high illiteracy rate among the commoners, 12 shapes of animals, such as mouse, tiger, and cow, were engraved in juxtaposition with the meters. No extant Joseon-dynasty sundials today were made during King Sejong's reign, none known to have survived past Imjin wars (임진왜란).

Research on weaponry

When King Sejong learned of reports that Korean melee weapons were duller and somewhat heavier than those of the neighboring countries, he sent Jang to Gyeongsang province, where he had spent his earlier life, to develop metal alloys for various weapons and tools. Since Jang used to be a gwanno (관노/官奴), a man-slave in government employ, he had already acquired much knowledge about metal working and also knew the geography of the area. Jang surveyed the available metals and their characteristics, and presented his research to the king and the generals, contributing to the development of Korean weaponry.

Rain gauge

The Korean economy during the Joseon dynasty was agriculturally based and was vulnerable to elongated or consecutively occurring droughts. Therefore, there was a need for better ways to manage water. Although rain gauges had been used in ancient Greece and India, Jang invented Korea's first rain gauge in 1441, called cheugugi (측우기/測雨器), and, by 1442, a standardized rain gauge with dimensions of 42.5 cm (height) and 17 cm (diameter) was introduced throughout the country to gather data on the yearly averages of precipitation throughout the different regions of the country.

Water gauge

To allow better water management, the king asked the scientists to figure out some ways to inform the farmers of the available amount of water. And, in 1441, Jang invented the world's first water gauge, called Supyo (수표/水標). It was a calibrated stone column placed in the middle of a body of water, connected by a stone bridge.

Jang's extraordinary accomplishments earned him much trust of the king. Some government officials were very jealous of Jang, especially when he had achieved so much regardless of his common origin. Furthermore, as Joseon's society was rooted in Korean Confucianism, scientists and engineers were held in low esteem in parallel with craftsmen.

In 1442, King Sejong ordered Jang to build a gama, an elaborately decorated Korean sedan chair. The gama broke while the king was traveling, and Jang was held responsible. Although the king was against the decree, Jang was jailed for a long time, and was expelled from the royal palace. Later events of his life, including the date of his death, were not recorded. It is unlikely, but possible that Jang Yeong-sil may have died during the reign of Joseon's 7th king, Sejo of Joseon (r. 1455-1468).

• Portrayed by Lee Chun-hee in the 2008 KBS2 TV series King Sejong the Great.
• Portrayed by Im Hyung-joon in the 2012 film I Am the King.
• Portrayed by Kim Seul-gi in the 2015 MBC TV series Splash Splash Love.
• Portrayed by Song Il-gook in the 2016 KBS1 TV series Jang Yeong-sil.