Chang Yŏngsil

Chang Yŏngsil (? – after 1442) was a Korean mechanical engineer, scientist, and inventor during the Joseon dynasty. He was born to a mother who was a government-registered courtesan and a father of Yuan dynasty descent. Although Chang was born a nobi, King Sejong gave him a court-technician position in the royal court.

Chang's inventions, such as the ch'ŭgugi (the rain gauge) and the water gauge, highlight the technological advancements of the Joseon dynasty.

Early years Chang Yŏngsil's precise date of birth is unknown and only recorded in the genealogy of the Chang family and his ancestry in the Veritable Records of the Joseon Dynasty. According to these records, his father, Chang Seong-hwi, was the eighth generation of the Chang family. Chang Seong-hwi was the third of five brothers, all of whom previously served as ministers in Goryeo. Historical records also mention his elder brother, Chang Seong-bal, who was born in 1344 and whose grave is located at Ui-seong in North Gyeongsang Province. Yŏngsil's mother was a government-registered courtesan, making Chang a government-owned nobia type of Joseon's lowest class.

Civil service Chang's fame gained him entry into the royal court in Hanseong (present-day Seoul), where select commoners displayed their talents before the king and his advisers. Sejong was impressed by Chang's craft and engineering skills and allowed Chang to work as a court technician in the palace. The talented scientists recruited under Sejong's new program worked at the Hall of Worthies.

Astronomical instruments

Chang was tasked to build a celestial globe to measure astronomical objects. Books obtained from Arabian and Chinese scholars contained incomplete instructions, for these devices could also be used for military purposes. After two months of study, Chang made a spherical device that performed with mediocre accuracy. In 1433, a year after his first attempt, Chang made an armillary sphere known as the honcheonui . 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 ) could measure time changes according to the seasonal variations. These instruments, along with the sundials and water clocks, were stationed around the Gyeonghoeru Pavilion in Gyeongbok Palace and put into use by the astronomers. The success of Chang'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 , 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 Ch'oe Yun-ŭi invented the world's first metal printing press in 1234 during the Goryeo period, Sejong asked scientists at Jiphyeonjeon to build a better printing press. In 1434, scientists accomplished building Gabinja , 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. Angbu-ilgu was bronze in composition, and consisted of a bowl marked with 13 meters to indicate time and 4 legs jointed by a cross at the base. Chang is often claimed to have invented Korea's first rain gauge in 1441, called cheugugi (측우기/), 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, Sejong asked the scientists to figure out some ways to inform the farmers of the available amount of water; and, in 1441, Chang invented the world's first water gauge, called Supyo (수표/). It was a calibrated stone column placed in the middle of a body of water, connected by a stone bridge.

Expulsion Chang's extraordinary accomplishments earned him the trust of Sejong. Some government officials were very jealous of Chang, especially when he had achieved so much despite his common origin.

In 1442, Sejong ordered Chang to build a gama, an elaborately decorated Korean sedan chair. The gama broke while Sejong was traveling, and Chang was held responsible. Although Sejong was against the decree, Chang was jailed for an extended period of time, flogged 80 times and expelled from the royal palace.