How it began
In late April 1441 the Crown Prince of Joseon, who later reigned as King Munjong, set a bronze vessel in the palace court and measured the depth of the rain it collected with a ruler. The experiment succeeded, and four months later the Ministry of Taxation proposed making iron cylinders about 16 centimeters across and 41 centimeters long for central and provincial offices. About a year later King Sejong ordered the system finalized, and the device received the name cheugugi. It is often wrongly believed that Sejong or the inventor Jang Yeong-sil made it.
Before it existed
Joseon was an agrarian society centered on rice farming, and harvests were tied directly to state finances, so the court needed to know how much rain fell in each region. A system for reporting regional rainfall already existed, but the method was to judge how deep the rain had soaked into the soil, for example by pushing a finger into the damp ground. The results varied with the person, the region and the properties of the soil.
How it works
The gauge is an open cylinder, at first iron and later also copper or ceramic, set on a stone stand high enough that splashed water cannot enter. The finalized 1442 vessel was about 14 centimeters in diameter and 31 centimeters deep. After rain, an official dipped a standardized ruler into the collected water and recorded the depth, along with the times when the rain began and stopped.
What changed first
A gauge was installed at the government office in charge of astronomy in the capital, and copies modeled on the ones cast in Seoul were placed in the courtyards of offices in each district. District heads measured and reported the rainfall, provincial governors collected the figures and sent them to the central government. Sejong's achievement, in one historian's summary, was the system that let the court monitor rainfall across Joseon.
What lasted
Official rainfall measurements made with the gauge survive for the period from King Jeongjo's reign to Emperor Gojong's. Only one old gauge is known to remain, the Geumyeong Cheugugi, which was installed at the provincial office in Gongju in 1837 and is a designated national treasure. Several of the stone stands also survive, including ones from Daegu and Changdeokgung Palace.
