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Jang Yeong-sil is often called the inventor of the world's first standard rain gauge. The court records of his time tell a more careful story. This post sets out what the Annals of the Joseon Dynasty say about him and the rain gauge, and why measuring rain as a depth was the clever part.
Who was Jang Yeong-sil?
The main source is the Veritable Records (Sillok) of King Sejong, published online by Korea's National Institute of Korean History. An entry for the 16th day of the 9th month of 1433 records King Sejong asking his ministers to give Jang the military rank of hogun.
In that entry the king explains Jang's background. His father came from the Suzhou and Hangzhou region of Yuan China and his mother was a gisaeng, a woman registered to the government as an entertainer. That made his social standing very low. The king says Jang's skill was beyond ordinary people, that the previous king Taejong had protected him, and that ministers had once argued over whether someone of his birth could serve in the royal workshop. Sejong then gives his reason for the promotion: Jang had built a self-striking water clock. The king says he guided the work, but that without Jang it could not have been made, and that it surpassed a self-striking clock reported from Yuan China.
What did the water clock do?
An entry for the 1st day of the 7th month of 1434 says a new water clock came into use that day. Water flowed from four supply vessels into two receiving vessels, where floating rods rose against marked scales. The king worried that the men who announced the hours would make mistakes, so he ordered hogun Jang Yeong-sil to build wooden figures that announced the time by themselves, without human help. The entry describes three figures that struck a bell, a drum and a gong, and twelve more that held up tablets for the hours. Small copper balls, released as the water level rose, set the machinery off.
The Annals name him in other work too. In 1434 he was one of the officials put in charge of casting a new set of metal printing type, more than 200,000 pieces. In 1438 the Heumgyeonggak, a hall of astronomical instruments beside the king's quarters, was completed. Jang built it; the record says its design came from the king.
Did Jang Yeong-sil invent the rain gauge?
The records do not say so. In an entry for the 29th day of the 4th month of 1441, King Sejong explains that the Crown Prince, worried about drought, used to dig into the ground after each rain to see how far the water had soaked in. Because that could not give an accurate amount, he had a copper vessel cast and placed in the palace to collect the rain.
Four months later, on the 18th day of the 8th month of 1441, the Ministry of Taxation (Hojo) pointed out the real problem. Provincial governors already reported rain, but soils differ in how dry or wet they are, so the depth soaked into the ground tells you little. The ministry proposed an iron vessel on a stone platform at the royal observatory, plus marked water-level posts in a Seoul stream and on rocks beside the Han river.
The final rule came on the 8th day of the 5th month of 1442. The iron vessel was named cheugugi, "rain-measuring instrument". It was 1 cheok 5 chon deep and 7 chon across, measured with a standard ruler called the jucheok. After every rain, officials were to measure the depth, write down the date and time the rain started and stopped, and report it. Every province received an iron gauge and a ruler, and each county made copies in porcelain or earthenware.
Jang Yeong-sil is not named in these three entries. He may have helped make the instruments, but these records do not show it.
Worked example: why a depth, not a volume?
Here is a classroom example with sample numbers. A cylindrical gauge 14 cm across collects 770 cm³ of water.
Area of the opening. r = 7 cm, so A = πr² = 3.1416 × 49 ≈ 153.9 cm².
Depth. depth = volume / area = 770 / 153.9 ≈ 5.0 cm, or 50 mm of rain.
A wider gauge. Double the diameter to 28 cm and the area becomes four times larger, so it collects about 3,080 cm³. The depth is still 3,080 / 615.8 ≈ 5.0 cm.
The depth does not depend on the gauge's size, so counties can compare records. The 1442 rule still fixed one design and one ruler, so everyone measured the same way.
Common mistakes: dividing the volume by the diameter instead of the area, and mixing centimetres with millimetres. A useful check: 1 mm of rain over 1 m² is 0.001 m³, which is 1 litre.
If unit conversions like this trip you up, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. Our guide to significant figures and units covers the habits behind them.
What happened to Jang Yeong-sil?
The Annals end his story sharply. In the 3rd month of 1442 a royal palanquin whose building he had supervised broke, and he was sent for questioning. The court's report asked for 100 strokes of the rod; the king reduced his sentence by two grades. Days later ministers advised that his official certificates be withdrawn, and the king agreed.
What can a student take from Jang Yeong-sil's way of working?
Make the machine do the boring part. His figures took the error-prone job of calling the hours away from tired people. In study, a fixed routine or checklist does the same for careless slips.
Measure what can be compared. Soaked-in depth varied with soil; collected depth did not. Choose quantities that mean the same thing everywhere.
Check the record. Even a famous name deserves a look at what the sources actually say.
Next step: repeat the worked example for a gauge 20 cm across that collects 1,570 cm³, and check that you get the same kind of answer.
