Concepts explained
The Geojunggi and Hwaseong Fortress — The Machines That Finished a Ten-Year Job in Under Three
Hwaseong Fortress was planned as a ten-year job and finished in two years and nine months. The credit usually goes to Jeong Yak-yong's geojunggi, but the geojunggi did not lift stones onto the walls. Three problems — loading cut stone, hauling it, raising it high — had three machines, and all three ran on one principle: you can save force, but nothing comes free.
How many stones go into a fortress
A wall is stacked stone, and each block weighs as much as dozens of adults. It must be split from the hillside, loaded onto a cart, hauled over the passes, and finally raised several times a person's height.
In old building work all of this was done by shoulders and rope, and a fortress commonly took about ten years. When King Jeongjo decided to build a new fortress at Suwon, ten years was the plan.
It was done in two years and nine months. Ample money and labour, wages paid by the work done, careful design — all played a part. So did a set of new machines.
Three problems, three machines
- Loading — lifting cut stone at the quarry onto a cart. This was the geojunggi's job.
- Hauling — keeping a heavily loaded cart from tipping on a slope. This was the yuhyeonggeo cart's job.
- Raising — lifting stone high onto the wall. This was the nongno's job: a long pole with a pulley at its tip.
A pulley is a wheel that shares the load
The geojunggi's secret is the — a grooved wheel with a rope over it, like the one at the top of a school flagpole.
Fixed in place, a pulley only changes the direction of pull; the force is the same. Hung on the stone so it rises with it, it changes everything: two strands now hold the stone and each carries half.
But nothing comes free
To raise the stone one span, all eight strands must each shorten one span, so your hands pull eight spans. Force falls to an eighth; distance grows eightfold. Force times distance — what science calls — is unchanged. A pulley does not reduce the work; it lets a weak force do it for longer.
Try it on the board below. Each time you change the pulleys, the number of workers needed and the spans pulled change together.
Tool The Pulley Board — How the Geojunggi Lifted a Thousand-Geun Stone Haul a thousand-geun stone onto a cart with a golden handle. Switch from a fixed pulley to a movable one, to Jeong Yak-yong's geojunggi, to a windlass, and twenty-five workers become four, then one — while the rope you pull grows longer.Add a windlass and one person lifts it
The rope was wound on a windlass rather than pulled by hand. A handle ten times the radius of the drum cuts the force by ten again — the .
With two four-wheeled pulley blocks, forty geun of force moves a thousand geun; add a windlass whose handle is ten times the drum, and forty geun moves twenty-five thousand.Jeong Yak-yong, general note to the Gijung doseol (paraphrased)
Forty geun is about a sack of rice; twenty-five thousand is three or four elephants. On paper the claim holds; on site, in rope and wheels would have taken its share. After the work, King Jeongjo is recorded as saying the machines saved forty thousand nyang.
The cart that hauled and the pole that raised
The yuhyeonggeo let its load bed tilt like a , keeping the stone level on slopes, since an overturned cart was the worst accident on site. The nongno stood a long pole at an angle with a pulley at its tip and wound the stone up to wall height; the reconstruction stands eleven metres tall.
A little further in
The building site 230 years later
A tower crane's hook block is reeved with two or four falls of rope. Four falls lift more but raise the hook more slowly — the geojunggi's bargain exactly.
A crane has one problem the geojunggi did not: tipping over. A load at the end of a long arm creates a turning effect that depends not only on weight but on distance from the mast — the . So cranes carry concrete counterweights, and the further out the load, the less they may lift.
Tool The Tower Crane Board — Why a Load Far Out Is Dangerous Hang a load on a tower crane and push the hook out along the jib. The further from the mast, the less it may lift, until the alarm sounds. Four falls of rope double the load near the mast but do nothing far out.A book rebuilt the fortress
When the work ended, the court recorded everything in the Hwaseong Seongyeok Uigwe: plans, machines drawn whole and taken apart, workers' names, dates and wages, quantities of stone and timber. Much of the fortress was damaged under colonial rule and in the Korean War; in the 1970s it was rebuilt from that record, and in 1997 it became a World Heritage Site.
Sharing a load among many pulleys is an old idea; Archimedes is said to have hauled a ship single-handed with a compound pulley. Jeong met the idea in a book and, like his contemporary Hong Dae-yong, reshaped knowledge from abroad to fit his own country.
What you would study to do this
- Designing machines — mechanical engineering: pulleys, gears and shafts that share force.
- Calculating structures — architectural and civil engineering, including where a crane may stand without tipping.
- Operating cranes — a tower crane licence and a cab tens of metres up.
- Crane signaller — the operator often cannot see the load; the signaller on the ground is their eyes.
- Working stone — heritage stonemasons who rebuild fallen walls the old way.
The last two are the unexpected ones, and lead straight from technical high school or vocational training.
The question that remainsThe pulley traded force for distance. Of the things that feel easy to us, which are quietly charging us something else?