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 fortress 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. That is why a single fortress commonly took about ten years. When King Jeongjo decided to build a new fortress at Suwon, ten years was the first plan too.
The work was finished in two years and nine months. What was different? Ample money and labour, wages paid for the work done, careful design — all of these are reasons. And one of them was a set of newly built machines.
Three problems, three machines
A stone has three hurdles to clear before it sits on the wall.
- Loading — stone split at the quarry has to be lifted onto a cart. This was the geojunggi's job.
- Hauling — a cart loaded with heavy stone has to roll over slopes without tipping. This was the job of a cart called the yuhyeonggeo.
- Raising — the stone has to be lifted high, up 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 run over it, the same wheel that sits at the top of a school flagpole.
If the wheel is fixed in place, only the direction of the pull changes. Pulling down to lift up is convenient, but it takes just as much force. This is a fixed pulley.
Hang the wheel on the stone so that it rises and falls with it, and the story changes. Two strands of rope now hold the stone, and each carries half the weight. Only half reaches your hand. This is a movable pulley.
But nothing comes free
There is one thing that must be pointed out here. If eight strands share the stone, then to raise the stone one span, all eight strands must each shorten by one span. So your hands have to pull eight spans of rope.
The force has fallen to an eighth, but the length pulled has grown eightfold. Force multiplied by distance — the quantity science calls — is unchanged. A pulley does not reduce the work. It only lets a weak force do the same work over a longer pull.
Put in words, this is hard to feel. Pull the rope yourself on the board below. Each time you change the pulleys, the number of workers needed changes, and so do the squares the stone has risen and the squares of rope you have pulled.
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 geojunggi had one more thing attached. The rope was not pulled by hand; it was wound onto a windlass. A windlass is a wheel in which a large turn of the handle gives a small turn of the drum at the centre. If the handle is ten times the size of the drum, the force falls again to a tenth. This is the principle of 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 of force moves twenty-five thousand.Jeong Yak-yong, general note to the Gijung Doseol (put into plain words)
Forty geun is about the effort of lifting a sack of rice. Twenty-five thousand geun is the weight of three or four elephants. This is the claim that sounded like boasting at the start. The pulleys divide the force and the windlass divides it again, so as a calculation it is correct. On the real site, the of rope rubbing on wheel would have made it take rather more.
After Hwaseong was finished, King Jeongjo is said to have remarked that the geojunggi had saved a great deal of money. The sum on record is forty thousand nyang.
The cart that hauled and the pole that raised
The yuhyeonggeo did not simply rest its load bed on the wheels. The bed could tilt like a , so that the load stayed level on the way up and down a slope. A cart of heavy stone overturning on a slope was the worst accident a building site could have.
The nongno was the machine for lifting high. A long pole was set up at a slant with a pulley at its tip, and a windlass at the bottom wound the rope to draw the stone up to the height of the wall. The rebuilt nongno stands eleven metres tall. The idea of hanging a pulley from the end of a long arm to lift high is the same as in today's tower crane.
A little further in
The building site 230 years later
The tower crane on an apartment site today rests on the same principle. Look closely at the block of pulleys above the hook and you will see the rope running in two strands or four. With four strands it lifts something heavier, but the hook rises and falls more slowly. It is exactly the geojunggi's bargain.
A crane has one more problem that the geojunggi did not have: falling over. Hang a load from the end of a long arm and a force arises that tries to topple the mast. It is set not by the weight alone but also by how far from the mast the load hangs. This is called the , or turning force. That is why a crane carries a heavy block of concrete on the opposite side of the arm, and keeps to a chart in which the further out the load is pushed, the less it may lift.
On the board below, drag a load onto the hook and push it out to the end of the arm. You will see where the alarm starts to sound.
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 on Hwaseong was done, the court set everything about it down in a book: the Hwaseong Seongyeok Uigwe. The plans of the fortress, the machines drawn whole and taken apart, the names of those who worked, their dates and wages, even the quantities of stone and timber that went in — all of it was written down.
That record was put to use again more than 150 years later. Hwaseong had collapsed in many places through the Japanese colonial period and the Korean War, and in the 1970s it was rebuilt to its original form with the Uigwe open on the table. In 1997 it became a UNESCO World Heritage Site. A fortress that had fallen and been rebuilt could become World Heritage because exactly how it was built had been preserved.
The idea of threading several pulleys together to share a force is very old. The story goes that Archimedes hauled a ship single-handed with a set of pulleys. Jeong Yak-yong met that idea in a book and, like his contemporary Hong Dae-yong, reworked knowledge that had come from outside to fit his own country's circumstances.
What it takes to do this work
- Designing machines — this is mechanical engineering. You calculate pulleys, gears and shafts and put together devices that share force.
- Calculating buildings and structures — architectural and civil engineering. Working out where a crane must stand so that it does not fall over belongs here too.
- Driving a crane — you earn a tower crane operator's licence and sit in a cab tens of metres up.
- Crane signaller — from the cab the hanging load is hard to see. The signaller watches the load from the ground and becomes the operator's eyes, by hand signal and radio.
- Working stone — heritage repair stonemasons. They rebuild fallen fortress walls by the old methods.
The fourth and fifth lines are the unexpected places in this field. A crane operator often cannot see the load being lifted, and without a signaller can move nothing at all. And Hwaseong was rebuilt not by engineers' calculations alone but by hands that knew how to dress stone. These are paths that lead straight on from a technical high school or vocational training.
The question that remainsThe pulley reduced the force and lengthened the pull in exchange. Among the things that feel easy to us, which ones are in fact making us pay more of something else?