When human strength runs short, what can make up the difference?

That time, that place

King Jeongjo decided to build a new fortress at Suwon. The design went to Jeong Yak-yong, thirty-one years old and back in his home village mourning his father.

The great problem in building a fortress is stone. Each block weighs as much as dozens of adults, and it has to be split from the hillside, loaded on a cart, hauled, and raised.

Until then that work was done by shoulders and rope. A bigger job simply meant bringing in more people. So the first estimate for the work was ten years.

Why this question

Jeongjo sent Jeong a book from the palace: the Qiqi Tushuo, in which the missionary Johann Schreck (Terrenz), working in China, had introduced Western machines in pictures.

Several of its drawings showed heavy things lifted by rope run over wheels. Pulleys. It is the same idea as in the story of Archimedes hauling a ship alone with a set of pulleys.

Jeong did not copy the drawings as they stood. He chose what a Joseon carpenter could build from wood and rope, and worked it out again.

Four wheels above and four below put eight strands of rope on the stone. Eight strands share the weight, so the hand carries one eighth. To this he added a windlass to wind the rope, dividing the force once more.

He set the calculation down in an essay, the Gijung Doseol, and presented it to the king: a force of forty geun could move twenty-five thousand.

What was found

Hwaseong was finished in two years and nine months. Jeongjo is recorded as saying the geojunggi had saved forty thousand nyang.

Two things are worth setting straight.

First, the geojunggi was not the machine that raised stone onto the walls. It was used mainly at the quarry, lifting stone onto carts. Hauling was the job of a cart called the yuhyeonggeo; lifting high was the job of a tall pole called the nongno. Three problems, three machines.

Second, the force did not come free. Cut the force to an eighth and you must pull eight times the rope. A machine does not remove the work; it lets a weak force do the same work over a longer pull.

The machines were not the only reason the work went fast. The labourers were not simply conscripted; they were paid for the work they did.

A few years later Jeongjo died, and Jeong Yak-yong was exiled to Gangjin, his brother Jeong Yak-jeon to Heuksando. He never got another chance to build machines. What remained was the official record of the works, the Hwaseong Seongyeok Uigwe, with its drawings of the geojunggi and its parts.