Slip sliding away: China understood friction long before the West (Image: Reuters/Guang Niu)

Fifteenth-century Chinese engineers didn’t so much reinvent the wheel as dispense with it altogether – opting to drag heavy stones for building the Forbidden City along a slippery artificial ice road instead of wheeling them.

That, at least, is the upshot of the latest analysis of a 17th-century Chinese text. Such a method would be 10 times as efficient as dragging the rocks along non-icy ground – suggesting that Chinese engineers had a more sophisticated understanding of friction than their Western counterparts at that time.

The Forbidden City, in what is now Beijing, housed China’s emperors for almost 500 years. Several of the massive stones incorporated into its design were extracted from the Dashiwo quarry. That is 70 kilometres away – and the stones weigh in excess of 100 tonnes.


Previous attempts to explain how such heavy stones made the long journey assumed sturdy wheeled vehicles were used. This is partly because large wheeled vehicles have been used in China for 2000 years, and partly because there are no images from China of colossal stones being dragged along the ground by teams of men. However, this was somewhat puzzling as even the biggest wheeled vehicles of the time could support no more than 95 tonnes.

Ice that road

Now a fresh translation and analysis of a Chinese text written in the year 1618 offers an explanation. The text describes how a 125-tonne stone was transported to the Forbidden City in 1557 – about 150 years after it was first built – to help with renovation work following a fire. Transportation occurred during winter, and the text explains that it involved digging wells every 500 metres along the route “to supply water for watering and running the sledge”.

Jiang Li at the University of Science and Technology Beijing, who helped translate the text, and Howard Stone at Princeton University, think they have worked out what the authorities were doing.

The mid-winter temperatures in China at the time were about -4 °C, so water poured onto the chilled ground would freeze to create an icy path. A wooden sledge carrying the heavy stone could then slip along the ice road.

Li and Stone calculated that the friction between the sledge and the ice would be low enough to require a team of about 340 people to move the 125 tonne stone – simply dragging the stone along a dirt road with no ice would have required a team of 1500, they calculated.

Formidable knowledge

What about the reference to watering the sledge? Pouring further cold water on ice lubricates the surface and would lower the friction between the sledge and the ice even more, says Li. The team’s calculations suggest that doing so would have allowed just 50 people to move the 125 tonne stone. “Sliding with additional water as a lubricant was the most likely method,” she says.

“It is possible this would have worked,” says Robert Wood, who researches tribology – the study of friction – at the University of Southampton, UK. “But they would have had to keep the blocks moving as stopping may cause the meltwater formed at the interface between stone/wood and ice to refreeze and make the block very difficult to move again.”

The findings suggest the 15th-century Chinese understood the properties of ice in a way that Western society could not match until the 19th century – even though they did not formalise that knowledge in a way that modern scientists and engineers would recognise.

Their practical knowledge of friction was formidable, says Wood. Chariots preserved with the 2200-year-old Terracotta Army showed they even used wheel bearings to reduce the friction as the wheels spun, he says.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1309319110