How Forbidden City was built: ice machine!

Not the moving parts kind, but the wedge/screw/lever kind. Want to move giant blocks of stone a few miles, but the locomotive and crane haven’t been invented yet? Nature‘s answer… by waiting until winter and freezing the streets:

Some of the largest stones used to construct Beijing’s Forbidden City beginning in 1406 were hauled from distant quarries on wooden sledges along ice roads, ancient Chinese documents have revealed. Calculations now reported in Proceedings of the National Academy of Sciences show that, on uneven, winding roads, this method is safer, more reliable and much easier than using wooden rollers or dragging the sledges over bare ground.

Many of the largest stones in the complex came from a quarry located about 70 kilometres from Beijing, says Howard Stone, a fluid mechanicist at Princeton University in New Jersey, and a member of the team that performed the study. “You go to the Forbidden City and see these massive rocks, and you ask yourself: ‘How in the world did they ever move this rock here?’” he says.

The ancient document reported that the monolith, a 49-cubic-metre slab estimated to weigh almost 112 tonnes, was hauled to Beijing in 1557 over the course of 4 weeks — which works out as an average speed of about 8 centimetres per second.

Dragging a 112-tonne sledge over bare ground would require more than 1,500 men, Stone and his colleagues estimate. Pulling the same sled across bare ice or across wet, wooden rails would require at least 330 men.

But if the stone-burdened toboggan were hauled along an ice road lubricated by a film of water, the researchers say, fewer than 50 men would be needed to tow the load.