The heavy stones of the Forbidden City were probably slid into Beijing on ice paths.
Chinese workers could have used wheeled carts to move the 100-ton stones. But the laborers probably preferred sliding the chunks across ice because wheeled carts could hit bumps in the road and cause damage to the expensive stone, researchers report November 4 in the Proceedings of the National Academy of Sciences.
The researchers translated a 500-year-old document describing how Chinese workers slid a 120-ton stone on water-lubricated wood along an ice path into Beijing in the winter of 1557 A.D.
Using the historical account, the scientists calculated that workers could slide the load at about eight centimeters per second, fast enough to move it forward before the water froze. The method would require only 50 men, compared to the 1,500 needed to pull the same load across the ground, the team suggests.