Stones in Beijing’s Forbidden City transported by ice sledge

New Scientist : The Forbidden City in Beijing, China, was built in the 15th century using large stones from a quarry 70 km away. Because some of the stones weigh more than 100 tons, and the wheeled vehicles of the time could not have supported so much weight, it has never been clear how the stones were transported. By studying a new translation of a text written in 1618, Jiang Li of the University of Science and Technology Beijing and Howard Stone of Princeton University believe they have the answer. The text describes how a 125-ton stone was moved by ice sledge for use during renovations some 150 years after the original construction. It indicates that wells were dug along the route to provide water to make ice on the roads, and that the sledge was “watered” to further decrease the friction as it slid. They calculate that the 125-ton stone could then have been pulled by a team of just 50 people. If their interpretation is correct, it appears that 15th-century China understood the properties of ice and friction in a way that wasn’t matched in the West until the 19th century.