Edited* by John W. Hutchinson, Harvard University, Cambridge, MA, and approved October 1, 2013 (received for review May 30, 2013)
In the history of mechanical engineering, it is commonly believed that transporting huge stones with sliding sledges hauled by men did not occur in ancient China, because there were well-developed wheeled vehicles in China around 1500 B.C. However, some books contain brief remarks that the Large Stone Carving in the Forbidden City in Beijing was pulled to the site with a sledge sliding on an artificial ice path. Here we investigate the Chinese case based on historical documents and modern experimental results. We demonstrate that wood-on-ice sliding is more reliable and efficient than other lubrication methods, and pouring water on the contact surface is necessary to make this type of transport feasible.
Lubrication plays a crucial role in reducing friction for transporting heavy objects, from moving a 60-ton statue in ancient Egypt to relocating a 15,000-ton building in modern society. Although in China spoked wheels appeared ca. 1500 B.C., in the 15th and 16th centuries sliding sledges were still used in transporting huge stones to the Forbidden City in Beijing. We show that an ice lubrication technique of water-lubricated wood-on-ice sliding was used instead of the common ancient approaches, such as wood-on-wood sliding or the use of log rollers. The technique took full advantage of the natural properties of ice, such as sufficient hardness, flatness, and low friction with a water film. This ice-assisted movement is more efficient for such heavy-load and low-speed transportation necessary for the stones of the Forbidden City. The transportation of the huge stones provides an early example of ice lubrication and complements current studies of the high-speed regime relevant to competitive ice sports.
Author contributions: J.L., H.C., and H.A.S. designed research; J.L. and H.C. performed research; J.L. analyzed data; and J.L., H.C., and H.A.S. wrote the paper.
The authors declare no conflict of interest.
↵*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1309319110/-/DCSupplemental.
