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Haul hundreds of tons of stone with your bare hands

Records say that in 15th-century China, workers hauled 100-ton-plus stone blocks 43 miles from a quarry to build the Forbidden City, over man-made paths.

The workers dug wells every half-kilometer to reach water, which they would pour to create ice roads in the deep winter. Then they hauled the stones themselves. Now, six hundred years later, a new study sheds light on exactly how that would have worked.

Engineers from two Beijing universities and Princeton University in the U.S. got to wondering about the details of this operation. The modern-day engineering team did the math, calculating how much friction different hauling methods—including log rollers, a popular ancient transportation method—created. Their numbers show sliding over ice was a good choice, the researchers say, and that workers likely poured water in front of their sledges as extra lubrication.

The team calculated that recorded winter temperatures in Beijing for that era were cold enough to maintain ice roads. In fact, it was so cold that friction alone wouldn't create a good lubricating layer of water between the bottoms of the wooden sledges and the surface of the ice roads. However, it was just warm enough that water poured onto the road would stay liquid for a few minutes.

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