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How'd They Do That? The Story Of A Giant Rock And A Road Of Ice

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How'd They Do That? The Story Of A Giant Rock And A Road Of Ice

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How'd They Do That? The Story Of A Giant Rock And A Road Of Ice

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Great works of ancient engineering, like the Pyramids or Stonehenge, inspire awe. And sometimes they inspire people to sit down and try to figure out how they were made. An engineer from Princeton University had such a moment at the Forbidden City in Beijing, when he first saw a carved, 300-ton slab of rock that forms a ramp up to a building.

NPR's Christopher Joyce has the story behind this giant rock and a road made of ice.

CHRISTOPHER JOYCE, BYLINE: The Forbidden City is a city within a city of palaces and temples built in the 15th and 16th centuries. Many building entrances have these huge ramps, some covered in carvings.

Howard Stone remembers his first visit there.

HOWARD STONE: We found ourselves standing in front of this enormous rock. And I just asked, well, how in the world did it get here?

JOYCE: Stone - and no, there will be no puns on his name in this story - is an engineer who studies fluid dynamics. Turns out that was a lucky thing. Stone got some of his Chinese colleagues involved. They discovered a 500-year-old document that said these slabs came from a quarry 45 miles away. And it said the slabs came on a road of ice. So Stone set about calculating if that was possible. First, why ice? The Chinese had had the wheel already for 3,000 years.

STONE: The roads were pretty bumpy and rough. And one thing ice does is give you a pretty smooth surface.

JOYCE: The document was sketchy about technique but it did say workers dug wells along the route. So that would have provided water to make ice. It also said the job took 28 days in January, the depth of winter. Stone did some heavy math to see if indeed people could drag a slab weighing about 120 tons over an ice road for 45 miles in 28 days. Everything hinged on the amount of friction between a wooden sledge the rock apparently sat on, and the ice below.

STONE: Given the friction and given the mass of the rock, given the temperature conditions of Beijing in the month of January when it was mostly done, what are the typical numbers of people you would need for the dragging, and is this a plausible number?

JOYCE: A thousand people, for example, would just be too unwieldy. But the math said you could do it with about 300 if these ice-roaders kept lubricating the ice with water. That lowered the friction enough so that 300 people could have move the slab 20 feet in per minute. That's fast enough to match the 28 days recorded in the Chinese document.

Writing in the Proceedings of the National Academy of Sciences, Stone says it was an amazing feat.

STONE: You had on the order of a month to pull this off at the temperature conditions of Beijing. I think this just says a lot about their ability to engineer, their ability to plan.

JOYCE: Ancient Egyptians pulled off a similar delivery, sliding a 60-ton statue on wooden planks to a temple using water to lower the friction. In 1999, engineers moved the Cape Hatteras lighthouse in North Carolina about a half-mile on steel rollers and soap.

But the Chinese, Stone believes, apparently were the first to move a chunk of mountain on a road of man-made ice.

Christopher Joyce, NPR News.

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