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20131104_atc_04.mp3?orgId=1&topicId=1024&p=2&story=242305465&t=progseg&e=243035679&seg=4&ft=nprml&f=242305465

Great works of ancient engineering, like the Pyramids or Stonehenge, inspire awe in every beholder. But some onlookers also get inspired to figure out exactly how these structures were made.

Howard Stone, an engineer from Princeton University, had such a moment in Beijing's Forbidden City — a city-within-a-city of palaces and temples built in the 15th and 16th centuries. A carved, 300-ton slab that formed a ramp to one structure particularly caught Stone's eye. "How in the world did it get here?" he wondered.

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 says the slabs came from a quarry 45 miles away, transported over a road of ice. So Stone set about figuring out what that would have required.

First, why rely on ice? The Chinese had had the wheel for 3,000 years already. "The roads were pretty bumpy and rough," Stone explains, "and one thing ice does is give you a pretty smooth surface."

The document is sketchy about the technique but it does say workers dug wells along the route. That would have provided water to make the ice. It also says the job took 28 days in January — during the depth of winter.

"Given the friction and ... the mass of the rock, ... the temperature conditions in Beijing in the month of January when it was mostly done, what are the typical numbers for people you would need for the dragging?" Stone asked himself. "And is this a plausible number?"

He did some heavy math to see how many people would be required to drag a slab weighing about 120 tons (the weight the document described) over an ice road for 45 miles in 28 days. Everything hinged on the amount of friction that existed between a wooden sledge the rock sat on and the ice beneath the sledge.

A thousand people seemed just too unwieldy a group to drag this block of stone along a road, the engineer thought. But the math said you could do it with about 300 people — if these ice-road workers kept lubricating the ice with water. Doing so would lower the "coefficient of friction" of a wooden sledge-on-ice enough, he figured, so that 300 people could move the slab 20 feet per minute. (Speed skaters understand this phenomenon well — the ability to skate faster when there's a film of water on the ice.)

Sure enough: 20 feet per minute turns out to be fast enough to match the 28 days recorded in the Chinese document for the slab's journey to the Forbidden City.

Writing in the Proceedings of the National Academy of Sciences, Stone says it was an amazing feat. "You had on the order of a month to pull this off at the temperature conditions of Beijing," Stone says. "I think this just says a lot about their ability to engineer, their ability to plan."

Ancient Egyptians pulled off a similar delivery, sliding a 60-ton statue on wooden planks to a temple. They, too, used water to lower the friction. And in 1999, engineers moved North Carolina's Cape Hatteras lighthouse — more than 4,800 tons — more than a half-mile on steel rollers greased with soap.

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