DEA/ W. Buss/De Agostini/Getty Images
The Large Stone Carving is the heaviest stone in the Forbidden City in Beijing. It was believed to have weighed more than 300 tons when it was first transported to the site between 1407 and 1420.
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.
The distance between the Forbidden City in Beijing (a) and the Dasiwo Quarry in Fangshan (b) is about 43 miles. Double lines on the map represent rivers; single lines represent roads.
The distance between the Forbidden City in Beijing (a) and the Dasiwo Quarry in Fangshan (b) is about 43 miles. Double lines on the map represent rivers; single lines represent roads. PNAS
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."
The Cape Hatteras Lighthouse in Buxton, N.C., was moved almost 3,000 feet inland on July 9, 1999, to protect it from the advancing ocean.
The Cape Hatteras Lighthouse in Buxton, N.C., was moved almost 3,000 feet inland on July 9, 1999, to protect it from the advancing ocean. Bob Jordan/AP
NYPL Digital Gallery
Judging from a carving found in an Egyptian tomb, experts say 172 men were used to transport the 60-ton Statue of Tehuti-Hetep, circa 1880 B.C. Water was used to lubricate the colossus as it slid along wooden planks.
Judging from a carving found in an Egyptian tomb, experts say 172 men were used to transport the 60-ton Statue of Tehuti-Hetep, circa 1880 B.C. Water was used to lubricate the colossus as it slid along wooden planks. NYPL Digital Gallery
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.
But the Chinese, Stone believes, appear to be the first to have moved a chunk of mountain along a road of man-made ice.
|ca. 1880 B.C.
||Statue of Tehuti-Hetep, El-Bersheh, Egypt
||Sliding on wooden planks
|ca. 400 A.D.
||A stone, Japan
||Log rollers on a dirt road
||A stone, Beijing, China
||Sliding on ice
||A team of men
||Mining equipment, Canada
||Sliding on ice
||Cape Hatteras Lighthouse, North Carolina
||Steel roller on steel tracks
||Five 30-ton jacks
Source: Jiang Li, Haosheng Chen and Howard A. Stone, PNAS