The Forbidden City's Ice Road

A slab of stone lies right where it has just been split from the mountain.

The slab is 9.6 meters long, 3.2 meters wide and 1.6 meters thick. It weighs about 123 tons. It lies in the Dashiwo quarry, in the Fangshan area. It still has 70 kilometers to go before it reaches the place that needs it. That place is the Forbidden City in Beijing, where a great round of repairs is under way.

That is the whole problem. It is hard enough to deserve a long think before the answer comes.

The first idea is a cart. The wheeled carts of the time could carry 95 tons at most. This slab is much heavier than that. The wheels would break, and the axle would snap. A stone of that weight tipping over onto the road would be dangerous for the people and dangerous for the animals. Most of all, it would be dangerous for the stone itself. The slab is one solid block. A single crack ruins the whole block, and no one can join it back together.

The next idea is to drag it straight along the ground. That sounds safer, but the number is frightening. To slide 123 tons of stone over bare earth, more than 1,500 people would have to pull at once. And 1,500 people could hardly even fit around a slab 9.6 meters long. The ropes would have to be hundreds of meters long. The people at the front and the people at the back could not see each other, so they could never heave on the same beat. Over 70 kilometers, a haul like that would never end.

What holds the slab back is not its weight. It is friction.

Friction is something everyone has met and few people name. Dragging a bin of rice across rough concrete is heavy work for the arms. Pushed with the same strength across a smooth floor, the same bin moves far more easily. The bin has not become any lighter. Only the surface under it has changed.

So the question changes shape completely. It is no longer about where to find enough people. It is about how to turn 70 kilometers of dirt road into a slippery surface.

The answer began with waiting.

They did not set out in spring, and not in autumn either. They waited for the middle of winter, when the northern cold had settled in deep enough. The trip took place in the winter of 1557, and in the depth of that winter the air averaged about minus 3.7 degrees Celsius.

But waiting for winter was not enough. Winter gave them the cold, but it did not give them the road.

So they made the road themselves. All along the 70 kilometers, about every half kilometer, they dug a well. They drew the water up and poured it over the road. Meeting the cold, the water settled, spread flat and hardened. Stretch by stretch, day by day, the dirt road turned into a long ribbon of ice running toward the capital. They did not go looking for a slippery road. They made one.

By now the slab could breathe much more easily. But the road was still not slippery enough.

Ice, however smooth, is still a hard surface. Dry wood pressed onto ice with no water on it still grips a little. On bare ice, pulling that slab would still take about 338 people.

And this is the best part of the whole story.

They drew up water once more, and poured a very thin layer over the ice that had just frozen. It was only a thin film of water, so thin that at a glance the ice simply looked a little wet.

Yet that film changed everything. The bottom of the wooden sled no longer touched the ice. It rested on water. It slid on water.

Anyone who has walked barefoot on a tiled floor just after mopping knows that feeling at once. On a dry floor, a step is just a step. On a floor still holding a thin film of water, the foot slides right out. Water is more slippery than tile, and water is more slippery than ice too. The workers of that time did not need to read a single page of physics. They only needed to know one thing that every foot has already learned. Wet is more slippery than dry.

With that thin film of water, the 123-ton slab needed far fewer hands.

About 46 people. Only 46. Just a little more than one school class.

The cold of minus 3.7 degrees turned out to be just right, too. It was cold enough to keep the ice below as hard as stone. Yet it was not cold enough to freeze the freshly poured water right away. The workers had a short window of time, and they used exactly that window to pull.

The procession moved very slowly. Each second, the slab crept forward about 8 centimeters, less than the span of an adult hand. In front, people carried water from the wells, poured it down, then stepped back to clear the way. Behind them, the ice had already stiffened again. It left no mark except a long, shining streak.

So it went, day after day. Pour the water, wait, then pull. After 28 days, the slab arrived.

All this might have been forgotten. But 61 years later, someone wrote it down in a book, telling how the slab had traveled. Thanks to those few pages, we can still work out every step they took.

And that 123-ton slab was not even the largest.

In the Forbidden City, behind the Hall of Preserving Harmony, lies a carved stone slab. It is the largest in the whole palace, and its face is covered in patterns. When it was brought in, it is said to have weighed more than 300 tons. That is more than twice as heavy. And perhaps it too glided there, slowly, on a winter road made of water.

The name Forbidden City has a familiar ring in Vietnam, because Hue has a Forbidden City as well, inside the Imperial City. The two places lie very far apart, and their stories are entirely different. Only the name has long been familiar.

And that way of thinking is more familiar still. To move a big stone or a heavy pillar, people slip a few round logs underneath so it can roll. When a door creaks, a drop of oil goes into the hinge. It is the same idea every time. Do not try to be stronger. Make the place where things touch more slippery.

A film of water left on a freshly mopped floor is so thin it is almost nothing. Yet nearly 500 years ago, in a faraway land of hard winters, a film of water just as thin carried 123 tons of stone the whole 70 kilometers.