Sliding heavy stones to the Forbidden City on ice.

Why Ice? From ancient Chinese literature, Li et al. (1) learned that the Large Stone Carving (>300 tons) was transported to the Forbidden City on a wooden sledge that was hauled by men across an icy road, during the period between the winter solstice (December 22 ± 1 d) and late January when the average temperature in Beijing at the time was found to be −3.7 ± 0.5 °C. Although a detailed account of that event could not be found, Li et al. found a record of winter transport in 1557 via sledge of a somewhat smaller stone (9.6 m × 3.2 m × 1.6 m, ∼123 tons) along an icy road over a distance of 70 km in 28 d. The ice was made from water drawn from a series of nearby wells. That the road could support the load follows from the fact that the pressure beneath the runners, the width of which was about 20% of the width of the sledge, was ∼180 kPa, lower by a factor of ∼67 than the indentation pressure or hardness of warm ice when slowly loaded (2). Li et al. (1) then ask three questions: Why did the Chinese use a sledge when wheeled vehicles were available? Was sledging on ice a good choice compared with other methods? What special features of lubrication were used? Regarding the first question, why the Chinese used a sledge when wheeled vehicles were available, the literature indicates that the carrying capacity of wheeled vehicles of the time was ∼95 tons. Had that method been used, failure would have endangered not only the men and mules that would have been employed, but also the expensive stones themselves: They were vulnerable to fracture when dropped. Sledging, although slower, is thus seen to have been the more reliable method. Regarding the second question, whether sledging on ice was a good choice compared with other methods, rolling on logs would have been the alternative choice. However, the unevenness of the road and the necessity of orienting logs perpendicular to the direction of movement of the advancing front was a challenge. So too, was the frictional resistance of logs rolling on a dirt road, even one lubricated with oil. The attendant coefficient of kinetic friction is ∼0.21: When