BEIJING, CHINA—Moving the massive stones used to build Beijing's Forbidden City in the 15th and 16th centuries required more than sheer manpower. According to medieval Chinese texts, teams of workers moved the stones on sledges in the depths of winter. Now a team of modern engineers says those workers likely dragged monoliths weighing more than 100 tons on ice roads lubricated with water. Princeton engineer and fluid mechanicist Howard Stone Sledges crunched the numbers, and estimates that it would take 1,500 men to drag a sledge carrying a 112-ton monolith over bare ground, while only 50 men would be needed haul it across a road covered in ice and a thin film of water. Though wheeled vehicles were introduced to China in the fourth century B.C., by the 1500s they still could not carry a load weighing more than 86 tons.