Technically it’s ancient technology. But now the two-millennia-old principle of the Greek mathematician Archimedes has been deployed at gargantuan scale. The Three Gorges Dam, China’s marvel on the Yangtze River, is one of the world’s largest engineering projects—the product of 37 million cubic yards of concrete. Its final feature, inaugurated in late 2016, is a new ship lift, a hydraulic seesaw that raises and lowers vessels as many as 371 feet to traverse the dam.
Archimedes’ notion was simple: The weight of a buoyant object is equal to the weight of water it displaces. Take two identical chambers filled with equal amounts of water. They will balance on a scale. Add an object—e.g., a ship—to one of them, and let water of an equal weight out. The two chambers will remain balanced. Remove water from one chamber, and that chamber will slowly rise.
A system designed to accommodate ships up to 3,000 metric tons is a little more complex. The dam first opened with a series of locks, similar to the Panama Canal’s. The new ship lift raises and lowers boats using cables, a basin, motors—and simple gravity. Concrete counterweights in addition to water keep the system balanced, as do high-tech safety stops.