“To see barges waiting ...at a lock affords a fine lesson in how easily the world may be taken."
Robert Louis Stevenson, An Inland Voyage (1878)
Locks interrupt a canal or river with stepped stretches of still water, thus reducing currents in the waterway and conserving deep water for passage. The forerunner of today's lock was the flash lock, already in use by the first century B.C.E. in China, whereby part of a dam would be temporarily opened to allow passage of a vessel. Those traveling downstream were carried on the resulting surge of water, whereas those sailing in the opposite direction hauled the vessel against the torrent. Such an arrangement was dangerous and resulted in the loss of large quantities of water downstream for every vessel passing, a circumstance not appreciated by mill owners reliant on the supply. In 984, during the construction of China's Grand Canal, engineer Qiao Weiyo noted that in placing two flash locks 750 feet (229 m) apart, he had created an intermediate stretch of water that could be held at the level of either the upper or lower reach of the waterway, and thus the pound lock, or chamber lock, was born. Following that breakthrough, a significant improvement was the development of mitered lock gates in sixteenth-century Italy, perhaps based on the designs of Leonardo da Vinci. The miter uses the pressure of the high water on the upper side of the gate to create a secure seal until the water levels have equalized. This allowed the constructions required to withstand pent-up water to be less massive.
