Grain thieves in Song China prompted the invention of the pound lock
In the 10th century a Chinese tax official named Qiao Weiyue kept losing grain barges on the West River near Huai'an. He found soldiers at a double slipway were colluding with bandits to wreck heavy imperial boats and loot the spilled cargo. His fix in 984, a pair of gates enclosing a short basin, became the pound lock.
Qiao set his two gates roughly 250 feet apart and roofed the whole structure like a shed. Crews raised or lowered wooden beams in each gate to fill the chamber from the canal above or drain it into the canal below. The polymath Shen Kuo mentioned such locks in his Dream Pool Essays of 1088. On the Grand Canal, successive locks lifting boats 4 or 5 feet at a time raised the water level by 138 feet in all.
Europe caught up centuries later. A large basin lock serving many ships at once was built at Vreeswijk in the Netherlands in 1385, but the first true pound lock on the continent opened at Damme near Bruges in 1396. Between 1452 and 1458 the engineer Bertola da Novate built 18 of them on a canal near Milan. Much earlier, the Greek historian Diodorus credited engineers of Ptolemy II with inventing a lock around 274 BC to keep salt water out of the Nile.
Passing through is simple. Heading downstream, a boat waits while the chamber fills, enters through the upper gates, then sits as paddles drain the water down to the lower level before the lower gates open. The whole cycle usually takes 10 to 20 minutes, so boaters are glad to meet a vessel leaving toward them, since it leaves the lock set at their level. On staircase locks the opposite holds, and travelling in convoy is quicker and uses less water.
Engineers tried bolder ideas. Around 1800 Robert Weldon demonstrated a caisson lock for the Prince Regent in which a sealed wooden box carrying a barge moved up and down inside an 18-metre-deep tank, wasting no water but plagued by engineering problems. At Dalmuir on Scotland's Forth and Clyde Canal stands the world's only drop lock, built so a busy road could cross on a fixed bridge.
Source: Lock (water navigation)
