The gates of most canal locks meet in a shallow V that points upstream, like a pair of doors that didn’t quite agree to shut flat. The usual date for the first ones in use is 1497, at the San Marco lock in Milan; Leonardo drew the design, though whether he invented it is still argued over. The V is the clever part. The higher water leans on the two leaves and presses them into each other, so the more water there is behind them, the tighter they close. No lock-keeper is strong enough to open them against that. You open a small sluice in the gate, let the two sides run level, and then a person can push them aside with one arm. A door that won’t move while there’s something to hold back, and gives as soon as there isn’t.