The Peterborough Lift Lock
Opened in 1904, with a lift of 19.8 meters, this is the highest hydraulic lift lock in the world and the first of two built in North America, both on the Trent-Severn Waterway. It operates on a balance principle. A constant water pressure supports each ram through connecting pipes below ground. When extra water is let into the upper chamber, a connecting valve is opened and the heavier chamber automatically descends forcing up the lower chamber to start a new cycle. Among innovations on this lift were the use of concrete, air-filled seals, drop gates, and cast-steel presses for the rams.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.