In central Scotland the Forth and Clyde Canal and the Union Canal sit 35 meters apart in height. Boats once crossed through an 11-lock flight that used about 3,500 tons of water per trip and took most of a day. Road and rail made it obsolete, the locks were dismantled in 1933, and parts of the Union Canal were filled in and piped under housing. The 78 million pound Millennium Link set out to reconnect Glasgow and Edinburgh by water, but its 1999 boat Ferris wheel design was rejected as boring. Architect Tony Kettle of RMJM got three weeks to redesign it, built gear concepts from his eight-year-old daughter’s Lego, and drew on a double-headed Celtic axe, a ship’s propeller, and a whale’s ribcage.

The site was an old fireclay mine, coal mine, and tar works laced with tar and mercury, so 30 piles 22 meters long were driven to bedrock, and an aqueduct once called unbuildable was reinforced with 40 millimeter rebar. The wheel was made in Derbyshire, trucked north on 35 lorries, and joined with more than 14,000 bolts instead of welds to absorb stress reversals. Each caisson holds 250,000 liters and always weighs 500 tons, because a boat pushes out its own weight in water, so ten hydraulic motors turn it on about 1.5 kilowatt hours. Just before a May 2002 opening by Queen Elizabeth II, vandals flooded the dry pit and caused 350,000 pounds in damage, delaying it a month. It has since drawn 5.5 million visitors.

• Why bolted joints let the structure flex safely and give it a design life of at least 120 years
• Archimedes’ principle keeping both caissons balanced whether or not boats are aboard
• An eight meter fixed central gear and idler gears that keep the caissons level through the whole turn
• A pumped-dry pit under the lower caisson so the arm never fights canal water buoyancy
• The wheel lifts boats 24 meters, so two old-fashioned locks still finish the climb to the Union Canal