Completed in 1894, London's Tower Bridge remains a masterclass in Victorian mechanical engineering. Designed to accommodate tall river traffic without blocking land transportation, the bridge features two central bascules that raise up to 86 degrees. Each bascule leaf weighs roughly 1,000 tons, making manual or simple gear operation impossible without immense mechanical advantage.
Engineers John Wolfe Barry and Sir William Arrol solved this by installing a counterbalanced hydraulic drive mechanism beneath each tower. The bascules rotate on massive 21-inch steel pivot trunnions, balanced by hidden 422-ton lead and iron counterweights tucked beneath the tail arm of each leaf. Because the bascules are almost perfectly counterbalanced around their pivot point, remarkably little effort is required to move them.
Steam engines originally burned coal to pump high-pressure water into massive hydraulic accumulators containing 60-ton weighted rams. When released, this pressurized water drove hydraulic engines, which turned giant drive pinions that meshed with curved rack gear segments attached to the bascule arms. This system could lift the mammoth structures to their maximum angle in just 60 seconds.
Today, oil-hydraulic pumps have replaced the steam engines, but the original steel bascule trunnions and counterweights operate on the exact same physical principles.