Tower Bridge

Tower Bridge is in London, the capital city of the United Kingdom.

It crosses a wide river called the Thames. It has two tall towers that look like a castle. When a tall ship comes, the road in the middle lifts up!

Tower Bridge crosses the River Thames in London, right next to the Tower of London, an old castle. It is about 240 meters long, and its two towers rise high above the river.

Tower Bridge is a bascule bridge. The road between the towers is split into two halves, called bascules, which tip up like giant seesaws to let tall ships through. It opened in 1894 and still lifts around 850 times a year, free of charge, for any registered boat with a mast or upper deck 30 feet (about 9 meters) tall or more.

Britannica describes Tower Bridge as a double-leaf bascule bridge across the Thames between the boroughs of Tower Hamlets and Southwark, about 240 m (800 ft) long, built to complement the adjoining Tower of London; the official site gives the central span between the towers as almost 66 m.

It is owned and run by City Bridge Foundation, the charity historically called Bridge House Estates, whose sole trustee is the City of London Corporation and which also maintains London, Southwark, Millennium and Blackfriars bridges. Under the Corporation of London (Tower Bridge) Act 1885, the bridge must open for registered vessels with a mast or superstructure of 30 feet or more, free of charge with 24 hours' notice, day or night; the official site counts around 850 lifts a year, Britannica about 800.

Each half of the road is like a seesaw.

When one end goes up, the heavy other end goes down inside the tower. Big gear wheels turn to lift it. Long ago, steam engines gave the power. Now electricity does.

Each bascule works like a seesaw. It turns on a pivot, with a heavy counterweight on the end hidden inside the bridge, so it balances and needs much less energy to lift. Eight big cogs, gear wheels about 1 meter across, turn to open and close the road.

At first, steam powered everything. Big boilers made steam to drive pumping engines, which pushed water into huge tanks called accumulators that stored energy like batteries. Since 1976, electric pumps and oil have done the job instead. High above the road, two walkways join the towers, and since 2014 they have had glass floors.

Each bascule pivots off center around a balanced axis, so lifting needs little energy and puts no load on the towers; the counterweights swing down into the bascule chambers. Eight cogs about 1 m across, four on each side, engage racks on the bascules.

Originally three boilers, stoked around the clock, fed steam pumping engines that charged six hydraulic accumulators with high-pressure water, released on demand to drive the opening engines. Lifts became rare as shipping moved downstream, and an electro-hydraulic system, installed from 1974 while the steam plant kept working, took over in 1976. The high-level walkways, 33.5 m above the road, house the suspension cable that supports the full length of the bridge; closed from 1910 for lack of use, they reopened in 1982 and gained glass floors in 2014.