Tower Bridge

The various incarnations of London Bridge were the only crossing over the Thames in London for around 1700 years. As the population of the city grew and expanded beyond the walls of The City, more bridges were constructed in the 19th century, however these were all to the west of London Bridge, as the vast expanse of docks to the east required sufficient clearance for boats to pass underneath them.
Horace Jones - architect of The City - met the needs of both the expanding population to the east and the docks, by incorporating a 'bascule' - or see-saw - design, so that the centre of the bridge could be lifted to allow boats to pass through, with high walkways between the towers to allow pedestrians to cross even when the bridge was raised.
Opened in 1892, the building took 432 workers six years to complete. The structure is steel, which is clad in granite and stone in order to fit in with the Tower of London which it approaches from the south.
Originally the bridge was lifted by steam and hydraulics, taking only a minute to rise to its fullest extent. The steam has now been replaced by oil and electricity.
In 1952, a London bus had to leap from one bascule to the other when the Bridge began to rise with the bus still on it.