TOWER BRIDGE TOWER BRIDGE In 1885 the British Parliament decreed that a new bridge had to be built across the River Thames.There were architectural and structural requirement. Architecturally the bridge had to be in harmony with the nearby Norman castle,the Tower of London's;structurally,it had to allow tall-masted ships through to the upper reaches of the Thames. The designers of Tower Bridge found ingenious solutions. This was possible also because in 1856 Henry Bessemer introduced a new method of purifying iron,the result of which was the production of steel. Steel, because it is stronger and more durable,started to replace cast and wrought iron in bridge construction. The use of iron with steel meant that building became an exact science. At first,a number of bridges and buildings collapsed because builders didn't know how to use iron with steel correctly. A steel framework provided the bridge a medieval flavour. The two central drawbridges,or bascules,could be raised by hydraulic engines to leave a space through which the largest of ships could pass.