The clock tower, designed by Augustus Pugin and Charles Barry for the rebuilt Houses of Parliament, was completed in 1859, and the clock was installed that April. Designed by lawyer and amateur horologist Edmund Beckett Denison with the Astronomer Royal, George Airy, and built by the Dent firm, it had to strike the first blow of each hour correct to within one second a day. It started on May 31, 1859. The Great Bell first chimed on July 11 and cracked that September.
The specifications were so demanding that many clockmakers said they could not be met. The delay in finishing the tower proved useful: it gave Denison years to experiment with the mechanism before installation, and he invented the double three-legged gravity escapement, which separates the pendulum from the forces of wind, rain and snow acting on the huge hands. It became standard in large tower clocks.
The bells were less fortunate. The first great bell, hung in New Palace Yard because the tower was unfinished, cracked beyond repair during testing. Its replacement, cast at the Whitechapel Bell Foundry and weighing about 13.7 tonnes, rang for only a few weeks in 1859 before it too cracked. Workmen chipped a square out of the rim, gave the bell an eighth of a turn and fitted a new hammer, and it has rung with a slightly different tone, crack unrepaired, ever since.
The tower, 316 feet high and renamed Elizabeth Tower in 2012, remains one of Britain's best-known landmarks, and the chimes have been broadcast by the BBC since the last day of 1923.
Key people: Edmund Beckett Denison, George Airy, Edward John Dent