Edmund Beckett Dennison, who might well be called an `amateur genius’, pushed his own design for the clock at Westminster against all opposition and his very original ideas were to prove most successful. Dennison advocated a gravity escapement, which is one in which the impulse is not given to the pendulum directly by the clock train, but indirectly through a second small weight or rod so that the impulse is kept constant.
This type of escapement, sometimes called a `remontoire’ ensures that the motion and therefore the time control of the pendulum is constant, so ensuring that the time is relatively constant. Dennisson had got the assistance of the clockmaker Frederick Dent and with his help had made a clock with an experimental gravity escapement which he submitted to the Astronomer Royal, George Biddell Airy, for testing at Greenwich. This performed so well that Airy gave his support to the incorporation of this escapement in the Westminster clock, Big Ben.
The final design was for a mechanism held in a flat-bed frame with arbors and barrels and wheels positioned along the frame and across the narrow width. Parts can be dismantled without the need to dismantle other parts of the clock. The whole has a double gravity-escapement. The pendulum beats every two seconds so that the minute hand jumps every two seconds. The minute hands are fifteen feet long and weigh over one hundred kilograms and the hour hand is nine feet long, so some idea of the forces involved can be appreciated. The striking mechanism was not too good at first owing to the time taken for the mechanism to gain momentum. It now commences the striking train for the hour at the fifth-eighth second, so that there is time for the strike to gain necessary momentum.
The bell involved proved to give as much difficulty as had the mechanism of the first bell cast in 1856 made at Stockton-on-Tees. The shape had to be modified so that it might fit in the tower as the bell was to be fourteen tonnes, one of the biggest ever cast. The modified bell was found to be some two tonnes over weight and when tested with the original hammer (over three hundred and fifty kilograms), produced too small a sound, so one of over six hundred and fifty kilograms was substituted. The bell when struck cracked.
A replacement was made by Mears of London, which proved satisfactory and the clock was started in May 1859. It was called Big Ben, after the portly Sir Benjamin Hall had made a long speech in Parliament asking for it to be given a dignified name. ‘Call it Big Ben’ and so it was. At 3.45 am on August 5, 1976 the clock was bought to a halt, although two wars had failed to stop its chime.
A shaft had failed and the striking weight fell down the tower, causing the barrel to jump out of the frame and the frame to crack. All assorted wheels were ripped out and the whole movement was severely damaged. Repairs took until May 4, 1977. The so-called Westminster chime was taken from a phrase in Handel’s Messiah and originally installed in a clock in Cambridge, Great St Mary’s Church. The familiar chime is now world renowned.
Dennison also made the large clock in St Paul’s.
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