patented his design, and it quickly became the standard on all new high-quality tower clocks. On top of the pendulum is a small stack of pre-decimal penny coins; these are to adjust the time of the clock. Adding a coin has the effect of minutely lifting the position of the pendulum's centre of mass, reducing the effective length of the pendulum rod and hence increasing the rate at which the pendulum swings. Adding or removing a penny will change the clock's speed by 0.4 seconds per day. It keeps time to within a few seconds per week. It is hand wound (taking about 1.5 hours) three times a week. The Keeper of the Clock is responsible for looking after the movement in addition to overseeing every aspect of maintenance around the Palace. A team of horologists are on call 24 hours a day to attend to the clock in the event of an emergency. On 10 May 1941, a German bombing raid damaged two of the clock's dials and sections of the tower's stepped roof and destroyed the House of Commons chamber. Architect Sir Giles Gilbert Scott designed a new five-floor block. Two floors are occupied by the current chamber, which was used for the first time on 26 October 1950. The clock ran accurately and chimed throughout the Blitz. Breakdowns and other incidents 19th century Before 1878: The clock stopped for the first time in its history, "through a heavy fall of snow" on the hands of a clock face. 21 August 1877 – January 1878: The clock was stopped for three weeks to allow the tower and mechanism to be cleaned and repaired. The old escape wheel was replaced. February 1900: The heavy build-up of snow on a clock face impeded the progress of the hour hand, causing the clock to stop for about eight hours. 20th century 1916: For two years during World War I, the bells were silenced and the clock faces were not illuminated at night to avoid guiding attacking German Zeppelins. The bells were restored at 11 a.m. on 11 November 1918 to mark the end of the war. 29 December 1927: Snow build-up on a clock face stopped the clock. Winter 1928: Heavy snow stopped the clock for several hours. 2 April 1934: The clock stopped from 7:16 a.m. to 1:15 p.m., when it was repaired. 23 September 1936: A painter painting the