DICTATION 10

       At 3:45 p.m. on August 6th 1976 Big Ben stopped working.

       The engineers who repaired the clock believe that the breakdown was the result of metal fatigue. Metals deteriorate owing to repeated stresses above a certain critical value.

       Part of the clock mechanism fractured. As a result, the speed of the gear wheels increased from above one and a half revolutions per minute to 16,000 rpm. The resulting centrifugal force threw pieces of the clock mechanism in all directions. It also damaged the mechanism which drives the clock’s hands.

       The metal fatigue had never been noticed, as fatigue cracks are invisible to the naked eye. The engineers have now fitted a device that will prevent the accident from happening again. Any increase in speed will cause a brake to be applied to the gear wheel. The engineers are confident that Big Ben will remain accurate and reliable for another 200 years.