Big Ben, one of London's best-known landmarks, which is famous for its accuracy and chimes ,stopped ticking for 90 minutes, an engineer said Saturday. Officials do not know why the 147-year-old clock on the banks of the River Thames stopped at 10:07 p.m. Friday. It continued keeping time, but stopped again at 10:20 p.m. and remained still for about 90 minutes before starting up again, a spokeswoman for the House of Commons said. A recent period of hot weather may have been to blame. Temperatures in London reached 90 Saturday, and forecasters called it England's hottest day in May in the past fifty three years, since 1953. Big Ben, which is operated by the Palace of Westminster, survived attack by German Luftwaffe bombers during World WarII,continued to mark the time within11/2 seconds of Greenwich Mean Time. However, the clock has experienced occasional problems. In 1962, snow caused the clock to ring in  the New Year 10 minutes late. In 1976, the clock stopped when a piece of its machinery broke. Big Ben also stopped on April 30, 1997, and once more three weeks later. Big Ben is actually the clock's 13-ton bell, which was named after Sir Benjamin Hall, the British commissioner of works at the time the clock was built. The official name for the Gothic tower holding Big Ben is St. Stephen's Tower. Standing 315 feet tall, it was completed in 1859 after an 1834 fire destroyed most of the Palace of Westminster. The probable reason for Big Ben stopping ticking for 90 minutes is   _  .
high temperature low temperature broken machine heavy snow