LONDON (Reuters) - Normally the traditional arbiter of precise time-keeping, Britain's 156-year-old Big Ben raised eyebrows this month by starting its familiar chimes up to six seconds late every hour.
The problem with the huge bell that forms part of the clock tower of the British parliament started around Aug. 15, say engineers who are still checking the pendulum, air pressure, temperature and gears.
"It may be that several small problems contributed to this unusual problem," they said on parliament's website.
But if the cause was a mystery, the answer was obvious - a pile of Britain's old one penny coins, of the type in circulation before the country switched to decimal coinage in 1971.
Adding or removing each penny to the pendulum of the venerable bell varies the rate of its swing and changes the clock speed by 0.4 seconds per day.
Simple, although climbing the 300-plus stairs to the top of the tower every time an adjustment has been needed over the past few days has not perhaps been so easy.
Now the clock is accurate to less than one second of the right time, although checks are continuing.
"The clock mechanics will continue to monitor the performance of the clock, and have increased their routine checks from three times per week to every day, including weekends," the website said.
Reporting by Honor Brocklebank-Fowler; editing by Steve Addison