The bells are rung in the traditional English style known as "change ringing".  Because of their great momentum, bells take about two seconds to rotate, so they can not be used to play ordinary melodic music. However, they can be made to follow one another in precise order each ringing once before the first rings again.  The order in which each bell rings can be constantly changed, thus the name "change ringing".  Experienced ringers test their abilities by ringing peals--  5,000 or more changes without repeating the order and without any break.  If one ringer makes a mistake there is no way to recover.  Everyone must stop and start over at the beginning.    Peals customarily last about three hours, however, the longest peal ever performed  outside the British Isles was accomplished here at the Old Post Office Tower.   It consisted of 11,418 changes and required 6 hours and 57 minutes to complete.