Bells are traditionally use to indicate time aboard a ship.
Instead of being struck hourly (with the number of rings indicating the hour), the bells are struck every half hour of a 4 hour watch.
when ringing the bell, the rings are grouped into pairs to make counting easier.
We use a simplified ships bell system to indicate the time (see right)
At the begining of our evening meeting the time is 7:30pm, so 7 bells are struck.
At the end of the evening it is 9:30pm, so we strike 3 bells.
If we were using the Royal Navy system, with a dog watch between 16:00 and 20:00 we would strike 3 bells at the begining and end of the evening.