The use of 30 minutes as the interval between bells originates from the use of a half hour glass to keep time. Each half hour, the glass would be flipped and the appropriate count of the bell would be rung.<ref>http://www.history.navy.mil/faqs/faq83-1.htm</ref> | The use of 30 minutes as the interval between bells originates from the use of a half hour glass to keep time. Each half hour, the glass would be flipped and the appropriate count of the bell would be rung.<ref>http://www.history.navy.mil/faqs/faq83-1.htm</ref> |