In the old days, few sailors could afford time pieces

So, at the beginning of each watch, the Quartermaster upended an hourglass containing 30 minutes of sand. When the sand ran out, he struck one bell and turned the glass. Another bell was struck each time the glass was turned. Thus, half-hours were marked by an odd number of bells, and an even number of bells marked each hour. Eight bells signalled the end of every four-hour watch- either 4, 8, or 12 o'clock.