Watches and Bells

On a ship the day is divided in six parts called watches. The watches are equal in length and therefore four hours long.
Tradition says we start at 8 in the evening with the First Watch, at midnight the Middle Watch takes over and stays till 4, from 4 till 8 is the Morning Watch. Then till noon the Forenoon Watch will be on, logically followed by the Afternoon Watch from 12 to 4, and then from 4 to 8 it's the Dog Watch turn and the circle is round.

There are other ways to divide the watch, but the above is the traditional one and the one that is still most often in use. But there are not six different groups of people taking those watches. There are just three. Everyone has two watches a day. The First Watch will also do the Forenoon Watch, the Middle Watch the Afternoon Watch, and the people of the Morning Watch will also do the Dog Watch.

The mates would always take the same watch day in day out, week after week, month after month. It's basically the same on all ships at sea. Chief Officer takes 4 to 8. Second Officer 12 to 4. Captain/Third officer 8 to 12. The Sailors might change which watches they work every week or month. Crew members like the Ship's Cook, Steward, Master at arms and Boatswain have their working hours arranged different from the watch system.

Now what are bells in this context? Bells are named after the ship's bell which was sounded by the watch to keep time. But unlike a land clock which sounds once for one'o clock, twice for two'o clock, twelve times for noon and midnight. They counted the hours of the watch, from 0 to 4 every watch again.

To calculate your position in the ocean you need to know the time. Or to be more precise the Ship Time compared to a known time of a known point, normally GMT, as the astronomical books would've been based on GMT too.

So in the early age of sail they used an hourglass to track time. The standard hourglass ran for half an hour, then the glass needed to be turned over. At that time the bell was struck once. The second time the glass was turned over the bell was struck double, marking the whole hour. This went on till the watch was over, and the next watch started at zero again.

So that would result in the following patern:

Nowadays there is no hourglass to be turned anymore but more technical advanced clocks, but these still ring these 'bells'

To know more about what time had to do with finding a position have a look overhere: Introduction to the Sextant.

So you could have an event recored at happening at "Three bells in Fore Noon Watch", which would be 9:30 in the morning, ship's time.

There are however other systems of watches and bells out there both current and historical. This is just one version I've explained here.