hourglass bell timer

The sound of the bell … this is what wakes the Camaldolese monks at 3:30 a.m., this it what announces the sacred silence – the end of the day at 7:45 p.m. This is what calls the brothers to prayer, marks the beginning and end of the time set aside for work or reading.
The vestry brother is responsible for the bell according to the Camaldolese constitutions.

Nowadays the bell is electrically driven and controlled by a clock mechanism, but how did it work in the past?. How did the sacristan know how long to pull the bell string? That’s what the tiny hourglass was used for. Interestingly, the time for the sand to pour is exactly one minute.