Stealing water: this is the origin of the word 'clepsydra'. The first clepsydras, in fact, worked by letting the water pass from one container to the next and the time they took to do so became a time unit. Only later, water was replaced by sand but not just any sand. In order to allow it to flow through a tiny hole, it must have the proper granulosity and it must not be clayey. Watching the sand flowing through an ampulla has an hypnotic charm, as if it were a challenge to the time passing.
Our pewter sand-glasses, with their classical shape, challenge the time they measure, still keeping their fascination and modernity.