A sand clock measures the passage of a few minutes or an hour of time. The sand clock is a clock used by Romans to measure a constant time called hour.
It consists of two chambers fitted over one another filled with an amount of sand and it takes exactly one hour to empty a chamber. Once the upper chamber is completely emptied, the chamber is turned upside down to record the time again. It is also called hourglass.

Sand clock

Some of the learners also asked:
Write the rate of heartbeats in a minute.