The “nilometer” was the device used in ancient Egypt to measure water levels in the Nile:
Only priests and rulers, whether pharaohs or later, Roman or Arab leaders, were allowed to monitor the nilometers, and their ability to predict the behavior of the Nile was used to impress the common people. (And to determine how much money would be collected in taxes.) This is why so many nilometers were built in temples, where only priests would be able to access the mysterious instrument.
Via Atlas Obscura