The Art Archive/Pharaonic Village Cairo/Dagli Orti

A small hole near the bottom of this Egyptian water clock (the example shown here is a reproduction) allows water to drain out at a constant rate. It takes about an hour for the water level to sink to each of 12 successive indentations-thus permitting ancient Egyptians to measure the passage of time without relying on the sun. The first such water clocks appeared in the late 16th century B.C.