picture description: This is a timekeeping device popular before the industrial revolution.  It is called an "hourglass," but may correspond to other units of time.   The substance contained inside is usually sand.  Because it takes almost exactly the same amount of time for the sand to run through the narrow opening in the middle of the object every time it is inverted, it is useful for keeping time.  Gravity acts on the sand consistently because its force is approximately the same over time.  Today, this object is still used to time the cooking of food, particularly boiled eggs.