Hourglass

Hourglass, ancient instrument to measure the passage of time. A quantity of fine, dry sand is contained in a bulb constricted at its center to a narrow neck. The device is turned so that all the sand is in the upper chamber. The time taken for the sand to trickle into the lower chamber depends on the amount of sand and on the diameter of the neck. Small hourglasses are used in the home as egg timers.

See also: Time.