Hour"glass` (?), n. An instrument for measuring time, especially the interval of an hour. It consists of a glass vessel having two compartments, from the uppermost of which a quantity of sand, water, or mercury occupies an hour in running through a small aperture unto the lower.

&fist; A similar instrument measuring any other interval of time takes its name from the interval measured; as, a half-hour glass, a half-minute glass. A three-minute glass is sometimes called an egg-glass, from being used to time the boiling of eggs.

• Glass construction with two halves separated/connected with a narrow passage. There is sand in it which flows from one side to the other, obstructed by the narrow passage. The time to pass through is always the same, so it can serve to 'measure' time or at least make sure that a duration is twice the same.