Hour'-glass, n.

(our'glàs), n. An instrument for measuring time, especially the interval of an hour, consisting 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 into the lower.

☞ 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.