\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.
Note: 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.