Source : Webster's Revised Unabridged Dictionary (1913)

Hourglass \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.