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The Collaborative International Dictionary of English v.0.48 (gcide)

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.
[1913 Webster]

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.
[1913 Webster]