Hourglass, a device for measuring time. In its usual form it consists of two cone-shaped or oval glass receptacles joined by a narrow neck. Sand or a liquid (such as water or mercury) in the uppermost section of a true hourglass will run through the neck into the lower section in exactly one hour. By turning the other end up, another hour may be marked, and the process may be continued indefinitely. When sand is used, the device is sometimes called a sandglass. A small sandglass, in which the sand passes from top to bottom in three minutes, is used for timing the boiling of eggs.
An early instrument working on the same principle was the clepsydra, or water clock.