Description	Diagram of a fancy clepsydra. Water enters and raises the figure, which points at the current hour for the day. Spillover water operates a series of gears that rotates a cylinder so that hour lengths are appropriate for today's date. The ancient Greeks and Romans had twelve hours from sunrise to sunset; since summer days are longer than winter days, summer hours were longer than winter hours.
Source	Abraham Rees (1819). “Clepsydra”, Cyclopædia: or, a New Universal Dictionary of Arts and Sciences. The image is the JPEG reproduction published 2007-02-01 by the Horological Foundation.
Date	1819
Author	The illustrator was probably John Farey, Jr. (1791–1851). The principal engraver for the encyclopedia was Wilson Lowry (1762–1824).