a mathematician of Alexandria, who flourished about 120 years before Christ. He was the first who invented the pump; and he also invented a clepsydra, or water-clock. His invention for measuring time by means of water was exceedingly ingenious. The water was made to fall upon wheels, which it turned; and the wheels communicated a regular motion to a small wooden image, which by a gradual rise pointed with a stick to the proper hours and minutes engraven on a column near the machine. This contrivance underwent many improvements; and the modern method of measuring time by an hour-glass is an imitation of the clepsydra of Ctesibius.