a mathematician of Alexandria, about 120 years before Christ. He was the first who invented the pump. He also invented a clepsydra, or water-clock. This invention of measuring time by water was wonderful and ingenious. Water was let drop upon wheels which it turned; the wheels communicated their regular motion to a small wooden image, which by a gradual rise pointed with a stick to the proper hours and minutes, which were engraven on a column near the machine. This ingenious invention gave rise to many improvements; and the modern method of measuring time with an hour-glass is in imitation of the clepsydra of Ctesibius.