a mathematician of Alexandria, about 120 years before Christ. He was the first who invented the pump. He also invented a clepsydra, or a 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 months, which were engraved on a column near the machine. This artful invention gave rise to many improvements; and the modern manner of measuring time with an hour-glass is in imitation of the clepsydra of Ctesibius.