1-157-337 - Ann Ronan Picture Library/Heritage-Images

Reconstruction of a clepsydra (water clock), invented by Ctesibius of Alexandria, c270 BC (1857). Ctesibius (fl270 BC) was an inventor and mathematician who initiated a tradition of great engineers in ancient Alexandria. None of his writings have survived, and his inventions are only known about from references to them made by others. This clock worked by water dripping at a constant rate and raising a float with a pointer. It was only superseded in terms of the accuracy of its timekeeping in the 17th century when Christiaan Huyghens showed how the pendulum could be applied to clocks