The clepsydra or water clock was invented to be used at night or on overcast days. It is much more consisten than sundials or shadow clocks because it does not rely on a changing factor. Water drips from the top basins to the bottom basin and the hours are marked off by hashes. Clepsydra comes from two Greek words which mean "thief" and "water". The clepsydra lost popularity as people began to realize that its consistency changes when the water is too cold, too hot or too dirty. You can make a clepsydra by following the simple instructions located in the hands-on section of this site.