Clepsydra

klep'si-dra, a contrivance for measuring time by the flow of water through an orifice. This is an ancient timepiece said to have been in use 2000 B. C. In its simplest form it is a hollow cylinder with an orifice in the bottom of such size that the water runs out in a certain number of hours. The descent of water in the tube or tank indicates the hour of the day or night. In its later forms it was furnished with a dial like that of a clock, the hands of which were operated by clockwork set in motion by means of weights and regulated by a float of wood or cork riding on the sinking surface of the water. Sometimes mercury was used instead of water. The clepsydra was superseded by the invention of the pendulum. See Hourglass; Clock