A clepsydra or "water clock" is described as a contrivance for measuring time by the graduated flow of water or other liquid through a small aperture. Many types of clepsydrae have been devised and have figured in horological history from the early ingenious examples of the Egyptians,' the Chinese,2 and those created in the Hellenistic period by Ctesibius and Vitruvius to more elaborate forms produced as novelties as late as the nineteenth century.3 Most of the clepsydrae of record utilized the flow or drip of water from or to a measurable container to a receptacle, and time was measured by the quantity emitted or accepted. There was a resurgence of interest in the subject during the seventeenth century, and writers on the sciences described and illustrated many of the old as well as some new types of the water clock. During this period of scientific exploration a " new " type was apparently introduced, consisting of a compartmented metal cylinder which rotated by the displacement of a contained fluid through one or more small openings in the partitions betwveen its compartments. Research has revealed, however, that although the compartmented cylindrical clepsydra appeared to be a particular preoccupation of seventeenth century horologists and writers, in actuality it had been invented many centuries earlier. It is known to have existed in Spain in the