Besides this, water clocks were used, and here again we must distinguish two kinds. The common water clock, which, like our hour-glass, marked a definite period of time by the flowing away of a certain quantity of water, is certainly a very ancient invention. This clock consisted of a vessel of clay or glass, in the shape of a jar or a basin, which was filled with water by an opening above, and a second cup-shaped vessel, on the top of which the former was arranged in such a way that the water poured out slowly through little sieve-like openings into the lower vessel. Water clocks of this kind probably existed in most households, but were not real clocks, since they did not indicate the hour of the day, but were only used for calculating some particular period of time. They were chiefly used in the law courts to mark the time allowed to each speaker, and when a speech was interrupted in order to hear witnesses, or to read out documents, or for any other purpose, the flow of the water was stopped, and it was set going again when the orator continued his speech. These water clocks were also used on other occasions wherever certain periods of time had to be calculated, and this might take place in any household. The same principle underlay the water clocks which were supposed to have been invented by Plato, and perfected by the Alexandrine Ctesibius, by means of which a long period of time could be subdivided into equal parts, and thus the hours of the night could be calculated, which was of great importance. These water clocks could only be constructed when it was possible to make transparent glass vessels large enough to hold a quantity of water sufficient to last for twelve hours and longer; on the glass there was a scale graven, which gave the relation of the hours to the height of the water. But as the length of the night decreases and increases in the course of the [Illustration: FIG. 93.]