The use of water clocks is attested in Babylon. In one set of tablets, for example, four maneh weight of water was poured into the clock for a daytime watch at summer solstice, and two for a night watch. The ratios were reversed at the winter solstice and the rest of the year. The change in ratio as the year passes was interpolated in a linear manner to adjust the weight of water to use each month. Presumably the scale for the opposite time of the year was used for night watches.
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