Ktesibios' water clock (see Section 1.2) is probably the first man-made system in which feedback was used in a deliberate manner. Its operations are shown in Figure P5.38(a). The clock indicates time progressively on scale D as water falls from orifice A toward vessel B. Clock accuracy depends mainly on water height hf in the water reservoir G, which must be maintained at a constant level hr by means of the conical float F that moves up or down to control the water inflow. Figure P5.38(6) shows a block diagram describing the system (Lepschy, 1992),