1. A CONTROL APPARATUS FOR AUTOMATICALLY CONTROLLING A CONTROL POINT AT WHICH FLOW OF AN INCOMPRESSIBLE FLUID IS TO BE CUT OFF IN A CONTROLLED INCOMPRESSIBLE FLUID PRESSURE SYSTEM COMPRISING A PROPORTIONAL CONTROL DEVICE COMPRISING, TWO CONTROL ELEMENTS RELATIVELY MOVABLE AXIALLY DEFINING INTERNALLY OF SAID DEVICE JOINTLY IN RELATIVE POSITIONS AN ORIFICE HAVING A VARIABLE FLOW AREA IN DEPENDENCE UPON RELATIVE OPERATIVE POSITIONS OF SAID CONTROL ELEMENTS, ONE OF SAID TWO CONTROL ELEMENTS BEING DISPOSED IN OPERATION TO SENSE AND RESPOND BY MOVEMENT TO VARIATIONS OF FLOW OF AN INCOMPRESSIBLE FLUID IN SAID CONTROLLED SYSTEM AND TO VARY THE FLOW AREA OF SAID ORIFICE IN DEPENDENCE UPON VARIATIONS OF SAID FLOW AND TO DEVELOP JOINTLY WITH THE OTHER OF SAID CONTROL ELEMENTS A PRESSURE DIFFERENTIAL ACROSS SAID ORIFICE VARYING IN DEPENDENCE UPON SAID VARIATIONS OF FLOW OF SAID INCOMPRESSIBLE FLUID, MEANS DEFINING A SEAT IN SAID DEVICE HAVING AN OPENING THEREIN THROUGH WHICH FLOW OF SAID INCOMPRESSIBLE FLUID IS TO BE CONTROLLED, A VALVE ELEMENT OPERABLE TO A SEATED POSITION ON SAID SEAT CLOSING SAID OPENING AND AN UNSEATED POSITION TION FOR ALLOWING FLOW OF SAID FLUID FROM SAID ORIFICE THROUGH SAID OPENING, AND ACTUATOR ELEMENT ACTUATED IN RESPONSE TO MOVEMENT OF SAID ONE OF SAID TWO RELATIVELY MOVABLE CONTROL ELEMENTS PROPORTIONALLY TO FLOW VARIATIONS OF SAID FLUID BEING SENSED, SAID ACTUATOR ELEMENT HAVING COUPLING MEANS TO UNSEAT SAID VALVE ELEMENT DURING TRAVEL OF SAID ACTUATOR ELEMENT REPRESENTATIVE OF A CHANGE OF SAID FLUID FLOW ABOVE A GIVEN LEVEL AND FOR SEATING SAID VALVE ELEMENT IN DEPENDENCE UPON A VARIATION OF SAID FLOW OF FLUID BEING SENESED PROPORTIONATE TO A DECREASE IN SAID FLOW BEING SENSED TO A GIVEN REDUCED FLOW LEVEL THEREBY CLOSING SAID OPENING WHEN SAID REDUCED FLOW LEVEL OBTAINS, AND MEANS TO LIMIT THE MOVEMENT OF SAID VALVE ELEMENT TO TRAVEL LESS THAN THE POSSIBLE TRAVEL OF SAID ACTUATOR ELEMENT AND TO ESTABLISH WITHIN SAID REDUCED FLOW LEVEL A SELECTED VALUE AT WHICH SAID VALVE ELEMENT SEATS INDEPENDENT OF A FLOW VALUE ESTABLISHED AT SAID ORIFICE.