Automatic depth control mechanism

Claim 1

IN THE COMBINATION COMPRISING A WHEELED VEHICLE, A GROUND-WORKING IMPLEMENT PIVOTALLY MOUNTED IN TRAILING RELATIONSHIP OF SAID VEHICLE FOR MOVEMENT VERTICALLY RELATIVE THERETO TO DIFFERENT GROUND-WORKING DEPTHS, MEANS INCLUDING FLUID PRESSURE OPERATED MOTOR MEANS OPERATIVELY INTERCONNECTING SAID VEHICLE AND IMPLEMENT TO CONTROL PIVOTAL MOVEMENT OF THE LATTER, AND MEANS INCLUDING A SOURCE OF FLUID UNDER PRESSURE AND CONTROL VALVE MEANS CONTROLLING THE SUPPLY AND EXHAUST OF FLUID TO AND FROM SAID MOTOR MEANS, SAID CONTROL VALVE MEANS HAVING A NEUTRAL POSITION HOLDING FLUID IN SAID MOTOR MEANS TO HOLD SAID IMPLEMENT IN A SUBSTANTIALLY FIXED POSITION RELATIVE TO SAID VEHICLE AND FIRST AND SECOND OPERATING POSITIONS RESPECTIVELY SUPPLYING FLUID TO AND EXHAUSTING FLUID FROM SAID MOTOR MEANS TO MOVE SAID IMPLEMENT RELATIVE TO SAID VEHICLE; THE IMPROVEMENT COMPRISING DEPTH CONTROL MEANS OPERABLE AUTOMATICALLY TO SHIFT SAID CONTROL VALVE MEANS AMONG SAID POSITIONS THEREOF TO MAINTAIN SAID IMPLEMENT AT A PRESELECTED GROUND-WORKING DEPTH AND INCLUDING AN ACTUATING PISTON RECIPROCABLY DISPOSED WITHIN A CYLINDER AND DEFINING THEREWITH A PRESSURE CHAMBER, SPRING MEANS URGING SAID PISTON IN ONE DIRECTION WITHIN SAID CYLINDER, A SOURCE OF FLUID AT SUBSTANTIALLY CONSTANT