1. IN POSITIONING APPARATUS, THE COMBINATION COMPRISING MEANS CONSTITUTING TRANSDUCER CHAMBERS, AND MANUAL CONTROL MEANS FOR CONTROLLING THE PRESENCE OR ABSENCE OF PRESSURE FLUID IN SAID CHAMBERS, SAID MANUAL CONTROL MEANS INCLUDING A PIVOTAL DIAL MEMBER THE PERIPHERY OF WHICH HAS NUMBERED STATIONS AND A RECESS ADJACENT EACH OF SAID STATIONS AND DETENT MEANS COOPERATING WITH SAID RECESSES TO YIELDINGLY HOLD SAID DIAL MEMBER AT ANY STATION SELECTED, SAID DIAL MEMBER HAVING PORT MEANS THEREIN ARRANGED TO VENT SAID CHAMBERS ACCORDING TO PREDETERMINED PIVOTAL POSITIONS OF SAID DIAL MEMBER CORRESPONDING TO SAID STATIONS.