1. A PNEUMATIC HAMMER WHICH COMPRISES A HOLLOW CASING DEFINING A GUIDE SLEEVE OPEN AT ITS FORWARD END, A PRIMARY AIR CHAMBER DISPOSED FOR COMMUNICATION WITH A SOURCE OF PRESSURIZED AIR, AND AN AUXILIARY AIR CHAMBER, A HAMMERHEAD PISTON SLIDABLY DISPOSED WITHIN SAID GUIDE SLEEVE FOR MOVEMENT RELATIVE THERETO FROM A RETRACTED POSITION TO AN EXTENDED POSITION FOR DELIVERING A HAMMER BLOW, A FIRST RESILIENT MEANS CONNECTED TO SAID CASING AND TO SAID HAMMERHEAD PISTON TO URGE SAME TOWARD ITS RETRACTED POSITION, A NORMALLY CLOSED PRIMARY VALVE DISPOSED FOR MOVEMENT INTO AN OPEN STATE TO ADMIT PRESSURIZED AIR FROM SAID PRIMARY CHAMBER IN TO SAID GUIDE SLEEVE BEHIND THE HAMMERHEAD PISTON TO ACCELERATE SAME AGAINST THE EFFECT OF SAID FIRST RESILIENT MEANS TOWARD SAID EXTENDED POSITION, AN AUXILIARY PISTON SLIDABLY DISPOSED WITHIN SAID AUXILIARY AIR CHAMBER, A HOLLOW ROD CONNECTING SAID PRIMARY VALVE AND AUXILIARY PISTON TOGETHER FORM MOVEMENT IN UNISON, SAID AUXILIARY PISTON AND PRIMARY VALVE EACH HAVING AN OPEN PASSAGE COMMUNICATING WITH THE HOLLOW PORTION OF SAID ROD FOR VENTING SAID GUIDE SLEEVE THERETHROUGH, A SECOND RESILIENT MEANS CONNECTED TO SAID CASING AND DISPOSED TO URGE SAID AUXILIARY PISTON, HOLLOW ROD AND PRIMARY VALVE INTO RESPECTIVE POSITIONS, CORRESPONDING TO THE NORMALLY CLOSED STATE OF SAID PRIMARY VALVE, AND A CONTROL VALVE DISPOSED FOR COMMUNICATION WITH A SOURCE OF PRESSURIZED AIR, FOR COMMUNICATION WITH SAID AUXILIARY CHAMBER, AND FOR COMMUNICATION WITH THE AMBIENT ATMOSPHERE, SAID CONTROL VALVE BEING OPERABLE TO SELECTIVELY ADMIT PRESSURIZED AIR INTO SAID AUXILIARY CHAMBER TO DISPLACE SAID AUXILIARY PISTON INTO A POSITION WHEREIN SAID PRIMARY VALVE IS SHIFTED TO ITS OPEN STATE THEREBY PRESSURIZING THE GUIDE SLEEVE AND ACCELERATING THE HAMMERHEAD PISTON TO DELIVER A HAMMER BLOW AT ITS EXTENDED POSITION, SAID CONTROL VALVE BEING ALSO OPERABLE TO SELECTIVELY VENT SAID AUXILIARY CHAMBER TO THE AMBIENT ATMOSPHERE TO RESTORE SAID PRIMARY VALVE TO ITS NORMALLY CLOSED STATE UNDER THE INFLUENCE OF SAID SECOND RESILIENT MEANS, AND TO RETURN SAID HAMMERHEAD PISTON TO ITS RETRACTED POSITION UNDER THE INFLUENCE OF SAID FIRST RESILIENT MEANS.