Percussion tool

Claim 1

IN A PERCUSSION DRILLING TOOL, A CASING,AN ANVIL ELEMENT AT THE FORWARD END OF SAID CASING AND A CONNECTION FOR A PRESSURED FLUID SUPPLY AT THE REARWARD END THEREOF, A HAMMER PISTON RECIPROCABLE IN SAID CASING FOR BEATING UPON SAID ANVIL ELEMENT, AND FLUID INLET AND EXHAUST PORTS IN THE WALL OF SAID CASING TRAVERSED BY SAID PISTON FOR ALTERNATELY ADMITTING PRESSURED FLUID TO AND EXHAUSTING THE SAME FROM SAID ENDS, SAID PORTS BEING POSITIONED TO BE SELECTIVELY COVERED AND UNCOVERED BY SAID PISTON TO ADMIT PRESSURED FLUID TO SAID FORWARD CASING END AND EXHAUST SAID REARWARD CASING END DURING THE LAST PART OF THE FORWARD STROKE OF SAID PISTON AND THE INITIAL PART OF THE SUCCEEDING RETURN STROKE, TO CUT OFF THE EXHAUST FROM SAID REARWARD CASING END AND TO ADMIT PRESSURED FLUID THERETO, INTERMEDIATELY DURING THE REARWARD STROKE OF SAID PISTON, AND TO CUT OFF THE PRESSURED FLUID SUPPLY TO SAID REARWARD END DURING THE LAST PART OF SAID REARWARD STROKE AND THE INITIAL PART OF THE SUCCEEDING FORWARD STROKE WHEREBY THE CHARGE OF PRESSURED FLUID IN SAID REARWARD END IS COMPRESSED AND EXPANDED TO CUSHION AND EXPEDITE THE TURN-AROUND OF SAID PISTON.