Hydraulic door closer

Claim 1

IN A HYDRAULIC DOOR CLOSER, A PAIR OF CONCENTRIC TELESCOPING INNER AND OUTER CYLINDERS HAVING CLOSED FLUID TIGHT OUTER ENDS, EACH OF SAID CYLINDERS HAVING A PORTION AXIALLY SLIDABLY ENGAGING THE OTHER OF SAID CYLINDERS, MEANS ON THE CLOSED OUTER ENDS OF SAID CYLINDERS FOR CONNECTING SAID CLOSED OUTER ENDS ONE TO A DOOR FRAME AND THE OTHER TO A COOPERATING DOOR HINGED TO SAID DOOR FRAME, THE INNER END OF SAID CYLINDER HAVING AN AXIAL OPENING THERETHROUGH, A PISTON IN SAID INNER CYLINDER, A PISTON ROD EXTENDING SLIDABLY THROUGH SAID AXIAL OPENING AND SECURED AT ONE END TO SAID PISTON AND AT ITS OTHER END TO THE CLOSED OUTER END PORTION OF SAID OUTER CYLINDER, AND YIELDING MEANS BIASING ONE OF SAID CYLINDERS IN ONE DIRECTION OF AXIAL MOVEMENT RELATIVE TO THE OTHER OF SAID CYLINDERS, WHEREBY TO IMPART RELATIVE AXIAL MOVEMENTS TO SAID PISTON AND INNER CYLINDER, SAID INNER CYLINDER CONTAINING LIQUID, AND VALVE MEANS OPERATIVELY ASSOCIATED WITH SAID PISTON FOR CONTROLLING FLOW OF SAID LIQUID FROM ONE END PORTION OF SAID INNER CYLINDER TO THE OTHER END PORTION THEREOF RESPONSIVE TO RELATIVE AXIAL MOVEMENTS, OF SAID INNER CYLINDER AND PISTON.