1. A HYDRAULIC MACHINE COMPRISING A HOUSING HAVING INLET AND OUTLET DUCTS FOR HYDRAULIC MEDIUM, A CLOSED TRAIN OF GEARS COMPRISING AT LEAST FOUR GEARS DEFINING A PATH THROUGH WHICH SAID MEDIUM PASSES, MEANS ROTATABLY MOUNTING AT LEAST ONE OF SAID GEARS IN SAID HOUSING IN RADIALLY FIXED POSITION, AND MEANS MOUNTING ANOTHER OF SAID GEARS FOR VARIABLE DISPLACEMENT OF SID OTHER GEAR IN A DIRECTION TRANSVERSE TO THE ROTATIONAL AXIS THEREOF SO AS TO VARY THE SPECIFIC DELIVERY QUANTITY AND DIRECTION OF SAID MEDIUM PASSING THROUGH SAID GEAR TRAIN, AT LEAST EACH ALTERNATE GEAR IN SAID GEAR TRAIN BEING VARIABLY DISPLACEABLE IN A DIRECTION TRANSVERSE TO THE ROTATIONAL AXIS THEREOF, EACH TWO ADJACENT MATING GEARS FO SAID TRAIN DEFINING TOGETHER TWO INSTANTANEOUS CIRCLES OF CONTACT OF VARIABLE DIAMETERS CORRESPONDING TO VARIABLE DEPTHS OF ENGAGEMENT OF THE MESHING GEAR TEETH THEREOF, AND MEANS FOR GUIDING THE DISPLACEABLE GEARS IN SAID HOUSING AND FOR ADJUSTABLEY VARYING THE SIZE OF SAID DIAMETERS IN RESPECTIVELY OPPOSITE SENSES TO A PREDETERMINED VALUE SO THAT RADIAL DISPLACEMTNT OF A DISPLACEABLE GEAR OF SID TRAIN IN ONE DIRECTION RESULTS IN A VARATION IN AN OPPOSITE DIRECTION OF THE DEPTH OF ENGAGEMENT OF THE TEETH OF THE GEAR MATING THEREWITH.