Abstract: A rotary compressor 10 of the rolling piston type is shown wherein the rolling cylindrical piston 28 is forced into rolling contact with the cylindrical interior 15 of the compressor chamber by a hydraulic piston 78 and cylinder 64 arrangement providing a resilient force on the rolling piston 28 to permit non-compressible matter such as liquid refrigerant to be present in the compressor chamber and also, through means 82 for varying the hydraulic pressure to the hydraulic piston and cylinder arrangement, permit varying the compressor discharge pressure while the rolling piston is continuously driven.