Abstract: A material-feeding apparatus having a casing provided with inlet and outlet openings. The casing encloses a rotor provided with circumferentially-arranged cylinders each fitted with a free piston. The cylinders progressively register with the inlet opening which is at or near atmospheric pressure and the outlet opening which is exposed to high pressure as the rotor is rotated intermittently by a drive mechanism. The casing and rotor are provided with a network of conduits which interconnect selected cylinders as the motor moves through one complete revolution and balances the pressure between the two extremes existing within the inlet and outlet openings. Fluid pressure slightly in excess of the pressure present within the outlet opening is fed to each cylinder is register with that opening to move the piston and discharge the material from the apparatus.