Abstract: A multi-cylinder Stirling cycle thermal device is provided having free floating piston members arranged without a solid mechanism connected to the piston members to derive power therefrom. The piston members are free to oscillate in the cylinders and define variable volume heat-absorbing and heat-rejecting chambers therein. Heat is arranged to be supplied to some of the heat-absorbing chambers so that the device functions as a Stirling cycle heat engine driving a Stirling cycle heat pump, heat being collected from all the heat-rejecting chambers to provide a heat input for a heating system.A reciprocating power output may also be provided by a device responsive to pressure changes in the working gas.