high tempera­ ture to output work and waste heat at a lower temperature. It also covers work-consuming machines used as refrigerating systems and heat pumps abstracting heat from a low temperature source and delivering this plus the heat equivalent of the work consumed to a higher tem­ perature. Finally it covers work-consuming devices used as pressure­ generators compressing a fluid from a low pressure to a higher pres­ sure. Very similar machines exist which operate on an open regen­ erative cycle where the flow of working fluid is controlled by valves. For convenience these may be called Ericsson engines but unfortunate­ ly the distinction is not widely established and regenerative machines of both types are frequently called 'Stirling engines'.