An engine for extracting energy from a heat source, comprising a support, a shaft rotatably coupled to the support and being rotatable in a first direction, a plurality of vessels coupled to and spaced about the shaft, a working fluid provided in the plurality of vessels, and a plurality of conduits connecting the vessels together in a circuitous fluid circuit. Each conduit has an outlet end connected to one of the plurality of vessels, an inlet end connected to another one of the plurality of vessels, and a one-way check valve configured to allow the working fluid to flow out of the one vessel via the outlet end, through the conduit and into the another vessel via the inlet end. The plurality of vessels and conduits are shaped and arranged about the shaft and the working fluid is selected so that when the one vessel is heated by the heat source the working fluid in the one vessel experiences an increase in vapor pressure causing at least part of the working fluid to flow from the one vessel into the another vessel located above the one vessel so as to produce a gravitational moment that encourages rotation of the shaft in the first direction.