An engine is configured to extract energy from a heat source as follows. A shaft is adapted to be rotatably coupled to a support and rotatable in a first direction. A plurality of vessels is coupled to and arranged about the shaft. At least a first vessel of the plurality of vessels includes a thermally insulative portion and a thermally conductive portion. A plurality of conduits connects the plurality of vessels together. Each of the plurality of vessels is in communication with at least one other of the plurality of vessels via at least one of the conduits. The plurality of vessels is arranged to allow the thermally conductive portion of the first vessel to encounter the heat source. The thermally conductive portion is capable of transferring heat to at least partially vaporize volatile fluid within the first vessel to cause a mass to at least partially move towards a connected vessel located above the first vessel. This produces a gravitational moment that encourages rotation of the shaft and the plurality of vessels in the first direction.