He reasons that if the flow of heat from a hot place to a cooler one can create motive power and raise a weight, then a machine can exist that does the opposite. In this machine, the motive power obtained from a falling weight will force heat to flow “uphill” from cool sink to hot furnace. There is a direct analogy with water mills and water pumps. The former uses the downward flow of water to produce motive power; the latter uses power to push water uphill.
