"Carnot considered that the steam engine was analogous to another prime mover, the water wheel. The analogy, and the upon which it was based, led Carnot to the incorrect conclusion that no heat was lost, or converted into mechanical energy during the operation of the steam engine. He thought that the same quantity of heat was given out by the boiler at the higher temperature as was received by the condenser at the lower temperature. However, the analogy led him also to the fruitful conception that the amount of energy produced... was solely dependent, in principle, upon the temperature difference between the boiler and the condenser and the amount of heat which passed from the one to the other. Thus it appeared that all heat engines in general, would have the same efficiency if they worked within the same temperature levels. He substantiated this... Carnot's principle, by pointing out that would be possible if it were not true. If two perfect heat engines operating between the same temperature levels did not have the same efficiency, it would be possible for the more efficient... to drive the less efficient engine backwards, pumping heat from the lower to the higher temperature, thus leaving the thermal conditions unchanged and yet generating a continuous excess of energy."