Sadi Carnot’s contribution to the second law of thermodynamics

We identify an operative principle in Sadi Carnot’s only publication that is closely related to a distinct version of the second law of thermodynamics. Although Carnot did not propose the second law of thermodynamics, he assumed its equivalent in proving Carnot’s theorem. We show that, in the absence of the first law, Carnot’s assumption is equivalent to Clausius’ version of the second law. Both Carnot’s assumption and Clausius’ version, in the absence of the first law, are more restrictive than Kelvin’s statement of the second law.