On the Stories Told by Indicator Diagrams and Carnot Diagrams

The "Carnot Diagram," so prevalent in conveying the Second Law of Thermodynamics, had a prehistory in the indicator diagrams used by some practical engineers to diagnose the ailments of steam engines and to improve their operation. These diagnoses can be understood in narrative terms, analogous to the case reports of physicians. A different narrative understanding can be extended to the series of theoretical works on the maximum power obtainable from heat engines by mathematical engineers and physicists: Sadi Carnot, Benoît Paul Émile Clapeyron, Rudolf Clausius, and William Thomson (Lord Kelvin). The narrative interpretation is important for understanding how versions of the Carnot Diagram functioned in their analytic reasoning and in their perspective on the directionality of natural processes, such as heat passing on its own from hot to cold and never the reverse.