In the history of thermodynamics, two dates stand out as especially important: 1824, when Sadi Carnot's brilliant memoirRéflexions sur la puissance motrice du feuappeared in print; and 1850, when Rudolf Clausius published his similarly titled paper ‘Ueber die bewegende Kraft der Wärme’. In this paper Clausius narrowly beat the Scottish physicist William Thomson to the solution of a puzzle which had been highlighted in the latter's recent publications: how could Carnot's theory, with all its intellectual attractions, be reconciled with the (...) newly discovered principle of the inter-convertibility of heat and work? Clausius's solution (as is well known) was to replace Carnot's axiom of heat conservation, with the axiom now known as the second law of thermodynamics. (shrink)