Which 1824 book by Sadi Carnot founded the theory of heat engines?

Sadi Carnot’s 1824 book was titled Reflections on the Motive Power of Fire.

Carnot published the French treatise Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance in Paris in 1824. The 118-page work examined how heat engines produce motive power at a time when steam engines were becoming economically and strategically important but lacked a rigorous general theory.

The book introduced ideas that became foundational to thermodynamics, including reversible operation, ideal cycles, thermal efficiency, and the principle now associated with Carnot’s theorem. Carnot still used the period’s caloric theory of heat, which treated heat as a conserved fluid-like substance; later scientists, especially Rudolf Clausius, recast his insights within the energy and entropy framework.

The title is sometimes rendered in English as Reflections on the Motive Power of Heat, so “fire” versus “heat” can create a translation issue. Both refer to Carnot’s landmark work, but the stored answer is the canonical English title used for the 1824 publication. Carnot published no other book during his lifetime.