In thermodynamics, mechanical action is a synonym for pressure volume work, defined by French engineer Émile Clapeyron in 1834 as the “the integral of the product of the pressure times the differential of the volume” during either the expansion or contraction of the gas and the resultant piston movement. [1] This is near synonym for motive power, used by French engineer Sadi Carnot in 1824, and mechanical work, used by German physicist Rudolf Clausius in 1850, and the shortened term "work" used often in the modern sense.

1. Clapeyron, Émile. (1834). “Memoir on the Motive Power of Heat”, Journal de l’Ecole Polytechnique. XIV, 153 (and Poggendorff's Annalender Physick, LIX, [1843] 446, 566).

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