Abstract: As he settled into his position as Professor of Natural Philosophy at Glasgow, Thomson would spend much of his time over the subsequent years pondering over the problem of heat and the difficulty of reconciling Joule's experimental determination of the mechanical equivalent of heat with Carnot's theory that seemed to demand that there could be no such figure. The eventual result of his ruminations was a series of papers presented to the Royal Society of Edinburgh in 1851 and published in their Transactions a few years later on the dynamical theory of heat. In these papers, Thomson outlined the basic principles of the new science of thermodynamics that had emerged from his attempts at making sense of Carnot and Joule's apparently contradictory positions. Over the next few decades, Thomson, Joule, and others would succeed in placing this new thermodynamic science at the very heart of 19th-century physics. To make sense of this achievement, it is important to start by going back to the 1820s and Carnot's essay Reflexions sur la Puissance Motrice du Feu. This chapter begins with a careful look at Carnot's essay and responses to it, followed by an account of Joule's experiments during the first half of the 1840s that led him to the mechanical equivalent of heat.

Keywords: William Thomson, Joule, Carnot, heat, Transactioins,