James Watt was certainly a ‘man o’ pairts’, but he was also a coherent whole. For too long our historical understanding of him has emphasized the many parts, seeing the whole only dimly, if at all. By pursuing the links between his practical and theoretical work I have tried to set this to rights. Watt was an engineer. He was certainly an expert, as J. D. Forbes put it in his discussions of these questions, at contrivance. Forbes admired Watt's invention of the parallel motion as showing Watt's genius for contrivance, but he argued that this in itself would never have been the basis for ‘reputation’. Watt's reputation derived from his quality as a philosophic engineer. I have argued that what gave his engineering its philosophic quality or dimension was largely chemical in character. Watt was a chemist, whose chemistry of heat provided the philosophical dimension to his engineering. That philosophical dimension was very different from the equivalent dimension of what became known in the mid-nineteenth century as ‘engineering science’. In engineering science heat was understood as a form of energy, convertible into other forms according to the fundamental laws of thermodynamics. All this was foreign to the intellectual world that Watt inhabited. However, because Watt was adopted as an icon and founding father of their field, the engineering scientists of the nineteenth century were not averse to smoothing over some of the differences between his world and theirs. Watt the chemist and engineer became Watt the mechanical engineer.

In my early chapters I showed how popular representation of Watt, which became a minor industry itself in the nineteenth century, presented the mechanical Watt. This was a natural and easy thing to do given the association of Watt in the popular mind with contrivance. The products of that genius for contrivance, or so it seemed, were everywhere in Victorian Britain, the engines, the linkage and control mechanisms to convert the power of those engines into useful work, the indicators designed to monitor, measure and adjust their performance.