So wrote Robert Burn in 1854. For a long time people had been puzzling over what was the actual driving force behind the steam engine. It is interesting that Savery stated that his engine would work by the ‘impellent force of fire’ and that the early atmospheric engines were called ‘fire engines’. Watt also was acutely conscious of the importance of heat in a steam engine which was shown by his use of the steam jacket and hence, in effect, lagging the cylinder. We have seen how this awareness was, in some degree, due to his understanding of the science of heat through his connections with the men of science of his day like Black who had just founded the new science of heat. Engineers after Watt, like Smeaton, Brindley, Ewart, and Fairbairn, had no such link with this science and they derived their theories from hydraulic engineering which thought in terms of water pressure or weight. The result was that often their concept of the operation of steam engines was deficient.

An example of these confused ideas was shown by the thinking of Farey. He could see that the role of heat was essential when he wrote in Rees's Cyclopaedia,