Some comments on James Watt’s published account of his work on steam and steam engines

In his study of the influence of James Watt’s expansive principle on Carnot JL and Clément, Dr Robert Fox draws attention to Watt’s claim, in a letter to Boulton in 1781, that the principle was conceived in 1767, and he points out that this conflicts with a statement in John Robison’s System of Natural Philosophy (1822) that the principle first occurred to Watt in 1769. He considers this to be a categorical statement by Robison, but the paragraph concerned is in italic type and this shows that it was in fact written by Watt himself in 1813 or 1814. He was then describing events that had happened more than forty years earlier, and it is not surprising that there is some confusion about dates. After Robison’s death in 1805, John Playfair agreed to write his biography and to edit for separate publication his numerous articles in the third edition (1797) of the Encyclopaedia Britannica(3). Poor health made it possible for Playfair to accomplish only the first part of this task, and Robison’s family asked David Brewster, one of his former students, to take over the editorial work (4).