The instrument maker James Short, whose output was exclusively reflecting telescopes, was a sustained and consistent supporter of the clock and watch maker John Harrison. Short's specialism placed his work in a tradition that derived from Newton's Opticks, where the natural philosopher or mathematician might engage in the mechanical process of making mirrors, and a number of prominent astronomers followed this example in the eighteenth century. However, it proved difficult, if not impossible, to capture and communicate in words the manual skills they had acquired. Harrison's biography has similarities with Short's but, although he was well received and encouraged in London, unlike Short his mechanical practice did not place him at the centre of the astronomers' agenda. Harrison became a small part of the growing public interest in experimental demonstration and display, and his timekeepers became objects of exhibition and resort. Lacking formal training, he himself came to be seen as a naive or intuitive mechanic, possessed of an individual and natural 'genius' for his work - an idea likely to be favoured by Short and his circle, and appropriate to Short's intellectual roots in Edinburgh. The problem of capturing and communicating Harrison's skill became acute once he was a serious candidate for a longitude award and was the burden of the specially appointed 'Commissioners for the Discovery of Mr Harrison's Watch', whose members included Short. Now the problem was one of transforming individual genius into a generally useful practice. It was a question that touched on the reputation of Short in the area of his own genius and it was familiar also to the astronomers, men who had engaged with making mirrors, had struggled to systematise and record their methods, and who now, as Commissioners, had to judge whether and how Harrison's very individual achievements might be shared. [ABSTRACT FROM AUTHOR]