Editing John Harrison (section)

==="Jefferys" watch===
He had already in the early 1750s designed a precision watch for his own use, which was made for him by the watchmaker [[John Jefferys (clockmaker)|John Jefferys]] {{circa}} 1752–1753. This watch incorporated a novel frictional rest escapement and was not only the first to have a compensation for temperature variations but also contained the first miniature ''[[going train]] [[Fusee (horology)|fusee]]'' of Harrison's design which enabled the watch to continue running whilst being wound. These features led to the very successful performance of the "Jefferys" watch, which Harrison incorporated into the design of two new timekeepers which he proposed to build. These were in the form of a large watch and another of a smaller size but similar pattern. However, only the larger No. 1 watch (or "H4" as it is sometimes called) appears to have been finished (see the reference to "H4" below). Aided by some of London's finest workmen, he proceeded to design and make the world's first successful marine timekeeper that allowed a navigator to accurately assess his ship's position in [[longitude]]. Importantly, Harrison showed everyone that it could be done by using a watch to calculate longitude.<ref>{{cite web | title = Harrison's Marine timekeeper (H4) | publisher=[[National Maritime Museum]] | url = http://collections.rmg.co.uk/collections/objects/79142.html | access-date = 25 February 2008}}</ref> This was to be Harrison's masterpiece – an instrument of beauty, resembling an oversized [[pocket watch]] from the period. It is engraved with Harrison's signature, marked Number 1 and dated AD 1759.