Discusses the source for Robert Dewar's transcript of Burns's impromptu, "At Roslin Inn," recorded without source in Kinsley's notes (1969, vol III, pp. 1238-1239), and prints a transcription of the Alexander Nasmyth letter from which it comes, as facsimiled in Poems and Letters in the Hand of Robert Burns (St Louis, 1908), while noting some previous references to the Nasmyth letter, and taking this as an example of how previously-known Burns sources slip from view when not fully referenced by later editors.