Dylan himself said on 'Biograph' that he heard the melody on an old 78 with bagpipes on it; he thought it was a Scottish air. (But he may have come to that conclusion because of the pipes; or he may have identified it as that on the record; hard to say without asking him). I am Scottish and never heard it before. I asked my guitar teacher, also Scottish, and who played the pipes as a youngster, and he didn't know the tune, but identified typically Scottish aspects in it. I heard Eddi Reader & Karen Msthieson (both Scottish) sing it recently at the Transatlantic Sessions & it was wonderful: as if from the DNA. I would love to know what the original melody was, so keep the topic going, folks!