One of my all-time favourite hymns. I first heard it sung by Pete Seeger on a friend's recording of his and Arlo Guthrie's great 'Precious Friends' album back in the early 1990's (although I think the album predates this). One of those songs that has stuck in my life. There's a truly horrible Christian worship rehash of the tune under the same name sadly popular on YouTube. I would urge anyone to avoid it like the plague. Pete's version of this song is still by far the best (although I prefer the 'Precious Friends' live recording):
Looking it up on Wikipedia the tune was composed by Robert Wadsworth Lowry and was first published in 1869. I found it's sheetmusic on this Folksong Collection.