In adapting Blake’s poem as a unison song, Hubert Parry deployed a two-stanza format, each taking up eight lines of Blake’s original poem. He added a four-bar musical introduction to each verse and a coda, echoing melodic motifs of the song. The word “these” was substituted by “those” before “dark satanic mills”.
Parry was initially reluctant to supply the music but knowing that his former student Walford Davies was to conduct the performance, and not wanting to disappoint either Robert Bridges or Davies, he agreed, writing it on 10 March 1916, and handing the manuscript to Davies with the comment, “Here’s a tune for you, old chap. Do what you like with it.” Davies later recalled,
We looked at [the manuscript] together in his room at the Royal College of Music, and I recall vividly his unwonted happiness over it … He ceased to speak, and put his finger on the note D in the second stanza where the words ‘O clouds unfold’ break his rhythm. I do not think any word passed about it, yet he made it perfectly clear that this was the one note and one moment of the song which he treasured.
Source: Wikipedia
The Far Mountains 