Tennyson follows his feelings in creating each line. He follows the music in his head. If you had asked him, at the end of the day, to describe the prosody of the poem to you, he would no doubt have had to think for a moment before he could answer you, not because he was ignorant of the terms, but because he had been writing a poem, not a metrical exercise. At every point, he was exerting his free will. And the outcome of that exertion was the form.