'Sky a tense diaphragm / Dusk hung like a backcloth / That shook where a swan swam' - Which metaphor has Seamus Heaney used in these lines from his poem, 'Twice Shy'?

A metaphor states that one thing is something else. This description, however, is too simple for the way metaphors often work in poetry, literature and speeches. You will often find something being described, or written about, as if it is something else, without the writer ever saying 'x is y' (do you see the mathematical metaphor there?).