Final answer:
The lines from "A Hymn to the Evening" contain a metaphor, comparing placid slumbers to a more heavenly and refined waking in the morning.
Step-by-step explanation:
The example in the given lines from "A Hymn to the Evening" is a metaphor. A metaphor is a figure of speech that compares two unrelated things by stating that one is the other. In this case, the comparison is between placid slumbers and a heavenly, refined waking in the morning. The poet is using the metaphor to express the idea that a peaceful sleep can lead to a more elevated and refreshed state of mind when morning comes.
Learn more about Metaphor in poetry