Final answer:
The poetic device exemplified in the given lines is a simile.
Step-by-step explanation:
The poetic device exemplified in the given lines is a simile, which is a figure of speech that compares two things using the words 'like' or 'as'. In these lines, the musk-rose blowing in a green island is compared to something tranquil. This comparison helps to convey a sense of peace and serenity.