Final answer:
The poetry device used in the line from Edgar Allen Poe's 'The Raven' is assonance, due to the repetition of the vowel sound 'ee' in 'dreary' and 'weary'.
Step-by-step explanation:
The poetic device used in the line "Once upon a midnight dreary, while I pondered, weak and weary" is assonance. The repetition of the vowel sound 'ee' in 'dreary' and 'weary' is a clear example of assonance, which is the repetition of vowel sounds within a word. Rhyme is also present in these lines, as 'dreary' and 'weary' end in the same sound, but the specific question asks for the repetition of vowel sounds, which points to assonance. Other poetic devices like alliteration and consonance involve repetition of consonant sounds, which is not what is being demonstrated in this line.