In this excerpt from Henry Wadsworth Longfellow's poem "The Day Is Done," one prominent poetic technique used is imagery. Longfellow employs vivid imagery by painting a picture of the transition from day to night, comparing the arrival of darkness to a feather being gently wafted down from an eagle's flight. This not only evokes a visual image but also conveys a sense of softness and tranquility associated with evening's approach.
Additionally, there is also the use of simile in the line "As a feather is wafted downward," where the descent of darkness is directly compared to the gentle falling of a feather, enhancing the serenity of the scene.