What figure of speech is "change of mood" in Robert Frost's "Dust of Snow"?

Question

From "Dust of Snow" by Robert Frost:

Has given my heart
A change of mood
And saved some part
Of a day I had rued.

What figure of speech, or poetic device, is used in the line, "A change of mood"?

Answer 1

There’s a straightforward double meaning (ambiguity) in this line. The word “mood” can mean something internal to a person:

mood, n., 3.a. A person’s humour, temper, or disposition at a particular time.

or external to them:

mood, n., 3.c. The pervading atmosphere or tone of a particular place, event, or period.

_{Oxford English Dictionary.}

So we can read the line two ways: the “dust of snow” shaken down by the crow has changed the atmosphere of the place, and also changed the speaker’s feelings.

Ambiguity does not fall under the remit of “figure of speech”, which usually encompasses devices in which words are not to be taken literally. But it is an important poetic device nonetheless:

The machinations of ambiguity are among the very roots of poetry.

_{William Empson (1930). Seven Types of Ambiguity, p. 3. London: Chatto and Windus (1947).}