Rhetoric is the art of persuasion, and rhetorical devices are those that help to persuade an audience to adopt a certain point of view or to act in a certain way. Literary devices go beyond the literal and factual to add color and weight to a piece of writing.
Garrison's passionate desire is to persuade people to support the immediate abolition of slavery.
Garrison leans on the rhetorical devices of ethos and pathos to stir his readers to his cause, and on the literary devices of imagery, analogy, and repetition to support his arguments in a way that appeals to the emotions of his audience.
Ethos is appealing to character or credibility. Garrison brings credibility and positive associations to his desire to free the slaves by aligning these desires with the cherished principles of the nation's founding. For example, he alludes to the Revolutionary War, a time remembered by whites in the U.S. as one of heroic glory in which Americans sacrificed to win freedom. He writes that he will:
lift up the standard of emancipation in the eyes of the nation, within sight of Bunker Hill and in the birth place of liberty.
Freedom for slaves is thus explicitly linked to the white American's cherished freedom from England. Garrison will drive the point further home by alluding to the Declaration of Independence and calling for blacks to be included in its fold.
Pathos is emotional appeal. Garrison reaches his audience's emotions, first, through imagery, which is description using the five senses of sight, sound, taste, touch, and smell. For example, he defends the urgency—what he calls the "severity" of his language—by supplying the following imagistic analogies to other urgent situations:
Tell a man whose house is on fire, to give a moderate alarm; tell him to moderately rescue his wife from the hand of the ravisher; tell the mother to gradually extricate her babe from the fire into which it has fallen
We can picture innocent, perhaps defenseless, people (women and babies) in situations of dire threat of harm and realize that moderation is not the best approach to saving them. These images make it seem reasonable to apply an analogous urgency to the abolition of slavery.
Similarly, Garrison's use of repeated words sounds like a drumbeat, and this raises our emotions. Repetition evokes a sense of urgency in the reader's mind, as when Garrison repeats "let" and "tremble" in the following:
Let southern oppressors tremble—let their secret abettors tremble—let their northern apologists tremble—let all the enemies of the persecuted blacks tremble.
The imagery of trembling—a motion in response to fear of God—impresses the mind with the dangers inherent in not working to free the slaves.
Finally, the poem at the end crystallizes and condenses Garrison's ideas, adding to the fervency of the piece.