The poetic device that was used by Emily Dickinson in this excerpt is;
Consonance is the repetition of consonant sounds among a group of closely related words. In the excerpt above, we see the repetition of the consonant sound 's'.
Consonance can be used for rhyming and emphasis. Emily wants her heart to be fast in forgetting memories of someone she once loved.
Consonance was used in this line, "Haste! lest while you're  lagging", to describe the urgency with which the poet wanted her heart to act. The rhythm formed through these words will hook the reader.
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