A metaphor is a type of figurative language that compares one thing to another without using the words "like" or "as." Usually, one thing is simply stated to be the other thing. Two notable metaphors in "Paul Revere's Ride" are explained below.
In the sixth stanza, the cemetery is compared to a military encampment with these words: "Beneath, in the churchyard, lay the dead, / In their night-encampment on the hill." Since this is a poem about the British military and a conflict that is about to occur, this metaphor is particularly apt. It makes one think about all the rows of graves as if they were the rows of tents in which soldiers would sleep. Like sleeping soldiers, the dead people in the graves were silent and still. Longfellow extends the metaphor by describing the wind as a "sentinel" that walks about among the tents, or graves.
In stanza 8 the flying hooves of Revere's horse are likened to a spark that ignites a raging fire: "And the spark struck out by that steed, in his flight, / Kindled the land into flame with its heat." Just as a single spark can cause a devastating forest fire, so the message that Paul Revere spread, and the battle that happened in response, began the Revolutionary War, an event that roused and involved the entire land occupied by the thirteen British colonies.
Each of these metaphors helps create the solemn and portentous mood that Longfellow imparts in this poem.