In Abraham Lincoln's Gettysburg Address, he uses rhetoric to move the audience to see things from his perspective.
The goal of rhetoric is to persuade towards a particular frame of view or a particular course of action.
There are two rhetorical devices that one can use with this result in mind: they are irony and metaphor.
A metaphor compares two dissimilar things that have similar characteristics.
In rhetoric, the job of the metaphor is similar in structure, but unique to its specific purpose in this situation. The rhetorical metaphor tries to share with the listeners...
...a new idea or meaning by linking it to an existing idea or meaning with which the audience is already familiar.
The rhetorical use of irony is to share with the listeners...
...an incongruity that is often used as a tool of humor in order to deprecate or ridicule an idea.
Irony is the difference between what is expected to happen and what really happens, or between what is said and what is meant. However, in using irony, President Lincoln does not use humor at all, but concentrates on the "incongruity" of a perception.
The first (and very famous) line of the Gettysburg Address uses a metaphor in which our "forefathers" gave birth to a new nation. Note the use of "brought forth" and "conceived."
Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
A second metaphor can be found in the reference to "consecration." This refers to making something holy or hallowed. This would refer to a church building or ground to be used for a cemetery. In this case, Lincoln says that the men who have died have consecrated the land because of their great sacrifice in giving their lives for a purpose to which they were so dedicated: the equality of all persons.
But, in a larger sense, we can not dedicate — we can not consecrate — we can not hallow — this ground. The brave men, living and dead, who struggled here, have consecrated it.
We might find an example of irony in the consecration as well. Usually this is something that is done by the living in terms of "making holy" a building or land. Without this dedication, consecration has not taken place. However, in this instance, President Lincoln states that the living, those left behind, cannot consecrate the land where these men have fallen, but the dead soldiers themselves have consecrated the ground by their great sacrifice. One would not expect the dead to be able to accomplish anything, yet in this case, Lincoln makes the argument that only the dead can hallow this ground.
Though the purpose of the speech may be to honor the men lost at the Battle of Gettysburg, it is also using the sense of the honorable men who consecrated (made holy) the land where they fell in battle as a reminder of the cause to which they were dedicated—the cause to which all Americans should be dedicated: freedom for all. It is this point that Lincoln uses in closing, to drive his point home to his audience that in order to make the sacrifice there meaningful, the commitment to freedom must endure:
...that we here highly resolve that these dead shall not have died in vain -- that this nation, under God, shall have a new birth of freedom -- and that government of the people, by the people, for the people, shall not perish from the earth.
In addition to metaphor, Lincoln uses antithesis. He contrasts the "death" of the soldiers in order for the nation to "live." This antithesis reflects the theme of his address-that we must honor the sacrifices made on the battlefield by preserving our ideals that this nation was conceived in liberty (a use of personification). There are several different examples of antithesis in his final paragraph: live/death, add/detract, and forget/remember.
Lincoln uses several other devices: alliteration, anaphora, personification and allusion. Choose the devices you feel most comfortable discussing and analyzing how they contribute to the persuasivness of the speech.