The world will little note, nor long remember what we say here, but it can never forget what they did here. What Rhetorical Device is this and why?

The rhetorical device used in this passage is antithesis.

Explanation:

Antithesis is a rhetorical device that achieves a contrasting effect by putting two opposite ideas together. Antithesis uses word parallelism so that the contrasting words, phrases, or ideas present similar grammatical structures.

The passage we are analyzing here is an excerpt from Lincoln's famous "Gettysburg Address." The first idea is that what he says is insignificant and will soon be forgotten. The second idea is that the death of soldiers during the Civil War will never be forgotten. Notice how one thing will not be remembered while the other will, and how they are placed side by side. This is a beautiful example of antithesis.

first of all the mood needs to be defined: it is obviously a mood of deceitful relief and serenity. why? because the cape horn is one of the most violent maritime regions of earth and although it has been left behind, it is a personification of ahab’s delirium and the fact that the end of the passage speaks about it about being “hidden” but “raving” immediately conveys the feeling the calm is not permanent but fleeting.

furthermore, the oxymoron “mild stun'sails” is the first instance of such feeling as the adjective “mild” is associated with “stunned” underscoring the fleeting nature of this “tranquil” moment. and the contrasts keep piling up, with the opposition between “dark den” and “blessed light and air” to end with the final conclusion that leaves the reader with the uneasy knowledge that this is just a reprieve.