What is an example of periphrasis in any famous speech?
Periphrasis is the use of excessive and long words to convey a meaning which could have been said with much more brevity.
It is important to not that periphrasis is a type of circumlocution, or roundabout and indirect way of speaking, but circumlocution is not always periphrasis.
Periphrasis is used to embellish sentences by creating vivid effects to draw the audience's attention.
This can be seen in Abraham Lincoln's famous Gettysburg Address:
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...
Lincoln could have simply said, "A long time ago, our forefathers said that all men are created equal." However, he did not. He used vivid periphrasis here, especially when he said, "Four score and seven years ago" (which is eighty-seven years, in case you were wondering).
Hope this helps!