Whittier used "We" and "Thou" as forms of repetition in his song of slaves in the desert.
We: He used we as to show that basically slaves WERE NOT alone in this at all. In a way at that time, every single slave was connected with each other no matter what.
Thou: From what i have seen, Whittier is using "Thou" to refer to God. For example: "We are fools, but Thou art wise"
For the devices, he's using Anaphora for "Thou" and maybe Conduplicatio for "We".
