In the triangular slave trade, which length was known as the middle passage?

Which stanza structure does emily dickinson use in this excerpt from "hope is the thing with feathers"? hope is the thing with feathers that perches in the soul, and sings the tune without the words, and never stops at all, and sweetest in the gale is heard, and sore must be the storm that could abash the little bird that kept so many warm.