This expression is a reference to Samuel Taylor Coleridge's poem "Rime of the Ancient Mariner," in which the Mariner inexplicably slays an albatross. The allusion may imply that Walton will play the role of Coleridge's Wedding Guest instead: he will listen to Victor's long, obsessive story that will ultimately be a confession of guilt, like the Ancient Mariner' tale. Since the poem was not published until September 1798, this reference also places the "17--" date of these letters as the summer of 1799. On the poem's role in the novel, see Beth Lau, "The Rime of the Ancient Mariner and Frankenstein," in Samuel Taylor Coleridge and the Sciences of Life, ed. Nicholas Roe (Oxford: Oxford University Press, 2001): 207-23.