Why does Walton make an analogy between himself and the "Ancient Mariner"?In which letter is it found in Mary Shelley's Frankenstein

In Letter II of Frankenstein, Walton writes in the latter
part of his missive to his sister that he is going to unexplored regions,
to

'the land
of mist and snow'; but I shall kill no albatross, therefore do not be alarmed for my
safety, or if I should come back to you as warn and woeful as the 'Ancient
Mariner'? 

Walton continues
by telling his sister a "secret":  He has long had an enthusiasm for the "dangerous
mysteries" of the ocean which has been influenced by his reading of Coleridge's poem. 
And, as in Coleridge's poem, Walton's expedition is to the polar regions.  In fact, in
"The Rime of the Ancient Mariner," the "polar gods" punish the sailor for his disrespect
for nature by his killing of the albatross.  Similarly, Walton displays disregard for
nature by later insisting that the ship remain stuck in the polar ice because he is so
obsessed with reaching the North Pole.  Fortunately, of course, Walton is dissuaded by
Victor Frankenstein and his crew, so he turns back from his
mission.