Fragment of a poem

Cafe Society
1

I am currently reading Passage by Connie Willis, and enjoying it immensely. one of the characters quotes a pair of lines from a poem:

“Four times fifty living men, they dropped down one by one.”

Any ideas what this bit came from?

2

Those lines appear in The Rime of the Ancient Mariner though not in succession.

3

The first Google entry will give you your answer. You may hear about that Google thing in the future.

4

The line’s also quoted by Iron Maiden in their song named after the poem.