The first lines of "On Seeing the Elgin Marbles" might catch readers off guard. This sonnet's title suggests that this will be a poem about a visit to a famous collection of 2,000-year-old statues: the titular Elgin Marbles, named for the British ambassador who spirited them away from the Parthenon in Greece and sold them to London's British Museum, where they remain to this day.
But the speaker doesn't begin by describing those ancient, stony gods and goddesses. Instead, he describes their effect on him—his experience of looking at them for the first time. And it seems as if that experience hasn't been altogether comfortable.
"My spirit," the speaker begins, "is too weak." An abrupt caesura after those first few words leaves readers wondering: too weak for what?
The speaker goes on:
[...] mortality
Weighs heavily on me like unwilling sleep,
In other words, looking at these statues has, for some reason, made the speaker feel terribly aware of his own inevitable death. Perhaps "mortality" itself is his spirit's "weak[ness]"—or perhaps he feels "weak" because thoughts of death leave him so shaken up.
Either way, he clearly feels powerless in the face of these thoughts. His simile here, in which "mortality" feels like "unwilling sleep," suggests that he worries he might nod off into an eternal slumber at any moment. The "heavi[ness]" of this sleep might put readers in mind of the way eyelids feel when you're trying hard not to fall asleep: so weighty they keep on shutting whether you like it or not. The enjambment at the end of line 1 evokes this heaviness as well, pulling the reader from one line to the next. Death, this speaker reflects, is just as involuntary and just as inescapable as that kind of helpless exhaustion.
The rest of this sonnet will explore why, exactly, the sight of some of the world's most beautiful statues should make the speaker so acutely aware of death—and what that awareness might suggest about the nature of art itself.