In both sonnets I can find that the author is comparing his feelings with features of the nature. In the sonnet On Seeing the Elgin Marbles for the First Time I could see that characteristic: “Of godlike hardship tells me I must die like a sick eagle looking at the sky”, he compares himself with a weak bird which is about to die. In other words, he is showing he is unpleasant due to the idea of mortality. Here the main topic is the death “mortality”. The author feels the need to get rid of the idea of dying which oppressed him, as another characteristic of Romanticism.
In the sonnet When I have Fears that I may Cease to Be the author also shows a need to express what he feels (fear). The topic here is “fear”. Here is a connection between what he feels and the feature of the nature: “Before high-piled books, in charactery, Hold like rich garners the full ripen'd grain;…”Here he may try to make readers to understand what he feels, although it is an uneasy quote.