Hello sophiatdantas!
Alliteration is a type of poetic/literary device. If 2 words in a text have the same letter in the beginning or of they have the same sound then we can say that there's alliteration. In the stanza that you have given, there are 2 alliterations. They are :-
- sinking silently (In the first example, 's' is repeated ('s' is in the beginning of both the words)
- Drops down In the econd example, ' is repeated ('d' is in the beginning of both the words)
See the attached picture to get a better idea on the topic.
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Ľùçàźź