alliteration. possibly consonance, but I doubt it.
Explanation:
repeated H sounds at the beginning of the words happy and heath, repeated S sounds at the beginning of the words smil'd and snow. alliteration is repeated sounds at the beginning of words.
consonance, on the other hand, is repeated consonant sounds close together. but the H and S sounds are separated by 2- 3 words, so that's why I doubt it.
