O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
As thou goest onwards still will pluck thee back,
She keeps thee to this purpose, that her skill
Others that the Fair Youth has been involved with are “withering”, whereas the boy seems to have grown and developed even more sweetly. 
 The noticeably gentle rhythm is continued, with the two balanced clauses that make up line four.
Sonnet 126