The brown arc Of the neck I cannot catch, (8-9)
At the beginning of the poem, the speaker's ride on Ariel is terrifying. She "cannot catch" the horse's neck; she's lost control. And she doesn't like it. (Can't say we blame her—this horseback ride sounds super-scary to us, too.)
Something elseHauls me through air— (15-16)
Here, we can once again sense the speaker's fear. We see that she has no power, that she's "haul[ed]" through air by the uncontrollable Ariel.
I unpeel—Dead hands, dead stringencies.
And now I Foam to wheat, a glitter of seas. The child's cry
Melts in the wall. (17-22)
In these lines, we can see the speaker's transformation. When she "unpeel[s] / Dead hands, dead stringencies," she lets go of the things that tie her down in life. The cry of a child (or perhaps her own childlike cry of fear) melts away. She's letting loose, and she likes it.