From which act in The Tempest does the quote below come, and what are the circumstances in which it is spoken?
But this rough magic
I here abjure; and when I have required
Some heavenly music (which even now I do),
To work mine end upon their senses that
This airy charm is for, I'll break my staff,
Bury it certain fathoms in the earth,
And deeper than did ever plummet sound
I'll drown my book.