These our actors,
As I foretold you, were all spirits and
Are melted into air, into thin air:
Monster, I do smell all horse-piss; at which my nose is in great indignation.
Prospero apparently approves of the union of Miranda and Ferdinand and before you know if a bunch of gods and nymphs (spirits, actually) turn up to celebrate. This is going along quite nicely until Prospero remembers that Caliban and some of the others are conspiring against him. They seek out Prospero so they can do their dirty deeds, unaware that he is watching them. His response is to set the dogs on them (spirit dogs, actually).