The scene opens with the Witches discussing the dreadful things they can do to men. However, there are limits to their powers. They can make the sea captain 'dwindle, peak, and pine' (line 23) but 'his bark cannot be lost' (line 24). As Macbeth and Banquo enter the heath, on their way home from their victorious battle, the Witches prepare themselves with a charm.
In the first two scenes, the world of the Witches has been separated from the world of men. The two worlds are now brought together. The Witches' conversation emphasises that although they cannot sink the sea-captain's boat, their evil and vindictive will cannot be thwarted by men, 'I'll drain him dry as hay' (line 18).
