Ruskin is quoting from Shakespeare 's Macbeth, Act IV, Scene 2, in which Macbeth contrives a second meeting with the three witches to learn his fate after murdering Duncan and usurping his position as king of Scotland. Macbeth 'conjures' the witches to answer him regardless of any disasters they might then choose to visit upon the natural world, including the possibility of 'yesty waves' which might 'Confound and swallow navigation up...'