chunk: Knot IX, A Serpent with Corners. The characters of Knot II, and Balbus and his two students, return to give three problems loosely connected by a narrative.
  Knot X, Chelsea Buns. Mad Mathesis and her niece return, as well as Balbus and his two students.
  Solution note: The puzzle called The Change of Day is never answered, as Carroll is "waiting for statistics" and is himself "so entirely puzzled by it."