When Mirando arrives for the debate, he is offered an ascent in Halran's balloon. Accepting, he quickly discovers himself Marko's captive, his safety dependent on all the philosophers being escorted to the coast and given charge of a steamship (another recent invention). The scheme goes without a hitch, and all the members of the convention proceed by sea to Mnaenn, which they seize from the witches.
Mirando arrives for a debate and is offered a ride in Halran's balloon. He becomes Marko's captive after accepting, his freedom tied to the safe transport of the philosophers to the coast and their assignment to a steamship (a new invention). The plan succeeds, and all the convention attendees travel by sea to Mnaenn, where they seize control from the witches.