In classic scientific style, Micah’s mathematical breakthrough is inspired by a turnip-related accident. Like the cinnamon roll that she is, she is seized with terror for Kaab’s people – they must be warned that their navigational techniques are flawed before their luck runs out and they start losing ships. What will the Kinwiinik do to protect their secrets?

It’s a rare Micah-focused week in Tremontaine. We join her in the cluttered student lodgings, frustrated by equations that never quite result in the right numbers, and certain that she’s missing something crucial. Her room mates are no help, constantly knocking her desk and intimidated by even the briefest brush with the mathematics she’s grappling with. Clearly what she needs is a bit more space and a tomato pie.

But the City is bustling with people and Micah is juggling difficult ideas, lots of papers and a sack of turnips. Inevitably (and hilariously), it’s a vision of turnips silhouetted against the sun as they arc across her line of sight when she takes a tumble that solves her dilemma. The world is not an ellipsoid. The world is round.

I’ve never seen a round turnip, but I’m not going to question genius at work.

There’s so much that delights me in this passage: Micah’s constant reduction of the world around her to a set of mathematical problems that would explain everything if only she could solve them. Her pure, straightforward view that just can’t recognize evasions or polite lies (why hasn’t Rafe taken her turnips to Tremontaine? She’s reminded him over a dozen times). Her instinctive reaction to her insight – terror for her friend, rather than a suspicion that Kaab might have lied to her. She is too pure to live (although if she doesn’t, there will be RAGE and TEARS and DRAMA. You’ve been warned).

Rafe rescues her from her attempt to prove her thesis in the streets, exhibiting his characteristic quick thinking and facility with lies to extract her from an irritated butcher and a bemused guardsman. Once he gets her story straight, it takes but a moment for him to draw the obvious conclusion that no sailor could enjoy such luck as Micah believes the Kinwiinik to have enjoyed; clearly, they know the world is round. And Rafe is a merchant’s son: he knows the value of this knowledge, and what a true trader would be prepared to do to keep it secret.

This delivers a series of gently comedic scenes, from the slapstick science to the running joke of how badly Micah lies (except, of course, that she has fooled the entire university into thinking her a boy). Because of course Kaab finds them in the streets while they are plotting how best to lie to her, fresh from a disappointingly short lesson with a disgruntled Vincent, and with her feet still far from the floor after a night with the beautiful Tess.

But Kaab is not just a Trader; she’s a spy. Rafe may be a better liar than Micah, but he’s nowhere near good enough to fool Ixkaab Balam. Thankfully, Kaab is reluctant to kill her friends to preserve her family’s secrets; she has another idea that I suspect Rafe will like little better than outright murder – she will involve the Duchess. As schemes go, it’s overcomplicated, it relies on the Duchess’s ability to control Rafe, and on Kaab’s ability to tease out Diane’s secret. What can possibly go right?

I enjoyed this week – once again, it feels fairly low key as the eddies swirl ever more tightly about the twin secrets of the Kinwiinik and the Duchess. The City comes to bewildering, colourful life through Micah’s eyes because we see the students and the bustling streets rather than the cool halls of Tremontaine. The cross-hatching of world-building and shading of plot reinforce what we know and jangle the nerves as we anticipate the confrontation that is surely just around the corner…