He and Isaac had walked from Trinity to the Fair once, to buy prisms, and along the way, Isaac had made some remarks about fluxions—the beginnings of the calculus. During his recent sea-voyage from Massachusetts, Daniel had summoned that ancient memory to mind, and brought it back to life in his head, remembering certain queer details, like the shapes of the aquatic plants in the river Cam, bent downstream by the sluggish fluxion of the water. It was now obvious that Isaac had been thinking hard, and recently, about the same memory.