In late December 1763, the astronomers complete the first of their tasks as assigned by the Commissioners: establish the southernmost limit of the city of Philadelphia. Erecting an observatory near Cedar Street they mark the line of the city, and the latitude they will follow west, as 12 miles south of the city. Dixon, in yet another coffee house, falls into conversation with Dolly over Mason's and Molly's shared melancholic personalities, and a series of puzzling measurements that seem to suggest Pennsylvania is moving by degrees each year.