He was a passionate advocate for preserving historic buildings. He urged the preservation of the Slate Roof House, a 1687 building that had housed Pennsylvania's founder, William Penn from 1699 to 1701, and served as the government offices for the Colony from 1699 to 1704. That fight was lost, but the shock of the building's 1867 demolition — after Watson's death — helped spark a historic preservation movement in Philadelphia.
