This paper discusses the attitudes toward theory (mathematical knowledge, rational mechanics) that prevailed around 1800 at Ponts et Chaussées, the corps of state engineers in charge of public works in France. Contrary to existing scholarship, it argues that Ponts et Chaussées was not the epicenter of scientific engineering in early-nineteenth-century France and proposes to reassess the role and meaning of theoretical tools in Ponts et Chaussées project-making. It focuses on one of the few projects in which theory played a significant role: the controversy surrounding the Ourcq canal. It considers the epistemological and social questions raised by the (mostly negative) reception of theories in that case. It concludes by drawing a few conclusions about the particular role of theory in the Ourcq case (with a particular influence on the issue of water purity) and, more generally, about the role of theory in the early-nineteenth-century Ponts et Chaussées.