The eighteenth century represents a major turning point in French engineers' conception and practice of calculation. This turning point can be described as a transition between the traditional use of arithmetical and geometric tools and the introduction of a new set of mathematical instruments based on calculus. This transition takes place within the epistemological frame provided by the Enlightenment analytical approach to science and technology. It is above all inseparable from a shift in the conception of technological efficiency that presents a strong connection to emerging political and economical values. Towards the end of the eighteenth century, engineering and political economy connect. This connection enables French engineers to present themselves as impartial judges of public interest.