Abstract: This paper describes a forgotten episode of the history of dimensional analysis. An article published in 1761 by François Daviet de Foncenex contains the first known attempt to derive a physical law – the parallelogram rule of forces – using the principle of homogeneity. The motivation of that work was the wish to provide an a priori proof of the basic laws of mechanics. The context and consequences of the paper are described. It is shown that this attempt was not grounded upon clear and solid assumptions and that its basic ideas were implicitly in conflict with the conceptions of that time. Keywords: dimensional analysis; parallelogram of forces; laws of mechanics; a priori proof; Foncenex, François Daviet de