Reflections on some general laws of nature which are observed in the effects of arbitrary forces
Euler proves that, given the static principle, one can derive the equilibrium conditions for a fluid, and from these, he obtains the integrability conditions for a "Pfaffian" form in three variables. He also looks at the equilibrium of a weight suspended from three elastic cords by looking, via analogy, to a special case of the problem for fluids. (Based on Clifford Truesdell's An idiot's fugitive essays on science: methods, criticisms, training, circumstances.)
Original Source Citation
Mémoires de l'académie des sciences de Berlin, Volume 4, pp. 189-218.
Opera Omnia Citation
Series 2, Volume 5, pp.38-63.