Equations of "rational mechanics" and general equilibrium
Pareto believed economics could be studied with the same mathematical rigor as physics, so it's quite possible he was speaking to the similarities between mathematical economics and classical mechanics (the latter of which was just coming into its own in the early 1900s).  I'm a physicist by training too, and those similarities helped me rapidly cover a great deal of ground my colleagues in economics had to learn fresh, so your intuition makes sense to me.