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Two prized papers, one by Augustin Cauchy in 1815, presented to the FrenchAcademy and the other by Hermann Hankel in 1861, presented to G\"ottingenUniversity, contain major discoveries on vorticity dynamics whose impact is nowquickly increasing. Cauchy found a Lagrangian formulation of 3D idealincompressible flow in terms of three invariants that generalize to threedimensions the now well-known law of conservation of vorticity along fluidparticle trajectories for two-dimensional flow. This has very recently beenused to prove analyticity in time of fluid particle trajectories for 3Dincompressible Euler flow and can be extended to compressible flow, inparticular to cosmological dark matter. Hankel showed that Cauchy's formulationgives a very simple Lagrangian derivation of the Helmholtz vorticity-fluxinvariants and, in the middle of the proof, derived an intermediate resultwhich is the conservation of the circulation of the velocity around a closedcontour moving with the fluid. This circulation theorem was to be rediscoveredindependently by William Thomson (Kelvin) in 1869. Cauchy's invariants wereonly occasionally cited in the 19th century --- besides Hankel, foremost byGeorge Stokes and Maurice L\'evy --- and even less so in the 20th until theywere rediscovered via Emmy Noether's theorem in the late 1960, but reattributedto Cauchy only at the end of the 20th century by Russian scientists.

Cauchy's almost forgotten Lagrangian formulation of the Euler equation for 3D incompressible flow