  A new analysis of basic Couette flow, is based on an Action Principle for
compressible fluids, with a Hamiltonian as well as a kinetic potential.
  An effective criterion for stability recognizes the tensile strength of
water. This interpretation relates the problem to capillary action and to
metastable configurations (Berthelot's negative pressure experiment of 1850).
We calculate the pressure and density profiles and find that the first
instability of basic Couette flow is localized near the bubble point. This
theoretical prediction has been confirmed by recent experiments.
  The theory is the result of merging the two versions of classical
hydrodynamics, as advocated by Landau for superfluid Helium II, but here
applied to fluids in general, in accord with a widely held opinion concerning
superfluidity.
  In this paper two-flow dynamics is created by merging two actions, not by
choosing between them, nor by combining the two vector fields as in the
Navier-Stokes equation . At rest, as contributions to the mass flow they
cancel, but a non-zero kinetic energy and kinetic potential as well as non-zero
angular momentum remain, manifest as liquid tension, as is well known to exist
by observation of the meniscus and configurations with negative pressure.
(Fronsdal 2020b in preparation).
  This theory gives a very satisfactory characterization of the limit of
stability of the most basic Couette flow. The inclusion of a vector field that
is not a gradient has the additional affect of introducing spin, which explains
a most perplexing experimental discovery: the ability of frozen Helium to
remember its angular momentum.
