  Observations of warped discs can give insight into the nature of angular
momentum transport in accretion discs. Only a few objects are known to show
strong periodicity on long timescales, but when such periodicity is present it
is often attributed to precession of the accretion disc. The X-ray binary
Hercules X-1/HZ Herculis (Her X-1) is one of the best examples of such
periodicity and has been linked to disc precession since it was first observed.
By using the current best-fitting models to Her X-1, which invoke precession
driven by radiation warping, I place a constraint on the effective viscosities
that act in a warped disc. These effective viscosities almost certainly arise
due to turbulence induced by the magneto-rotational instability. The
constraints derived here are in agreement with analytical and numerical
investigations into the nature of magneto-hydrodynamic disc turbulence, but at
odds with some recent global simulations.
