  The results of hydrodynamic simulations of the Virgo and Perseus clusters
suggest that thermal conduction is not responsible for the observed temperature
and density profiles. As a result it seems that thermal conduction occurs at a
much lower level than the Spitzer value. Comparing cavity enthalpies to the
radiative losses within the cooling radius for seven clusters suggests that
some clusters are probably heated by sporadic, but extremely powerful, AGN
outflows interspersed between more frequent but lower power outflows.
