  A semi-analytic model of cluster cooling flows is presented. The model
assumes that episodic nuclear activity followed by radiative cooling without
mass-dropout cycles the cluster gas between a relatively homogeneous, nearly
isothermal post-outburst state and a cuspy configuration in which a cooling
catastrophe initiates the next nuclear outburst. Fitting the model to Chandra
data for the Hydra cluster, a lower limit of $284\Myr$ until the next outburst
of Hydra A is derived. Density, temperature and emission-measure profiles at
several times prior to the cooling catastrophe are presented. It proves
possible to fit the mass $M(\sigma)$ with entropy index $P\rho^{-\gamma}$ less
than $\sigma$ to a simple power-law form, which is almost invariant as the
cluster cools. We show that radiative cooling automatically establishes this
power-law form if the entropy index was constant throughout the cluster gas at
some early epoch or after an AGN activity cycle. To high precision, the central
value of $\sigma$ decreases linearly in time. The fraction of clusters in a
magnitude-limited sample that have gas cooler than $T$ is calculated, and is
shown to be small for $T=2\keV$. Similarly, only 1 percent of clusters in such
a sample contain gas with $P\rho^{-\gamma} < 2\keV\cm^2$. Entropy production in
shocks is shown to be small. The entropy that is radiated from the cluster can
be replaced if a few percent of the cluster gas passes through bubbles heated
during an outburst of the AGN.
