  Three comments are made on AGN heating of cooling flows. A simple physical
argument is used to show that the enthalpy of a buoyant radio lobe is converted
to heat in its wake. Thus, a significant part of ``cavity'' enthalpy is likely
to end up as heat. Second, the properties of the repeated weak shocks in M87
are used to argue that they can plausibly prevent gas close to the AGN from
cooling. As the most significant heating mechanism at work closest to the AGN,
shock heating probably plays a critical role in the feedback mechanism. Third,
results are presented from a survey of AGN heating rates in nearby giant
elliptical galaxies. With inactive systems included, the overall AGN heating
rate is reasonably well matched to the total cooling rate for the sample. Thus,
intermittent AGN outbursts are energetically capable of preventing the hot
atmospheres of these galaxies from cooling and forming stars.
