  Mechanical active galactic nucleus (AGN) feedback plays a key role in massive
galaxies, galaxy groups and clusters. However, the energy content of AGN jets
that mediate this feedback process is still far from clear. Here we present a
preliminary study of radial elongations $\tau$ of a large sample of X-ray
cavities, which are apparently produced by mechanical AGN feedback. All the
cavities in our sample are elongated along the angular (type-I) or jet
directions (type-II), or nearly circular (type-III). The observed value of
$\tau$ roughly decreases as the cavities rise buoyantly, confirming the same
trend found in hydrodynamic simulations. For young cavities, both type-I and II
cavities exist, and the latter dominates. Assuming a spheroidal cavity shape,
we derive an analytical relation between the intrinsic radial elongation
$\bar{\tau}$ and the inclination-angle-dependent value of $\tau$, showing that
projection effect makes cavities appear more circular, but does not change
type-I cavities into type-II ones, or vice versa. We summarize radial
elongations of young cavities in simulations, finding that $\bar{\tau}$
increases with the kinetic fraction of AGN jets. While mild jets always produce
type-II cavities, thermal-energy-dominated strong jets produce type-I cavities,
and kinetic-energy-dominated strong jets produce type-II cavities. Our results
suggest that some AGN jets are strong and dominated by thermal energy (or
cosmic rays). However, these jets do not dominate in AGN feedback. If most jets
are dominated by non-kinetic energies, they should be mainly mild jets. If most
jets are strong, they must be mainly dominated by the kinetic energy.
