  Second sound phenomenon and ballistic heat conduction, the two wave like
propagation modes of heat, are the two most prominent, experimentally observed
non-Fourier effects of heat conduction. In this paper we compare three related
theories by quantitatively analyzing the crucial NaF experiments of Jackson,
Walker and McNelly, where these effects were observed together. We conclude
that with the available information the best comparison and insight is provided
by non-equilibrium thermodynamics with internal variables. However, the
available data and information is not the best, and further, new experiments
are necessary.
