THIRD SOUND IN SUPERFLUID HELIUM FILMS.

A detailed discussion of the present state of understanding the third sound phenomenon is presented describing the hydrodynamic theory of third sound in flat films, experimental results, the problem of surface roughness of the substrate, third sound resonators-theory and experiments, mixed He**3-He**4 films, the energy content of a third sound wave, the contribution of normal fluid motion to the attenuation of third sound, and, some attempts to go beyond hydrodynamics in describing third sound. Refs.