  The propagation of acoustic-gravity waves (AGWs) in the stratified isothermal
atmosphere is analyzed using methods of the oscillation theory. It is shown
that AGW in the atmosphere can be considered as an oscillatory process
occurring at two eigenfrequencies. This consideration makes it possible to
explain some of the observed properties of AGWs. The solutions for perturbed
hydrodynamic velocity versus time and spectral characteristics are obtained in
a real, but not complex, variables.
