Аннотация:Concepts of a "solid body motion" and "fluid flow" are compared taking into account the condition of inobservability of a "fluid particle". General properties of the fundamental set of fluid mechanics equations, accepted for describing fluid flows, are analyzed taking into account the compatibility condition. Hierarchy of periodic flows is classified basing on the order of linearized set of governing equations. Results of theoretical analysis of infinitesimal periodic flows in a stably stratified fluid including periodic internal waves and accompanied family of small scale components are given. Calculations of periodic internal waves propagation and generation in a fluid with arbitrary stable profile of buoyancy are compared with data of schlieren observations in laboratory. Fine flow structure observed behind uniformly towing strip is discussed in context of a given model. Some conclusions and recommendations on improvement techniques of a fluid dynamics experiment are presented.