  Stability of cylindrical interface between two ideal incompressible fluids,
including the magnetic field, surface tension and gravitational field is
studied in linear approximation. We found that helical waves arising both in
plasma comet tails and on the vertical cylindrical water jet in the air are
described by the same dispersion equation where the comet tail magnetic field
plays the same stabilizing role as surface tension for water jet. Hence they
represent the same phenomenon of Kelvin-Helmholtz instability. Thus helical
waves in comet tails and astrophysical jets may be simulated in the laboratory.
The resonance nature of the Kelvin- instability damping is demonstrated.
