  A new set of laws of motion for turbulent jets propagating in an
intergalactic medium characterized by a decreasing density is derived by
applying conservation of momentum flux both in the classical and relativistic
framework. Two characteristic features of radio-galaxies, such as oscillations
and curvature, are modeled by a classical helicoidal jet. A third feature of a
radio-galaxy, the appearance of knots, is explained as an effect due to the
theory of images.
